Complete Collection of Questions and Answers
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MLA Citation: Bloomfield, Louis A. "Complete Collection of Questions and Answers" How Everything Works 22 Apr 2018. Page 1 of 1. 22 Apr 2018 <>.
1. Are you accelerating when your speed decreases?
Yes! If you are walking east and you come to a stop, it is because you accelerated to the west! By "deceleration" we mean acceleration in the direction opposite our direction of motion. Thus in a car, when you stomp on the brake and decelerate, you are actually accelerating toward the rear of the car (in the direction opposite its direction of motion).

2. As the Space Shuttle falls, does it accelerate forever and does it go faster and faster?
Yes to the first part, no to the second part. Remember that acceleration can change the direction of velocity without changing the magnitude of velocity (the speed of the object). When the space shuttle accelerates, its speed doesn't change, only its direction of travel. Although it accelerates endlessly, it never goes faster or slower. Actually, if the shuttle's orbit isn't circular, its speed does increase and decrease slightly as it orbits the earth in an ellipse, but that's an unimportant detail. For a circular orbit, the shuttle's speed is constant but its velocity (speed and direction) is not constant!

3. Does a bullet go from 0 to maximum speed instantly?
The bullet accelerates gradually, like everything else. However, the forces that push on the bullet when the gun is fired are extremely large and it accelerates extremely rapidly. It goes from 0 to maximum speed in about a thousandth of a second.

4. Does air resistance affect a horizontally thrown ball?
Yes. A ball thrown horizontally gradually loses its downfield component of velocity. For that reason, you must throw a ball somewhat below the 45° angle from horizontal in order to make it travel as far as possible. Actually, the air has even more complicated effects on spinning balls.

5. Doesn't weight have resistance to acceleration?
No, weight measures a different characteristic of an object. Mass measures inertia (or equivalently resistance to acceleration). But weight is just the force that gravity exerts on an object. While an object that has great weight also has great mass and is therefore hard to accelerate, it's not the weight that's the problem. To illustrate this, imagine taking a golf ball to the surface of a neutron star, where it would weigh millions of pounds because of the incredibly intense gravity. That golf ball would still accelerate easily because its mass would be unchanged. Only its weight would be affected by the local gravity. Similarly, taking that golf ball to deep space would reduce its weight almost to zero, yet its mass would remain the same as always.

6. How can an object in space "fall"?
Gravity still acts on objects, even though they are in space. No matter how far you get from the earth, it still pulls on you, albeit less strongly than it does when you are nearby. Thus if you were to take a ball billions of miles from the earth and let go, it would slowly but surely accelerate toward the earth (assuming that there were no other celestial objects around to attract the ball—which isn't actually the case). As long is nothing else deflected it en route, the ball would eventually crash into the earth's surface. Even objects that are "in orbit" are falling; they just keep missing one another because they have large sideways velocities. For example, the moon is orbiting the earth because, although it is perpetually falling toward the earth, it is moving sideways so fast that it keeps missing.

7. I can accept that weight is a force, but it doesn't seem to follow common sense to me.
It would seem like a force if you had to lift yourself up ladder. Imagine carrying a friend up the ladder; you'd have to pull up on your friend the whole way. That's because some other force (your friend's weight) is pulling down on your friend. But when you think of weight as a measure of how much of you there is, then it doesn't seem like a force. That's where the relationship between mass and weight comes into play. Mass really is a measure of how much of you there is and, because mass and weight are proportional to one another, measuring weight is equivalent to measuring mass.

8. I don't understand the horizontal component of a ball thrown downfield. Does it have constant velocity and/or acceleration, even at the start?
Until you let go of the ball, you are in control of its velocity and acceleration. During that time, it does accelerate and its velocity isn't constant. But as soon as you let go of the ball, everything changes. The ball's motion in flight can be broken up into two parts: its vertical motion and its horizontal motion. Horizontally, the ball travels at a constant speed because there is nothing pushing or pulling on it horizontally (neglecting air resistance). Vertically, the ball accelerates downward at a constant rate because gravity is pulling down on it. Thus the ball travels steadily forward in the horizontal direction as it fall in the vertical direction. Of course, falling can begin with upward motion, which gradually diminishes and is replaced by downward motion.

9. I don't understand the relationship between mass, acceleration, and force in Newton's second law.
First off, force causes acceleration. The stronger that force, the more the acceleration. In fact, the two are exactly proportional to one another: double the force and you double the acceleration. Secondly, mass resists acceleration. The more mass an object has, the less it accelerates. The two are exactly inversely proportional to one another: double the mass and you halve the acceleration. These two ideas can be combined into one observation: the force you exert on an object is equal to the product of its mass times the acceleration it experiences. Look at that relationship: if you double the force you exert on an object, you double its acceleration, so that part checks out. If you double the object's mass and leave the force unchanged, then the acceleration must be halved, so that part checks out. Thus Newton's second law is simply a sensible relationship between the force you exert on an object, its mass, and its acceleration.

10. If a projectile released or hit at a 45° angle above horizontal should go the farthest, then why, in the game of golf, does the three iron (20° loft) hit a golf ball so much farther in the air than, say, a seven iron (approximately 45° loft) if the same technique and force are produced by the golfer? Is it backspin, shaft length, etc.?
It's backspin! Air pushes the spinning ball upward and it flies downfield in much the same way as a glider. When you throw a glider for distance, you concentrate your efforts on making it move horizontally because the air will help to keep the glider from hitting the ground too soon. Similarly, the air holds the spinning golf ball up for a remarkably long time so that giving the ball lots of downfield speed is most important for its distance. That's why a low-loft club like a three iron sends the ball so far.

11. If force causes only acceleration and not velocity, does a machine (i.e. an engine) that causes a constant velocity in an adjacent object not exert a force?
If that adjacent object is free of any other forces, then no, the machine does not exert a force on it! This is a wonderful question, because it points toward many of the issues concerning energy and work. The bottom line is this: if some object is truly free moving (no other forces on it), it will move along at constant velocity without anything having to push on it. For example, if your car were truly free moving (no friction or air resistance), then it would coast forever on a level surface and the engine wouldn't have to do anything. You could even put the car in neutral and turn off the engine. The only reason that you need an engine to keep pushing the car forward is because friction and air resistance push the car backwards.

12. If the Space Shuttle is always falls toward the center of the earth, how does it get to outer space? If something accelerates, doesn't it go faster and thus have its speed increase?
The second question first: no, an object can accelerate without going faster. In fact, a stopping object is accelerating! If an accelerating object can speed up or slow down, it can certainly maintain a constant speed. If you swing a ball around in a circle on a string, that ball is accelerating all the time but its speed isn't changing.

Now the first question: for the space shuttle to reach orbit, it needs an additional force in the upward direction. It obtains that force by pushing exhaust gas downward so that the exhaust gas pushes it upward. During the time when it's heading toward orbit, it's not falling because it has an extra upward force on it. However, the Space Shuttle can leave its orbit and head off into outer space by traveling faster than it normally does. It acquires this increased speed by firing its rocket engines again. Its usual speed keeps it traveling in a circle near the earth's surface. If it went a bit faster, its path wouldn't be bent downward as much and it would travel more in a straight line and away from the earth. It would still be falling toward the earth (meaning that it would still be accelerating toward the earth), but its inertia would carry it farther away from the earth. If the Shuttle had enough speed, it would travel to the depths of space before the earth had time to slow its escape and bring it back.

13. If you drop a penny from the Empire state building - could it really puncture a hole in a car because of its constant acceleration?
Probably not. If the penny were to fall sideways, so that it had as little air resistance as possible, it would reach about 280 km/h (175 mph). That speed ought to be enough to drive the penny into the car if its top were thin enough. However, studies have shown (see that coins tumble as they fall and experience substantial air resistance. As a result, you could probably catch a falling penny in your hand, although it might sting a bit. A falling ballpoint pen, because of its aerodynamic shape, is another matter.

14. If you dropped a bullet and at the same time, fired a bullet directly at the ground, wouldn't the bullet fired at the ground hit the ground first?
Sure it would. The fired bullet will only hit the ground at the same time as the dropped bullet if the fired bullet is shot exactly horizontally. If you fire the bullet at the ground, then it starts out with an enormous downward component to its velocity. The falling bullet doesn't have this initial downward component to its velocity and never catches up.

15. If you fire a bullet horizontally and drop an identical bull at the same moment, will they both hit the ground at the same time?
Yes. The fired bullet may travel farther, but it will fall just as quickly as the dropped bullet and they'll hit the ground at the same moment. This effect explains why you must aim above the target when shooting at something far away. The faster the bullet travels to the target, the less it will drop. An arrow travels slowly enough that it will fall a considerable distance en route. You must aim quite high when shooting an arrow.

16. If you had an object in an empty sphere with a radius of a few miles, surrounded by equally distributed and very concentrated mass, what effects of gravity would the object feel?
As long as the mass isn't so concentrated that the laws of general relativity become important, the object won't feel any gravity at all. The forces from opposite sides of the surrounding mass will cancel exactly. For example, if you were at the center of the earth in a large spherical opening, you would be perfectly weightless. The force from the north side of the earth would balance the force from the south side. This effect is quite remarkable and depends on the fact that gravity becomes weaker as the inverse square of the distance separating two objects. That way, even if you aren't in the exact center of the earth, the forces still cancel.

17. If you jump off of a diving board, are you exerting force on the board or is it exerting force on you?
Actually, as you stand on the end of the board or as you push off from its end, you are pushing on the board and it is pushing back on you. The forces you exert on one another are exactly equal in amount but opposite in direction. That observation is called Newton's third law of motion and is the real meaning behind the phrase "for every action there is a reaction."

18. If you shot a gun and dropped a bullet at the same time, how could they land at the same time? Wouldn't the acceleration behind the bullet keep it in the air longer?
If you shot the bullet horizontally, it really would hit the ground at the same time as the bullet you simply dropped. During the firing, the bullet would accelerate like crazy, but only horizontally. It would leave the gun with a velocity that was only in the horizontal direction. With no forces pushing on it horizontally after that (we'll neglect air resistance), the bullet will make steady progress downfield. But at the same time, it will begin to fall. The vertical component of its velocity will gradually increase in the downward direction as it falls. Like the dropped bullet, it will drift downward faster and faster and the two will hit the ground together.

19. In what sense is the Space Shuttle falling toward the earth?
When the space shuttle circles the earth, it's experiencing only one force: the force of gravity. As a result, it's perpetually accelerating toward the earth's center. If it weren't moving initially, it would begin to descend faster and faster until...splat. But it is moving sideways initially at an enormous speed. While it accelerates downward, that acceleration merely deflects its sideways velocity slightly downward. Instead of heading off into space, it heads a little downward. But it never hits the earth's surface. Instead, it arcs past the horizon and keeps accelerating toward the center of the earth. In short, it orbits the earth—constantly accelerating toward the earth but never getting there.

20. Is it possible for a ball to fall to earth at a different angle from the one at which it rose?
If the ground is level and there were no air resistance, the answer would be no. The flight of the ball is perfectly symmetric. It rises to a maximum height in a parabolic arc and then returns to the ground as the continuation of that same parabolic arc.

However, if the ground isn't level, then the angle it hits the ground at might be different. For example, if you toss a ball almost horizontally off a cliff, it will hit the ground almost vertically. Horizontal and vertical are two very different directions.

Air resistance also tends to slow a ball's motion and it's particularly effective at stopping the downfield component of its velocity. Gravity makes sure that the ball descends quickly, but there is no force to keep the ball moving downfield against air resistance. The result is that balls tend to drop more sharply toward the ground. When you hit a baseball into the outfield, it may leave your bat at a shallow angle but it will drop pretty vertically toward the person catching it.

Finally, if the ball is spinning, it can obtain special forces from the air called lift forces. These forces can deflect its path in complicated ways and are responsible for curve balls in baseball, slices and hooks in golf, and topspin effects in tennis.

21. Is it possible for a skydiver who jumps second from a plane to put himself in an aerodynamic position and overtake a person who jumped first?
Yes. When you skydive, your velocity doesn't increase indefinitely because the upward force of air resistance eventually balances the downward force of gravity. At that point, you reach a constant velocity (called "terminal velocity"). Just how large this terminal velocity is depends on your shape. It is possible to increase your terminal velocity by rolling yourself into a very compact form. In that case, you can overtake a person below you who is in a less compact form.

22. Is there a fixed amount of force in the universe?
No, forces generally depend on the distances between objects, so that two objects that are moving together or apart will experience different amounts of force as they move about. As a result, the total amount of force anywhere can change freely. But there are quantities that have fixed totals for the universe. The most important of these so-called "conserved" quantities is energy.

23. Isn't there "some" acceleration at the very start and very end of an elevator ride? Why does one's stomach take a flop when the elevator stops and not when it starts?
Yes, there is acceleration at the start and stop of an elevator ride. As the car starts, it accelerates toward the destination and as the car starts, it accelerates in the opposite direction. Your stomach takes a flop whenever you feel particularly light, as when you are falling or otherwise accelerating downward. As you accelerate downward, your body doesn't have to support your stomach as much as normal and you feel strange. In fact, you feel somewhat weightless. You have this feeling whenever the elevator starts to move downward (and therefore accelerates downward) or stops moving upward (and there accelerates downward).

24. What is the difference between mass and weight?
Mass is the measure of an object's inertia. You have more mass than a book, meaning that you are harder to accelerate than a book. If you and the book were each inside boxes, mounted on wheels, I could quickly determine which box you were in. I would simply push on both boxes and see which one accelerated most easily. That box would contain the book and you would be in the box that's hard to accelerate. Weight, on the other hand, is the amount of force that gravity (usually the earth's gravity) exerts on an object. You weigh more than a book, meaning that the earth pulls downward on you harder than it does on the book. Again, I could figure out which box you were in by weighing the two boxes. You'd be in the heavier box. So mass and weight refer to very different characteristics of objects. They don't even have the same units (mass is measured in kilograms, while weight is measured in newtons. But fortunately, there is a wonderful relationship between mass and weight: an object's weight is exactly proportional to its mass. Because of this relationship, all objects fall at the same rate. Also, you can use a measurement of weight to determine an object's mass. That's what you do when you weigh yourself on a bathroom spring scale; you are trying to determine how much of you there is-your mass-but you are doing it by measuring how hard gravity is pulling on you—your weight.

25. When you pushed the baseball and bowling ball with an equal force, the baseball went farther on the table because it has a smaller mass. If gravity also exerts an equal force on the 2 balls, like your push, then why do they fall at equal speeds?
The answer is that gravity doesn't exert equal forces on the 2 balls! It pulls down harder on the bowling ball than it does on the baseball. Suppose the bowling ball has 10 times the mass of the baseball. Then gravity will also exert 10 times the force on the bowling ball that it exerts on the baseball. The result is that the bowling ball is able to keep up with the baseball! The bowling ball may resist acceleration more than the baseball, but the increased gravitational force the bowling ball experience exactly compensates.

26. When you shoot a bullet straight upward, doesn't it accelerate upward?
When you shoot a bullet upward, is does accelerate as long as it's in the gun. The burning gases push upward on the bullet and it accelerates upward. But as soon as it leaves the gun, it's a falling object, with the only force on it being gravity (and air resistance).

27. When you throw a ball upward, what force pushes it upward?
To throw the ball upward, you temporarily push upward on it with a force greater than its weight. The result is that the ball has a net force (the sum of all forces on the ball) that is upward. The ball responds to this upward net force by accelerating upward. You continue to push upward on the ball for a while and then it leaves your hand. By that time, it's traveling upward with a considerable velocity. But once it leaves your hand, it is in free fall. Nothing but gravity is pushing on it—it's carried upward by its own inertia! In fact, it's accelerating downward at 9.8 m/s^2. It rises for a while, but less and less quickly. Eventually it comes to a stop and then it begins to descend.

28. While gravity supposedly makes all objects accelerate at the same rate, feathers do not seem to comply. What factors affect the feather's acceleration, besides air resistance (which should affect all objects equally)?
Actually, air resistance doesn't affect all objects equally. The feather has so much surface area that it pushes strongly on the air through which it moves and the air pushes back. For an object with very little mass and weight, the feather experiences an enormous amount of air resistance and has great difficulty moving through the air. That's why it falls so slowly. If you were to pack a feather into a tiny pellet, it would then fall just about as fast as other objects. Similarly, you fall much more slowly when your parachute is opened because it then interacts with the air much more effectively.

29. Why do objects on earth accelerate downward at the same speed regardless of their mass?
What you mean here is that they accelerate downward at the same rate ("speed" has a particular meaning that isn't so well suited to discussions of acceleration). This fact comes about because, although massive objects are harder to accelerate, they also experience more weight. Thus a huge stone will fall at the same rate as a small rock because the stone will be pulled downward more strongly by gravity and that extra pull will make up for the stone's greater inertia.

30. Why do two objects of unequal mass fall and hit the ground at the same time?
If one object has twice the mass of the other, then it is twice as hard to accelerate. To make it keep pace with the other ball, it must experience twice the force. Fortunately, gravity pulls on it twice as hard (it has twice the weight of the other ball), so in falling, it does keep pace with the other ball. The two fall together. Just for fun, imagine stepping off the high diving board with two friends. The three of you have essentially identical masses and weights and also fall at the same rate. Now imagine that two of you hold hands as you fall. You are now a single object with twice the mass of your other friend. Nonetheless, you still fall at the same rate. So an object with twice the mass of another falls at the same rate as that other object.

31. Why do you feel no acceleration in free fall, even though you are accelerating?
This wonderful question has many answers. The first, and most direct, is that you do feel the acceleration. You feel an upward fictitious force (not a real force at all, but an effect of inertia) that exactly balances your downward weight. The feeling you experiences is "weightlessness." That's why your stomach feels so funny. You're used to having it pulled downward by gravity but the effect of your fall is to make it feel weightless.

32. Why does a ball fall 4.9 meters during its first second of falling?
As a simple argument for that result, think about the ball's speed as it falls: it starts from rest and, over the course of 1 second, it acquires a downward speed of 9.8 m/s. Its average speed during that first second is half of 9.8 m/s or 4.9 m/s. And that is just how far the ball falls in that first second: 4.9 m. By holding the ball 4.9 m above the floor, you can arranged for it to hit one second after you drop it.

33. Why does an object accelerate when it changes direction?
What you mean by "changes direction" is that the direction part of its velocity changes. For example, instead of heading east at 10 m/s (or 10 miles-per-hour, if that feels more comfortable), it heads north at 10 m/s (or 10 miles-per-hour). This change in direction involves acceleration. The car must accelerate toward the west in order to stop heading east, and it must accelerate toward the north in order to begin moving north. Actually, it probably does both at once, accelerating toward the northwest and shifting its direction of motion from eastward to northward.

34. Why is 45° above horizontal the ideal angle to throw something the greatest distance if gravity is acting on the vertical direction but not the horizontal?
The 45° angle is ideal because it gives the ball a reasonable upward component of velocity and also a reasonable downfield component of velocity. The upward component is important because it determines how long the ball will stay off the ground. The downfield component is important because it determines how quickly the ball will travel downfield. If you use too much of the ball's velocity to send it upward, it will stay off the ground a long time but will travel downfield too slowly to take advantage of that time. If you use too much of the ball's velocity to send it downfield, it will cover the horizontal distances quickly but will stay of the ground for too short a time to travel very far. Thus an equal balance between the two (achieved at 45°) leads to the best distance. Note that this discussion is only true in the absence of air resistance.

35. Why is force = mass * acceleration an exact relationship (i.e. why not force = 2 * mass * acceleration)?
The answer to this puzzle lies in the definition of force. How would you measure the amount of a force? Well, you would push on something with a known mass and see how much it accelerates! Thus this relationship (Newton's second law) actually establishes the scale for measuring forces. If your second relationship were chosen as the standard, then all the forces in the universe would simply be redefined up by a factor of two! This redefinition wouldn't harm anything but then Newton's second law would have a clunky numerical constant in it. Naturally, the 2 is omitted in the official law.

36. Why on Pg. 6, 2nd full paragraph, it says the car is accelerating if the slope of the road changes but in the "not accelerating" list it says a bicycle going up a hill is not accelerating. Aren't those the same situation?
Here is why the two situations are different:

In the first case, the car is traveling on a road with a changing slope. Because the road's slope changes, the car's direction of travel must change. Since velocity includes direction of travel, the car's velocity must change. In short, the car must accelerate. Picture a hill that gradually becomes steeper and steeper—the car's velocity changes from almost horizontal to almost vertical as the slope changes.

In the second case, the bicycle is climbing a smooth, straight hill at a steady speed. Since the hill is smooth and straight, its slope is not changing. Since the bicycle experiences no change in its direction of travel or its speed, it is traveling at a constant velocity and is not accelerating.

37. How do you push a shopping cart and have the cart exert the same force on you, if you are still traveling forward? Friction? Air Resistance?
When you push a shopping cart straight forward down an aisle, you are pushing it forward and it is pushing you backward. If nothing else were pushing on the two of you, the cart would accelerate forward and you would accelerate backward. But the cart is experiencing friction and air resistance, both of which tend to slow it down. They are pushing the cart backward (in the direction opposite its motion). So you must keep pushing it forward to ensure that it experiences zero net force and continues at constant forward velocity. As for you, you need a force to keep yourself heading forward; otherwise the cart's backward force on you would slow you down. So you push backward on the ground with the soles of your shoes. In return, the ground pushes on you (using friction) and propels you forward. As a result, you also experience zero net force and move forward at constant velocity.

38. How does a surface know how hard it must push upward on an object to support that object?
If you put a piano on the sidewalk, the piano will settle into the sidewalk, squeezing the sidewalk's surface until the sidewalk stops it from descending. At that point, the sidewalk will be pushing upward on the piano with a force exactly equal in magnitude to the piano's downward weight. The piano will experience zero net force and will not accelerate. It's stationary and will remain that way.

But if the sidewalk were to exert a little more force on the piano, perhaps because an animal under the sidewalk was pushing the sidewalk upward, the piano would no longer be experiencing zero net force. It would now experience an upward net force and would accelerate upward. The piano would soon rise above the sidewalk. Of course, once it lost contact with the sidewalk, it would begin to fall and would quickly return to the sidewalk.

For an example of this whole effect, put a coin on a book. Hold the book in your hand. The book is now supporting the coin with an upward force exactly equal to the coin's weight. Now hit the book from beneath so that it pushes upward extra hard on the coin. The coin will accelerate upward and leap into the air. As soon as it loses contact with the book, it will begin to fall back down.

Thus, if the sidewalk pushed upward too hard, the piano would rise upward and leave the sidewalk's surface and if the sidewalk pushed upward too weakly, the piano would sink downward and enter the sidewalk's surface. A balance is quickly reached where the sidewalk pushes upward just enough to keep the piano from accelerate either up or down.

39. If a falling egg weighs only 1 newton, how can it exert a force of 1000 newtons on a table when it hits?
As the egg falls, it is experiencing only one force: a downward weight of 1 N. But when it hits the table, it suddenly experiences a second force: an upward support force of perhaps 1000 N. The table is acting to prevent the egg from penetrating its surface. The net force on the egg is then 999 N, because the upward 1 N force partially cancels the downward 1000 N force. If the egg could tolerate such forces, it would accelerate upward rapidly and wouldn't enter the table's surface. Because the egg is fragile, it shatters. The force that the egg exerts on the table is also 1000 N, this time in the downward direction. The egg and table push on one another equally hard. The table doesn't move much in response to this large downward force because it's so massive and because it's resting on the floor. But if you were to put your hand under the falling egg, you would feel the egg push hard against your hand as it hit.

40. If every force always has an equal and opposite force pushing against it (like the bowling ball and your arm in today's lecture), how can anything at all accelerate? Wouldn't forces always cancel each other out?
The two equal but opposite forces are being exerted on different objects! In many cases, those two objects are free to accelerate independently and they will accelerate—in opposite directions! For example, when I push on a bowling ball, it pushes back on me with an equal but opposite force. If my force on the bowling ball is the only force it experiences, it will accelerate in the direction of my force on it. Since it exerts an opposite force on me, I will accelerate in the opposite direction—we will push apart!

41. If it takes less force to push something up a ramp, why doesn't it also take less work?
When you lift an object using a ramp, the uphill force you exert on it is less than its weight but the distance you must travel along the ramp is more than if you simply lifted the object straight up. Since the work you do on the object is the product of the force you exert on it times the distance it travels in the direction of that force, the work isn't changed by using the ramp. For example, if you lift a cart weighing 15 N straight up for 0.2 meters, you do 3 newton-meter or 3 joules of work on it. To raise that cart that same 0.2 meters upward on the ramp, you'd have to exert a 3 N force on it as you pushed it 1.0 meter along the ramp. The work you'd do to raise the cart by pushing it up the ramp would be 3 joules again. No matter how you raise the cart to the height of 0.2 meters, you're going to do 3 joules of work on it.

42. If Newton's third law is true - then how can you move anything? If it exerts the exact same amount of force on you that you exert on it, wouldn't the net force be zero and the object wouldn't move?
The total force on the two of you (the object you're pushing on and you yourself) would be zero, but the object would be experiencing a force and you would be experiencing a force. As a result, the object accelerates in one direction and you accelerate in the other! To see this, imaging standing on a frozen pond with a friend. If the two of you push on one another, you will both experience forces. You will push your friend away from you and your friend will push you in the opposite direction. You will both accelerate and begin to drift apart. Each of you individually will experience a net force. (It's true that the two of you together will experience zero net force, which means that as a combined object, you won't accelerate. The way this appears is that your overall center of mass won't accelerate. It will remain in the middle of the pond even as the two of you travel apart toward opposite sides of the pond.)

43. If the downward motion of lifting a weight transfers energy to you, why does your arm get tired?
Your body is unable to store working that's done on it and also wastes energy even when it is not doing any work. When you lower a weight, the weight does transfer energy to you, but your body turns that energy into thermal energy. You get a little bit hotter. If you were made out of rubber, you might store it as elastic potential energy (like a stretched rubber band). Instead, your muscles don't save the energy in a useful form. As for getting tired, your muscles turn food energy into thermal energy even when you aren't doing work. That's what happens during isometric exercises. There's nothing you can do about it. It's like a car, which wastes energy when it's stopped at a light.

44. Is it impossible to do work on a ball while carrying it horizontally, or were you only referring to the force of gravity in the demonstration? Or must you be "pushing" the ball?
When I carried the ball horizontally at constant velocity, I did no work on the ball. That's because the force I exerted on the ball was directly upward and the direction the ball moved was exactly horizontal. Since work is force times distance in the direction of that force, the work I did was exactly zero. But when I first started the ball moving horizontally, there was a brief period during which I had to push the ball forward horizontally. That's when I "got the ball moving." During that brief period, I did do work on the ball and I gave it kinetic energy. It needed that kinetic energy to move horizontally. When I reached my destination, there was a brief period during which I had to pull the ball backward horizontally. That's when I "stopped the ball from moving." During that brief period, I did negative work on the ball and removed its kinetic energy.

45. What forces are involved when a football player who is running is tackled by another player?
If the two players collide hard, they will both exert enormous forces on one another. The player running toward the right will experience a force to the left and will accelerate toward the left (slowing down). The player running toward the left will experience a force to the right and will accelerate toward the right (slowing down). The forces involved would cause bruises if they weren't wearing pads. The pads reduce the magnitudes of the forces on their skin by prolonging the accelerations (smaller forces exerted for longer times). If one player simply trips up the other player, then the player who falls will still come to a stop. However, that player will be experiencing most of the stopping force from the ground by way of sliding friction.

46. What happens with things like liquids "falling" onto objects like sponges? Does the sponge exert an upward force onto the liquid?
When liquids fall onto sponges, the sponges do exert upward forces on the liquids. Otherwise, the liquids would continue to fall. When a raindrop hits your hair, you can feel it push on your hair and your hair pushes back, stopping the raindrop's descent.

47. When a falling egg hits a table and breaks, did it fail to push equally on the table?
No. It pushed hard against the table and the table pushed hard against it. The forces exerted were exactly equal but in exactly the opposite directions. Each object experienced a strong push from the other object. But as they say, "whether the rock hits the pitcher or the pitcher hits the rock, it's bound to bad for the pitcher." The egg couldn't take the push and it broke.

48. When a person bumps into something or has something dropped on them and a bruise forms, does it form because of the object hitting the person or from the person exerting a force on the object to keep that object from pass through their skin?
The bruise forms because of the force exerted on the person by the object. When an object hits you, it's obvious that the object pushes on you. But the object also pushes on you when you hit it. In fact, it's a matter of perspective which is hitting which. To a person standing next to you when you're hit by a ball, the ball hit you. To a person running along with the ball, you hit the ball. In each case, the ball pushes on you and gives you a bruise. You also push on the ball, causing it to accelerate away from you.

49. When you drop a glass on a hard floor, why does it sometimes break and sometimes not?
When the glass hits the floor, the floor exerts all of its force on the part of the glass that actually touches the floor. That small part of the glass accelerates upward quickly and comes to rest. The remainder of the glass isn't supported by the floor and continues downward. However the glass is relatively rigid and parts of it begin to exert forces on one another in order to stop the whole glass from bending. These internal forces can be enormous and they can rip the glass apart. Glass is a remarkable material; it never dents, it only breaks. As the glass tries to come to a stop, the internal forces may bend it significantly. It will either tolerate those bends and later return to its original shape or it will tear into pieces. Which of the two will occur depends critically on the precise locations and amounts of the forces. If the forces act on a defect on the glass's surface, it will crack and tear and the glass is history. If the forces all act on strong parts of the glass, it may survive without damage.

50. When you push up on an object, are you creating thermal energy or does that only occur when something does work on you?
When you lift a heavy object, you do work on that object. After all, you exert an upward force on it and it moves in the direction of that force. However your muscles are inefficient and you consume more food energy (calories) during the lifting process than you actually transfer to the heavy object. Whatever energy you consume that doesn't go into the object remains in you as thermal energy. Any time you tighten your muscles, whether you do work on something, it does work on you, or neither does work on the other, you end up wasting some food energy as thermal energy.

51. Why doesn't an egg break when it falls into a pile of feathers? Isn't the pile of feathers exerting the same force on it (perhaps 1000 newtons) that a table would if it were to hit that table?
The egg doesn't break because the feathers exert a much smaller force on the egg than the table would. The feathers can move so when the egg first hits them, the feathers don't have to stop the egg so quickly. To keep the egg from penetrating into the table, the table has to stop the egg's descent in about a thousandth of a second. That required a huge upward force on the egg of perhaps 1000 N. This large upward force, exerted on one small point of the egg, breaks the egg. But when the egg hits the feathers, the feathers can stop the egg's descent leisurely in about a tenth of a second. They only have to push upward on the egg with a smaller force of perhaps 10 N. This modest force, exerted on many points of the egg, shouldn't break the egg. During this tenth of a second, the feathers and the egg will both move downward and the egg will come to a stop well below the place at which it first touched the feathers.

52. Can you give me an example of when the angular acceleration is in a different direction from the torque applied?
When an object isn't symmetric, it can rotate in very peculiar ways. If you throw a tennis racket into the air so that it is spinning about an axis that isn't along the handle or at right angles to the handle, it will wobble in flight. Its axis of rotation will actually change with time as it wobbles. If you were to exert a torque on this wobbling tennis racket, its angular acceleration wouldn't necessarily be along the direction of the torque.

53. Given a lever long enough, could you move the world?
Yes. Of course, you would need a fixed pivot about which to work and that might be hard to find. But you could do work on the world with your lever. If the arm you were dealing with was long enough, you could do that work with a small force exerted over a very, very long distance. The lever would then do this work on the world with a very, very large force exerted over a small distance.

54. How can cats turn their bodies around to land on their feet if they fall and how can people do tricks in the air when they are skydiving if you're supposed to "keep doing what you've been doing" when you leave the ground?
Cats manage to twist themselves around by exerting torques within their own bodies. They aren't rigid, so that one half of the cat can exert a torque on the other half and vice versa. Even though the overall cat doesn't change its rotation, parts of the cat change their individual rotations and the cat manages to reorient itself. It goes from not rotating but upside down to not rotating but right side up. Overall, it never had any angular velocity. As for skydiving, that is mostly a matter of torques from the air. As you fall, the air pushes on you and can exert torques on you about your center of mass. The result is rotation.

55. Is moment of inertia determined only by mass, as inertia is in translational motion?
No, moment of inertia embodies both mass and its distribution about the axis of rotation. The more of the mass that is located far from the axis of rotation, the larger the moment of inertia. For example, a ball of dough is much easier to spin than a disk-shaped pizza, because the latter has its mass far from the axis of rotation.

56. Shouldn't the seesaw be completely horizontal in order to be balanced? How can it be balanced if it's not horizontal?
A balanced seesaw is simply one that isn't experiencing any torque—the net torque on it is zero. Because there is no torque on it, it isn't undergoing any angular acceleration and its angular velocity is constant. If it happens to be horizontal and motionless, then it will stay that way. But it could also be tilted or even rotating at a steady rate.

57. What exactly are angular speed and angular velocity?
Angular speed is the measure of how quickly an object is turning. For example, an object that is spinning once each second has an angular speed of "1 rotation-per-second," or equivalently "360 degrees-per-second." Angular velocity is a combination of angular speed and the direction of the rotation. For example, a clock lying on its back and facing upward has a minute hand with an angular velocity of "1 rotation-per-hour in the downward direction." The downward direction reflects the fact that the minute hand pivots about a vertical axis and that your right hand thumb would point downward if you were to curl your fingers in the direction of the minute hand's rotation.

58. What is the difference between right and left hand rules?
The rule that's used in the mechanics of rotation is always the right hand rule and that's important. It represents a choice made long ago about how to describe an object's rotation. Having made that choice, it says that the minute hand of a clock (which naturally rotates clockwise) points into the clock. You know that because if you curl the fingers of your right hand in the direction that the minute hand is turning, your extended thumb will point into the clock. There is no left hand rule because that was not the choice made long ago.

59. When a lacrosse stick acts as a lever, does it convert a big force to a small one or vice versa?
The lacrosse stick converts a big force into a small one. As you flip the stick, you do work on it—you push part of it forward while that part moves forward. You use a large force and the place on which you push moves forward a small distance. The stick, in turn, does work on the ball. It exerts a small force on the ball but moves that ball through a large distance. The products of force times distance are essentially equal (the stick itself takes some of the energy). The result is a very fast moving lacrosse ball that sails across the field.

60. When you exert a torque on a merry-go-round, how does it exert one on you? I have to exert a lot of torque to get it going but it doesn't feel like torque is being exerted back on me.
When you spin a merry-go-round, you exert a torque on it and it exerts a torque back on you. If you were free to rotate, this torque on you would be quite apparent. Suppose that the merry-go-round was located on an ice skating rink and that you were attached to the central pivot of the merry-go-round by a strap that went around your waist. As you spun the merry-go-round clockwise, you would begin to spin counter-clockwise. In fact, because your moment of inertia is much smaller than that of the merry-go-round, you would experience a much larger angular acceleration and would end up spinning much faster than merry-go-round. The reason that you don't rotate like this after spinning a playground merry-go-round is that your feet touch the ground. As the merry-go-round exerts its torque back on you, you exert that same torque on the ground. The result is that the earth undergoes angular acceleration in the opposite direction from that of the merry-go-round. Because the earth's moment of inertia is so huge, you can't tell that it undergoes angular acceleration at all. It really does, just as the earth undergoes acceleration when you jump-you push down hard and the earth as it pushes up hard on you and you both accelerate away from one another. Since the earth is much more massive than you are, it doesn't accelerate nearly as much as you do.

61. You said that when you were spinning around in circles, you were actually causing the earth to move, but it was too tiny a motion to notice. If everyone on the planet got together in one area and started spinning around at exactly the same time and with the same angular velocity, could the effect of the people causing the earth to move be noticed?
I don't think that it would be possible to detect any change in the earth's rotation. The earth has a mass of about 6,000,000,000,000,000,000,000,000 kg, which is about 20,000,000,000,000 times the mass of all the people on earth. The earth's moment of inertia is even more different than that of the people because much of the earth's mass is located far from its rotational axis. So if all of the people gathered together and started spinning one way, the effect on the earth would be to make it spin the other way about 1/1,000,000,000,000,000,000 as much. The result might be that the day would change lengths by about a trillionth of a second. (1/1,000,000,000,000 s). That change is less than the natural fluctuations in the earth's rotation rate, so no one would ever notice. You might find it interesting that the earth's rotation rate changes slightly with the seasons because of snow in the mountains. When there is lots of snow in the northern hemisphere (during its winter), the earth's moment of inertia increases just enough to slow its rotation. The day is a tiny bit longer than during our summer. People might be able to duplicate this effect by all climbing to the tops of mountains.

62. I didn't understand how a car (or wagon) starts its motion.
A wagon starts its motion when you pull it or push it. If its wheels weren't touching the ground, they would simply move along with the wagon and would not turn. However, they are touching the ground and the ground exerts a backward frictional force on them to keep them from sliding on the ground. This backward frictional force causes the wheels to begin turning.

A car starts its motion when the engine of the car exerts a torque on its wheels. These wheels begin to rotate. However, the wheels are again touching the ground and the ground exerts a frictional force on the wheels to keep them from skidding. This frictional force not only opposes the wheels' angular acceleration, it also causes the wheels and the car to which those wheels are attached to accelerate horizontally.

63. In the book, you discussed pushing on a file cabinet that was resting on the sidewalk. Why doesn't the file cabinet move when you push even a little — you're making the net force greater than zero?
When you exert a small horizontal force on the file cabinet, it doesn't move because static friction between the ground and the file cabinet exerts a second horizontal force on the file cabinet that exactly balances your force. If you push the file cabinet west, the ground will exert a static frictional force on the file cabinet, pushing it east. The file cabinet will thus experience a net force of zero. You'll have to push very, very hard before static friction will be unable to match your force. One you do exceed the limit of static friction, the friction will no longer be able to balance your force and the file cabinet will experience a net force in the horizontal direction. The file cabinet will then accelerate in the direction of your force.

64. Is a spinning toy top a perfect example of angular momentum?
Yes. If you spinning it about a vertical axis (so that gravity doesn't exert a torque on it about its point), it will spin at a steady angular velocity almost indefinitely. Sliding friction does slow it gradually but if the point is very sharp, sliding friction there exerts very little torque on the top about its rotational axis. Because it's unable to exert a torque on the ground, the top can't exchange angular momentum with the earth. It spins on until it slowly gets rid of its angular momentum through sliding friction and air resistance.

65. What exactly is the different between momentum and inertia?
Inertia is a concept—the property of an object to resist any change in its velocity. Momentum is a vector quantity—the product of an object's mass times its velocity and an important characteristic of a moving object. Momentum is important because it's conserved and it's conserved in large part because of inertia and related concepts.

66. What's going on with the wheels when a car accelerates?
As a car heads forward, its freely turning wheels begin to rotate. The torque that starts them rotating comes from static friction with the ground. The ground pushes backward on the bottoms of the wheels to keep them from sliding and this backward frictional force exerts a torque on the wheels. They begin to rotate so that their bottom surfaces head backward and their top surfaces head forward.

The car's powered wheels turn for a different reason: they are driven by a torque from the car's engine. As you step on the accelerator, the engine exerts a torque on the wheels and they begin to turn. They would skid backward across the ground where it not for static friction between the wheels and the ground. This static friction opposes the skidding by exerting a forward force on the bottom surface of the wheels. This static frictional force produces a torque on the wheels and that torque partly balances the torque from the engine. The wheels don't skid and they're angular velocities increase relatively slowly. However, the forward frictional force on the wheel's bottom surface isn't balanced elsewhere in the car and the car experiences a forward net force. The car accelerates forward.

67. Where does energy go when you try to push a heavy object and it doesn't move? Thermal energy isn't made, so why do people get tired?
While it's true that there is no thermal energy made by static friction, since the object doesn't slide, your body can still make thermal energy directly. You get tired because your muscles must turn useful food energy into thermal energy whenever they are under tension. If you are doing work, they also convert food energy into that work, but even when they aren't doing work, they still convert food energy into thermal energy.

68. Why are tires filled with air instead of something less likely to go flat?
This is an interesting question with several answers. First, a solid rubber tire would have a huge mass and would require consider work to accelerate. Because it rotates as the car moves, a tire stores twice as much kinetic energy as the other parts of the cars. By reducing the mass of the tires, the car reduces the amount of energy it must put into the tires to get them moving and the amount of energy it must remove from the tires to stop them from turning.

Secondly, a solid rubber tire would be so hard that it would give the car a very rough ride. The air in the tires cushions the car against many of the rough spots it drives over. Without the air cushion, the wheels and axles would bound up and down with every pebble in the road.

Lastly, a solid rubber tire would be very expensive. The materials used in a tire are expensive and a tire's cost should be roughly proportional to its weight. Since a solid tire would weigh much more than an air-filled one, it would also cost much more. Its tread would still wear out, so it wouldn't last any longer than an air-filled tire.

69. Why is it that we can use energy without doing work? Where does this energy go? For example, you could push on a wall until your arms fell off, but you wouldn't have done any work.
When you are pushing on something without doing any work, your energy is being converted directly into thermal energy inside your body. Your muscles are inefficient and they convert food energy into thermal energy whenever they are under tension. It's like a car, which uses gasoline even when it's stopped at the light. The engine keeps running but it does no work. Similarly, if you simply burned your cereal in your breakfast bowl, you would turn its energy directly into thermal energy without doing any useful work. Your body is also able to burn up that food energy and create thermal energy, albeit a little less visibly.

70. Why is the frictional force on a wagon's wheel in the opposite direction from the frictional force on a car's wheel?
When you pull a wagon forward, friction from the ground starts the wheel turning and it does this by pushing backward on the bottom of the wheel. Friction is thus preventing the wheel from skidding across the pavement. When you step on a car's accelerator, the car's engine starts the wheel turning and friction from the ground pushes forward on the bottom of the wheel to prevent the wheel from skidding across the pavement. In the first case, friction is trying to help the wheel to turn while in the second case friction is trying to keep the wheel from turning. That's why the forces (and the resulting torques) on the wheel are in opposite directions for the two cases.

71. How can you measure weight and/or mass through distance?
With a spring scale, the distortion of the spring is proportional to how much force it is exerting. If you measure that distortion, you can determine how hard it is pulling or pushing on whatever is attached to it. If it's supporting the weight of an object, you can determine that object's weight by measuring how far the spring distorts while supporting it.

72. If a spring scale measures weight, what does a mass scale use to figure out mass? Are weight and mass measured the same way?
A spring scale measures weight. It does this by reporting how much upward force it needs to exert on an object to keep that object from accelerating. Since this upward force exactly balances the object's weight (assuming the object isn't accelerating), the upward force reported by the scale is exactly equal to the object's weight. If the scale reports that the object has a certain mass (in kilograms), then it is taking advantage of the fact that, near the earth's surface, each kilogram of mass weighs 9.8 newtons. But it is still measuring weight and using the relationship between mass and weight to determine the object's mass. If you were to move the "mass" scale to a new location, such as the moon's surface, the scale would read incorrectly because the relationship between mass and weight would have changed.

73. If you hang a weight from a scale ten feet up and the weight descends 2 feet, is the loss in gravitational potential energy equal to the elastic potential energy gained?
Not quite. When you first let go of the weight, it falls freely because the spring isn't stretched and doesn't exert any upward force on the weight. The spring won't support the weight fully until the weight has fallen 2 feet. By then, the weight has acquired a lot of kinetic energy and it overshoots the 2-foot level. The weight begins to bounce up and down around that 2 foot point and takes a while to settle down. The weight is vibrating up and down because it has too much energy at the 2-foot point. Eventually, it converts its extra energy into thermal energy and becomes motionless at the 2-foot point. At that point, it has turned exactly 1/2 of the missing gravitational potential energy into elastic potential energy and the other 1/2 into thermal energy. This 50/50 conversion is a remarkable result related to the exact proportionality between the spring's distortion and the force it exerts.

74. If you lifted an object with a hanging scale on earth and it read 15 N, would it read the same on Jupiter? What about the gravitational force pulling the object down? Wouldn't that alter the reading on the scale? Would you have to calibrate another scale to measure mass on Jupiter?
No, the scale would not read the same on Jupiter, and there would be nothing wrong with the scale! On Jupiter, the object would simply weigh more than on earth. Its mass wouldn't have changed and it would still contain the same number of atoms, but Jupiter would pull on it harder. As a result, the scale would have to pull upward on it harder and the scale would read a larger number of newtons. You wouldn't want to recalibrate the scale because it would be doing its job: it would correctly report that the object weighed about 40 N.

75. Is it true that gravity is stronger at the north pole than at the equator. Does that mean that a person would be able to jump higher at the equator?
Yes. Because of its rotation, the earth isn't quite spherical and people near the poles of the earth experience stronger gravity than at the equator. At the equator, they would experience an apparent weight that was 1% less than at the poles and would be able to jump higher as a result. The Olympic committee should take note.

76. When you transfer momentum between two objects, why is it that the change in total momentum is 0?
Suppose you are standing motionless on extremely slippery ice. If you now take off your shoe and throw it northward as hard as you can, you will transfer momentum to it. Since you and your shoe were initially motionless, your combined momentum was 0. Neither of you nor the shoe was moving, so the product of mass times velocity was 0. But after you throw the shoe, both you and the shoe have momentum. Your momentum is equal to your mass times your velocity, so your momentum points in the direction you are going. The shoe also has momentum, equal to its mass times its velocity. But since it is heading in the opposite direction from you, it has the opposite momentum from you. Together, your combined momentum remains exactly 0—it didn't change. In general, momentum is transferred from one object to another so that any change in momentum in one object is always compensated for by an opposite change in momentum in the other object.

77. How do rubber bouncing balls work? Does the table exert more force than is applied, causing an upward acceleration?
The table never pushes up on the ball harder than the ball pushes down on the table. That would violate Newton's third law and is just not the way our universe works. As the ball strikes the table, the two objects dent. The ball dents most and has work done on its surface—the table pushes the surface inward and work is force times distance in the direction of that force. The ball stores this work/energy as a deformation of its elastic surface and a compression of the air inside the ball. The ball then rebounds from the table as this stored energy reemerges as kinetic energy in the ball. Throughout the bounce, the upward force that the table exerts on the ball is much larger than the ball's downward weight. As a result, the ball accelerates upward the whole time. It starts the bounce heading downward and finishes the bounce heading upward.

78. If all the laws of physics always happen the same, then what relevance does the frame of reference have?
If you observe the world from an inertia frame of reference—meaning that you aren't accelerating—then all of the laws of physics will apply properly to the objects you see. Energy will be conserved during activities, momentum will be transferred between objects without being created or destroyed, and so on. So it's true that any inertial frame of reference will do. However, there is often a "best" reference frame from which to observe a situation. A good example of this is the situation in which a ball bounces from a bat. The best inertial reference frame from which to watch that bounce is the frame of the moving bat. In that special inertial reference frame, the bat doesn't move and the ball bounces off the stationary bat.

79. If I'm a WWF Wrestler, and I sling-shot myself off the ropes, and my momentum carries me as I put a flying shoulder block on my opponent, is my momentum conserved and do I feel any momentum against me?
As you bounce off the ropes, you exchange momentum with the ropes (and the earth). As a result, you normally reverse your momentum and head back into the ring. When you hit your opponent, you begin to exchange momentum with him/her. If you hit your opponent feet first and jump backward, you will reverse your direction of travel again and your opponent will receive an enormous amount of forward momentum. All of this transfer of momentum means that your personal momentum will change often but the total momentum of the earth and its population won't change. That momentum will just be rearranged amount the various objects.

80. If you throw a dead ball at a baseball, would the baseball not roll as far as if you throw a super ball at it?
Your right. The dead ball transfers less momentum to the baseball than the lively super ball does. That's because the dead ball transfers momentum only one, essentially coming to a stop on the baseball's surface. The bouncy ball transfers momentum twice because it also pushes on the baseball as it rebounds. Overall the baseball receives more momentum (and also more energy) from the super ball than from the dead ball. The dead ball turns much of the collision energy into thermal energy.

81. What forces are involved when hitting the sweet spot of a baseball bat?
If the ball bounces from the sweet spot, the two push on one another hard. The ball slows to a stop and then reverses its direction, rebounding from the bat at high speed. The bat accelerates in the opposite direction, and begins to rotate slightly about its center of mass. This rotation is just right to keep the bat's handle from accelerating either toward or away from the ball. That's why the hit feels so clean and neat. The handle doesn't accelerate. The force from the ball on the bat also doesn't cause the bat to vibrate, because the sweet spot is a vibrational node.

82. When a bowling ball hits a wall, is it doing work on the wall?
If the wall doesn't move at all, no. Work requires both a force and a movement in the direction of that force. But in reality, the wall will certainly move at least a short distance. When it does, it moves in the direction of the force on it and the ball is doing work on the wall.

83. When the falling ball bounced off the rising board, why did the ball go upward very quickly? Because of your frame of reference?
The frame of reference from which you observe the situation doesn't cause the rebounding ball to move quickly, but it does help you to understand why the ball rebounds so quickly. Instead of describing the ball bounce from the rising board, let's look at the ball bouncing from a horizontally moving bat. That way, we won't have to worry about gravity—we can pretend it doesn't even exist for a moment. Let's begin from the fan's inertial frame of reference as a pitched ball heads toward a bat at home plate. As the ball approaches the bat, the bat approaches the ball. Both objects are moving, which makes things complicated. So we'll now shift to the bat's frame of reference for a while. In this frame of reference, the bat is stationary and the ball is approaching at high speed. (This rapid approach speed reflects the fact that the two objects are each moving toward the one another in the fan's reference frame.) The ball now bounces from the bat. Because it approached the bat at such a high speed, the ball rebounded at a high speed, too—it heads away from the bat at high speed. Now we'll shift back to the fan's reference frame. The ball is still going away from the bat at high speed, but now we must notice that the bat itself is heading toward the outfield at a high speed, too. So the ball must really be heading toward the outfield fast—it's outrunning the bat toward the outfield. And that is the case. The ball heads toward the outfield at a much higher speed than it had when it was heading toward the bat originally. In the fan's frame of reference, there is a large transfer of energy from the bat to the ball

84. Why do some objects bounce off the ground (balls) whereas others would break (eggs)?
Some objects can deform elastically, storing energy in the process, while others can't. The surface of a rubber ball is made up of long, flexible molecules called polymers that can bend and stretch without breaking. As the ball's surface dents during an impact, these polymer molecules move about and begin to exert forces on one another (storing energy in the process). As the ball rebounds, these molecules release their stored energy and push the ball back into the air. An egg, on the other hand, is made of hard, crystalline material that shatters during the deformation. Whole rows of atoms and molecules rip apart from one another and are unable to return. The egg doesn't store the impact energy. Instead, it turns that energy into thermal energy. The shell just crumbles.

85. Why does a basketball bounce higher than a bowling ball?
When a ball bounces from a rigid surface, the ball's surface distorts inward and then pops back outward. During the inward motion, the ball stores energy—pushing its surface inward takes energy. During the outward motion, the ball releases that stored energy. But not all the energy invested in the ball emerges as useful work. Some of that energy is turned into thermal energy and never reappears. A properly inflated basketball returns a good fraction of the energy it receives while other balls may not. In fact, a bowling ball bounces pretty well from a hard surface such as cement. But when it hits a softer surface such as wood, the wood receives much of its energy and wastes that energy as thermal energy.

86. Why does a rubber ball transfer more forward momentum as the ball rebounds off an object?
As the ball hits a wall and stops, it transfers its forward momentum to the wall. The ball pushes the wall forward for a certain time and thus provides a forward impulse to the wall. As the ball rebounds from the wall, it also pushes the wall forward for a certain time and thus provides an additional forward impulse to the wall. The ball ends up traveling in the opposite direction from that which it had initially, so its momentum points in the opposite direction. This reversal of momentum required an enormous transfer of forward momentum to the wall; so large that the ball actually ended up with a negative amount of forward momentum (which is equivalent to a positive amount of backward momentum).

87. Why when you play baseball is it easier to hit a home run off a fast ball than off a slow ball?
The speed of the ball's rebound from the stationary bat (let's adopt the bat's inertial frame of reference for the moment) depends on the speed at which the ball and bat approach one another. The faster the ball approaches the bat, the higher the ball's rebound speed will be. Since a fastball approaches the bat faster than a slow ball, the fastball also leaves the bat at a higher speed and is more likely to fly out of the outfield for a home run. You can even consider the case in which the batter tries to bunt and holds the bat stationary. A fastball will approach the bat faster and will bounce back faster than a slow ball will. If the pitch is fast enough, the rebounding ball could conceivably fly past the outfield for a home run, too.

88. Would a baseball bat do more damage on a person if the point of contact was the very end of the bat (torque=force x lever arm) or at the sweet spot? (assuming the bat was swung with a constant angular momentum)
The sweet spot. Hitting someone with the bat is very similar to hitting a ball. When you hit a ball with the sweet spot of the bat, the bat slows down and begins to rotate slowly. The slowing is good because it means that some of its kinetic energy has been transferred to the ball. The rotation is bad, because it means that the bat has put energy into rotation (spinning objects have kinetic energy). If the ball hit the bat's center of mass, the bat wouldn't rotate and the transfer of energy would be better; except for one new problem: the bat would begin to vibrate and that vibration would use energy. By hitting the ball on the sweet spot, you keep the bat from vibrating and wasting some of its energy. The transfer of energy and momentum to the ball is maximized. The same occurs when hitting any other object, including a person.

89. Can you explain the term centripetal?
Centripetal means "directed toward a center." A centripetal force is a force that's directed toward a center. For example, a ball swinging around in a circle at the end of a string is experiencing a force toward the center of the circle—a centripetal force. Because the ball accelerates in the direction of the force, it accelerates centripetally. And because it experiences a fictitious force in the direction opposite its acceleration, it experiences an outward fictitious force away from the center of the circle. That fictitious force is called centrifugal "force." However, you should always recognize that this outward "force" is not a force at all, but an effect caused by the ball's inertia—its tendency to travel in a straight line.

90. If all the kids on the merry-go-round are clustered around its center while it is spinning at a constant angular velocity, then if all the kids were to "cautiously" move away from its pivot to the outer edges (while still spinning), would that cause the merry-go-round to slow down faster than if they had remained in the center?
Yes. When the kids move away from the center, the merry-go-round will slow down. If they then return to the center, the merry-go-round will speed up!

91. If the fictitious force you experience on a loop-the-loop isn't greater than your weight, will you fall?
Yes. If you go over a loop-the-loop too slowly, so that you don't accelerate downward quickly enough, you will leave the track and fall. That's why some roller coasters strap you in carefully before taking you upside-down slowly. Without the supports, you would fall out of the car.

92. If you feel fictitious force upward on a loop the loop, how can that fictitious force make objects fall upward? Is fictitious force fictional or real?
As you travel over the top of the loop the loop, you observe the world from an inverted perspective. The sky is below you and the ground is above you. If you were to take a coin out of your pocket and release it, you would see it fall toward your seat. From that observation, and the feeling of being pressed into your seat, you might think that gravity is suddenly pulling you toward the sky. It isn't. Gravity is still pulling you toward the ground, but you are in a car that is accelerating rapidly toward the ground. As a result, the car is having to push you toward the ground with a force on the seat of your pants. You feel pressed into your seat because the car is pushing you downward hard. When you release the coin, it seems to fall toward the sky, but it's really just falling more slowly than you are. With the car pushing you downward, you're accelerating toward the ground faster than the coin and you overtake it on the way down. It drifts toward the seat of the car because the car seat accelerates toward it. As you can see, the only forces around are the force of gravity and support forces from the car. There is no outward or upward force here. The fictitious force is truly fictional; a way of talking about the strange pull you feel toward the outside of the loop.

93. When a ball swings in a horizontal circle at the end of a string, what's the force on the ball pulling it straight? What's the force pulling it out?
Let's neglect gravity, which isn't important in this horizontal motion problem. When a ball swings in a circle at the end of a string, there is only one force on it and that force is inward (toward the center of the circle). We call such a force a centripetal force, meaning toward the center. There are many kinds of centripetal forces and the string's force is one of them. As for the ball's tendency to travel in a straight line, that's just the ball's inertia. With no forces acting on it, it will obey Newton's first law and travel in a straight line. There is no real force pulling the ball outward. But a person riding on the ball will feel pulled outward. We call this feeling a fictitious force. Fictitious forces always appear in the direction opposite an acceleration. In this case (an object traveling in a circle) the outward fictitious force is called centrifugal "force." But remember that it's not a real force; it's just the object's inertia trying to make it go in a straight line.

94. When you spin an object around a fixed point, a sling for example, does the object at the end build up energy that causes it to shoot out quickly when released?
Yes. As you whip the object around on a string, you are doing work on it. You do this by making subtle movements with your hand, exerting forces that aren't exactly toward the center of the circle. When you do this, the object begins to travel faster and faster, so its kinetic energy increases. Traveling in a circle doesn't change this kinetic energy because kinetic energy is proportional to speed squared, and doesn't depend on direction. Finally, when you let go of the string, the object stops circling and begins to travel in a straight line. It carries with it all the kinetic energy you gave it by whipping it about.

95. Why is the outward force in a loop-the-loop a "fictitious" force? Why isn't it a "real" force?
A real force causes acceleration. If the outward "fictitious" force on a circling object were "real," that object wouldn't circle. It would accelerate outward. When you swing an object around on a string, you feel the object pulling outward on the string. But it isn't itself being pulled outward by anything. What you're feeling is the object's inertia trying to make it travel straight. The inward force you're exerting on it isn't opposing some real force, it's causing the object to accelerate inward.

96. Have you heard about the Egyptian Lighthouse (one of the seven wonders of the ancient worlds) that was found in the sea? Well, people are proposing lifting the 70-ton sections of the lighthouse with balloons. Can a balloon do this? What special aspect of the balloon will lift 70-ton bricks? What kind of balloon would be used?
I would guess that they intend to use balloons in the water. The blocks are at the bottom of the sea, so they must be lifted up to a boat. A balloon experiences an upward net force that is equal to the difference between the upward buoyant force on it and its downward weight. If the balloon displaces air, then the buoyant force on it is rather small and it would have to be extraordinarily big to displace enough air to lift a 70-ton block. But if it displaces water, then the buoyant force on it is much greater. To displace 70 tons, it would only have to displace about 65 cubic meters of water. That's not hard at all. The balloon could be made of heavy reinforced canvas and still work just fine underwater.

97. How can a balloon support the air around it (pressure wise) and still rise?
If you could have filled a balloon with nothing at all, it would float very nicely because it would have had no weight and the only force on it would have been the buoyant force upward. But an empty balloon will be crushed by the surrounding air, which will push inward on its surface with enormous forces. To keep the balloon from crushing, you must fill it with gas. Since this gas will weight the balloon down, you should choose the lightest gases around: hot air or helium. In the case of hot air, a relatively small number of air molecules create the pressure needed to keep the balloon inflated. With helium atoms, lots of helium atoms are needed to create the pressure but helium atoms are very light and their total weight is less than that of an equal volume of air. Thus the upward force on the helium filled balloon is more than its weight and floats upward.

98. How do clouds exist? If oxygen molecules, which must weight less than water vapor are drawn toward earth, then why aren't the clouds?
First, water molecules are lighter than the average air molecule so a balloon filled with water vapor would actually float in air. But that isn't what you're asking. Clouds exist because when water condenses from vapor to liquid, it often forms extremely tiny water droplets. These droplets are so small that they experience lots of air resistance as they try to move about. They begin to fall but quickly reach terminal velocity at perhaps a millimeter per second. The water droplets drift downward so slowly that they hardly move. That's what's happening in a cloud. The water droplets are trying to fall, but air resistance is slowing their descents.

99. How do submarines sink if they have air inside?
The net force on the submarine depends on its average density, not on the density of one of its constituents. If the average density of the submarine is less than that of water, it will float upward in water. If the average density of the submarine is more than that of water, it will sink downward in water. To determine the submarine's average density, you need to divide its overall mass by its overall volume. While the submarine does contain air, a low-density material, it also contains steel, a high-density material. The submarine's average density turns out to be very nearly that of water. Fine adjustments to its density, made by pumping water in and out of ballast tanks, determines whether the submarines floats or sinks.

100. How does altitude affect the presence of molecules?
As you travel upward, the air around you has less and less pressure. That's because it's supporting less and less weight above it. As long as the temperature isn't changing much, this decrease in pressure is caused by a decrease in density: the air molecules are become less tightly packed together. The result is that at high altitude, each breath you take delivers fewer air molecules into your lungs. Actually, air usually gets colder at higher altitudes, a change which keeps the air's density from decreasing so rapidly. The higher you go, the colder the air gets and the more molecules you need in each liter of air to maintain its pressure.

101. If you don't want your tires to blow up from too much thermal energy, then why aren't tires white (to absorb less sunlight and thus receive less thermal energy)?
That's an interesting question. It's probably difficult to manufacture white tires and they'd probably look terrible after they'd been driven a while. The old white-wall tires where difficult to keep clean. Because the pressure in a tire varies with its temperature, your tires will probably go over their optimum pressure on a hot sunny day. But driving them on the road also heats them, probably more than sunlight does. Because their temperatures increase during hard use, the tires are evidently capable of handling pressures much higher than their normal fill pressures.

102. Many large boats seem to taper down toward the water line. If their hulls follow this trend, their centers of mass will be high above their centers of buoyancy, making the boats unstable (like standing in a canoe). How do these things stay upright?
You're right that the boats must keep their centers of gravity lower than their centers of buoyancy. A boat with its center of gravity above its center of buoyancy will flip over, just as an upright broomstick will flip over if you support it only from below. But because a boat with a narrow tapered hull will go deeper into the water than one with a wide flat hull, the boat with the tapered hull may actually have a lower center of gravity than the boat with the wide flat hull. For example, imagine adding a long thin vertical plate to the bottom of a canoe, effectively converting the canoe's wide flat hull into a thin tapered hull. That canoe will be much more stable than before. So the shape of a boat's hull isn't as important as where the boat's weight located is relative to its center of buoyancy (the effective location of the buoyant force on the boat).

103. What is the buoyant force?
When you displace a volume of air and replace that volume with something else, the air around the volume still pushes on it as before. If that volume had remained air, then it would have just floated there, suspended by a force from the surrounding air. Now that the volume has been replaced by something else, it still experiences the same suspending force. That suspending force is the buoyant force. It's actually created by a slight imbalance in the pressures around the volume. The pressure at the bottom of the volume is slightly higher than on top, so the air exerts a net upward on the volume. This pressure imbalance is in turn created by gravity and the fact that the air near the ground must support the air above it against the force of gravity.

104. Why do high altitude places have different cooking temperatures than sea-level places?
The air pressure is lower at high altitudes than it is near sea level because there is less atmosphere overhead to support. This decreased air pressure affects the way water boils. Molecules can always evaporate from the surface of a pot of water, even when that water is cold, but above a certain temperature, water molecules can begin to evaporate from the interior of the water as steam, the gaseous form of water, in a process we call boiling. The temperature at which boiling occurs depends on the ambient air pressure because it can only proceed when there is enough pressure inside the steam bubbles to make them grow larger. At high altitudes, the lower air pressure makes it easier for these steam bubbles to form and grow, so they occur at lower temperatures. That's why water boils at a lower temperature at high altitudes. Once water reaches its boiling temperature, any heat that you add to it tends to cause the water molecules to boil away rather than to make the water hotter—so it's hard to heat water-containing food hotter than the boiling temperature of water. Since food needs high temperatures to cook, they don't cook as easily at high altitude where the low boiling temperature of water tends to keep the food from getting very hot.

105. Why does a balloon collapse without air inside it while a vacuum bell jar does not?
A balloon's skin is too weak to support the air around it. If you don't put any gas inside the balloon, the atmosphere around it will push it inward and it will collapse. But a vacuum bell jar is made of an extremely strong plastic that easily supports the weight of the atmosphere on top of it. Even with nothing inside it, it remains full size.

106. Why don't soap bubbles float forever (I believe they fall when not influenced by wind currents) if you have the "same" air both inside and outside the bubble?
The average density of an air-filled soap bubble is just slightly higher than that of the surrounding air. That's because the soap film itself is denser than air and because the air inside the bubble is very slightly compressed, thus having a slightly higher density than the surrounding air. Because the bubble's average density is slightly higher than that of the surrounding air, the bubble will slowly sink in still air and will eventually reach the ground.

107. Can air have gravitational potential energy?
Yes. However, you often don't notice this because as you lower a volume of air downward, you displace a similar volume of air upward. Thus you can't just raise or lower air to observe changes in its gravitational potential energy. You'd have less trouble if you compressed the air tightly together, perhaps turning it into a liquid, and then raised or lowered it. It's gravitational potential energy would then be much more noticeable.

108. How does water move toward your mouth through a straight straw if you don't suck on the straw?
If the straw is horizontal and the water wasn't moving to begin with, it won't move toward your mouth unless you suck. To make the water accelerate, it must experience net force and the two ways to achieve that net force are (1) to create a pressure imbalance on the water's ends and (2) to have the water's weight accelerate it. In a horizontal force with no pressure imbalance on it, there is no net force on the water and it doesn't accelerate.

109. I was wondering about the change in pipe sizes within a house. In many cases, water pipes coming to a house are very large, only to drop to small pipes when they reach the house. Does this mean that the water from the water company is slow velocity, high pressure, and houses turn this water into fast velocity, low pressure?
Yes, but the effect is not so extreme. As the water from the water company enters the narrower pipes in your house, it does have to speed up slightly and its pressure does drop slightly. But its pressure is still well above atmospheric pressure. However, the fact that the water must move faster through the narrower pipes in your house means that this water loses energy relatively quickly in your house. And the more water you draw through your house's plumbing, the larger the fraction of its energy it loses. That's why drawing a huge amount of water out of one faucet will diminish the flow through another faucet—increasing the flow by opening that first faucet wastes the energy of the water reaching the second faucet and it flows out more slowly.

110. In a siphon, what makes water flow from one container to the other without a pump?
The water is propelled by a pressure imbalance. When the water level in one container is higher than that in the other container, the pressures at the two ends of the siphon aren't equal. There is more pressure on the high water side than on the low water side. As a result, the water accelerates toward the low water side and the water levels gradually become equal.

111. In the book section on Water Distribution, there was a question (exercise 5) about a novelty straw. The answer says that the straw can't be taller than 0.5 meters. I thought you could suck liquid up a straw 10 meters tall? Why can this straw only be 0.5 meters tall?
The question itself said that the straw was only 0.5 meters tall. In the answer I was intending to point out that you can have as much tubing as you like in that straw, because it's only 0.5 meters tall overall. I didn't intend to mean that straws taller than 0.5 meters but shorter than 10 meters wouldn't work. Just that a short straw will work no matter how much tubing it contains. Sorry for an imperfect answer in the book. I'll change it in future editions.

112. Is air a fluid or a gas or both?
Air is both a gas (a material composed of many independent particles that normally expands to fill its container) and a fluid (a material that can be reshaped easily to take on the shape of its container).

113. Please define the 3 types of energy that flowing water has?
Whenever water (or any incompressible fluid) passes fixed obstacles in a laminar flow, its total energy is conserved (we're neglecting friction effects—viscous drag). That total energy consists of (1) the water's gravitational potential energy (how high up it is), (2) the water's pressure potential energy (how hard it pushes on surfaces), and (3) the water's kinetic energy (how fast it's moving). Since the water's total energy doesn't change, a change in one of these forms of energy necessitates a change in one or both of the other forms. For example, if water speeds up during its flow, the water's pressure or height or both must decrease.

114. Water seeks areas of lowest pressure. Is this the concept behind low-pressure weather systems bringing precipitation and high pressure bringing clear, dry conditions?
Not really. Fluids do accelerate toward lower pressures, so a low-pressure weather system does attract surface winds (the air near the surface of the earth accelerates toward regions of lower pressure). But the precipitation issues are generally related to temperature changes. Hot air can hold more moisture than cold air, so if a low-pressure system attracts air and causes hot and cold airs to mix, the new air temperature and moisture may be incompatible. When that happens, the moisture emerges from the air as water droplets and it rains.

115. When kinetic energy goes down (like in the Bernoulli tube), does potential energy go up?
Yes. When a fluid that's in steady state flow (moving smoothly and continuously past stationary obstacles) loses kinetic energy, its potential energy goes up—either its pressure rises or it moves upward against gravity. That assumes that the kinetic energy isn't being lost to thermal energy because of some terrible friction problem.

116. Why are water towers larger on top than on the bottom?
The goal of the water tower is to store water high in the air, where it has lots of gravitational potential energy. This stored energy can be converted to pressure potential energy or kinetic energy for delivery to homes. Since height is everything, building a cylindrical water tower is inefficient. Most of the water is then near the ground. By making the tower wider near the top, it puts most of its water high up.

117. Why can't you pull the water up above a certain point without a pump?
When you draw water up through a pipe (or straw) by removing the air inside that pipe, you are allowing the atmospheric pressure around the water to push the water up the pipe. The water experiences a pressure imbalance between the pressure around it (atmospheric pressure) and the pressure in the pipe (less than atmospheric pressure), so it accelerates into the pipe. But as the water column inside the pipe grows taller, a new problem appears: gravity. The water's weight pushes downward and begins to oppose the pressure imbalance. At a certain height, the two effects balance and the water stops accelerating upward. When the water's height reaches 10 m, atmospheric pressure can't overcome this weight problem, even if all the air has been removed from the pipe.

118. Why does water stay in the straw when a finger is pressed over one end? How does sealing off the one end make the pressure less?
When you fill a straw with water and then seal one end with your finger, you can then hold the straw vertically without any water falling out of the straw. That's because the air pressure above the column of water decreases until the upward force caused by the unbalanced pressure at the top and bottom of the water column is exactly equal to the weight of the water column. The drop in pressure above the water column occurs because the water initial does fall downward. When you first tip the straw from horizontal to vertical, the air pressures above and below the water column are equal and there is no pressure force to opposite the weight of the water. The water begins to fall. As it does, it creates a relatively empty region above the water column and below your finger. The air molecules in that region become sparser and their pressure decreases as a result. The water descends just far enough to lower the pressure inside that trapped air region until the pressure force balances the water's weight. Actually, the water column bounces up and down briefly, just like a weight at the end of a spring or a person at the end of a bungee cord. But after a second or so, the water column just hangs there motionless in the straw, supported against gravity by the pressure imbalance. If air could work its way through the water column and enter the trapped region between the water column and your finger, the water column would be able to descend further. But the straw is so narrow and the water sticks to tightly to itself (a phenomenon called surface tension) that it prevents air bubbles from working their way up the straw.

119. Why is high pressure air/fluids slow moving, while low-pressure fluids/air are fast moving?
First, I should point out that high pressure air/fluids can move either fast or slow, depending on the situation. The same holds for low-pressure air/fluids. What Bernoulli's equation tells us is that when air/fluids slows down, its pressure rises (assuming that it isn't moving up or down so that gravity is out of the picture) and when air/fluids speed up, its pressure drops. Here are two common examples.

First, when you spray water from a garden hose against your hand, the water goes from moving quickly through the air at atmospheric pressure to moving slowly on your hand at more than atmospheric pressure. You know that this pressure increase has occurred because you feel the water pushing hard on your hand. The water is exchanging kinetic energy for pressure potential energy and its pressure is rising.

Second, when you put your thumb over the end of the garden hose and allow only a fine spray to emerge, the water goes from slow moving water at high pressure inside the hose to fast moving water at atmospheric pressure in the air. You know that this pressure drop has occurred because you feel the water in the hose pushing hard against your thumb. The water is exchanging pressure potential energy for kinetic energy and its pressure is dropping.

120. Why is it that when I am in my dorm room with my window open and the door closed, there isn't a change in temperature and no wind comes in or blows around. But if I open the door, the room becomes cold and wind is felt throughout the room?
When the wind blows into your room, it comes to a stop and experiences a rise in pressure. This is an consequence of Bernoulli's equation, which recognizes that energy is conserved and that in a fluid, energy can exist either as kinetic energy (energy of motion), pressure energy, or gravitational potential energy. In this case, the wind's kinetic energy becomes pressure energy as it slow down in your room. As the pressure in your room rises, it prevents more air from entering, so you have high pressure but no movement inside your room. As soon as you open the door, the high-pressure air in your room accelerates toward the relatively low-pressure air in your hall. The pressure in your room drops and the wind can get in now. Soon the wind is blowing right through your room, as though you were part of a wind tunnel. If the wind is cold, you will be too.

121. Why is there a relationship between speed and pressure? What is that relation? Why are they inverses of each other?
When a fluid is flowing smoothly and steadily through a stationary environment, its energy is conserved. As long as it doesn't lose much energy to frictional effects, you can count on its total energy remaining essentially constant as it flows downstream. Since it only has three forms for its energy: gravitational potential energy, pressure potential energy, and kinetic energy, you can expect that a decrease in one of these forms of energy will be accompanied by an increase in one of the other forms. That's when speed and pressure are inversely related. When the fluid slows down, its kinetic energy drops so its pressure potential energy (and its pressure) must rise.

122. Does air create a friction force on water? Would a gutter be much quicker at having water flow through it, rather than a pipe?
Air does exert frictional forces on water, but much less than a surface would. Thus a gutter would be a better water carrier than a pipe. It would have one less surface to slow the flow of water.

123. Does super cooled helium act in a viscous or non-viscous manner?
Below 2.17 K, liquid helium behaves very differently than normal fluids. It behaves as though it were made of two intermingled fluids: one that is normal in every way and the other that is completely without viscosity. Depending on what sort of experiment you do, you will see one or the other fluid. If you swirl the liquid helium with a stick, you will see the viscous fluid component swirling and splashing. If you pour the liquid helium through a filter made of tightly packed dust, you will see the non-viscous component rushing through. No normal fluid can travel through packed dust, because its viscosity slows its travel until it doesn't move at all. But the viscosity-free component of liquid helium can flow easily through any holes, no matter how small. It can flow through holes that even helium gas has trouble passing.

124. How does Jell-O work? How come it congeals when it is cooled?
Jell-O is composed of long, stick-like molecules. When you dissolve it in hot water, those molecules separate, but as the liquid cools, they begin to stick together like a giant heap of straws. The water flows slowly through these straws because of frictional effects. The result is a stiff material that is given its structure by the straw heap. If you leave the Jell-O long enough, the water will seep out and make puddles on the plate.

125. What are vortex rings?
These rings (also called smoke rings) are moving portions of fluid that are moving relative to the surrounding fluid. They form a remarkably stable structure. The inner edge of the ring heads forward, while the outer edge head backward and the ring pulls itself through the air. Fluid dynamicists study these sorts of objects.

126. What is the difference between "thickness" and viscosity? Is viscosity just a fancy word for thickness?
Viscosity is a measurable quantity—a liquid has a specific viscosity as measured in units of poise or pascal-seconds. Thickness refers to the same characteristic as viscosity, but isn't a specific quantity. It's certainly correct to say that a thick liquid is a liquid with a large viscosity.

127. When you were showing us water faucets during class, each faucet had a corner immediately preceding the opening through which the water came out. Does that corner help slow the pressure of the water?
Most faucets do have a turn just before the water comes out and that turn is there to slow the water down. Unless the faucet is opened for maximum flow, the pressure of the water emerging from the valve part of the faucet is pretty close to atmospheric pressure, so there isn't any need to control that pressure. But the water emerging from the valve may be traveling very fast and it could easily spray across the room if there were nothing in its way. To prevent such sprays, most faucets are bent so that water spraying out of the valve will hit the bend and become turbulent. The turbulence will help it to convert its kinetic energy into thermal energy so that it will emerge from the faucet at low speed and atmospheric pressure. (Great question!—I'd never thought of this before).

128. Why does a hose squirt further when you cover the hole with your thumb?
The water entering the hose has a certain amount of energy per liter. That energy can be in one of three forms: pressure potential energy, gravitational potential energy, or kinetic energy. If you let it flow freely through the hose, most of that energy will become kinetic energy and the water will move quickly through the hose. But it will encounter frictional effects as it slides past the walls of the hose (its viscosity participates here) and it will convert much of its kinetic energy into thermal energy by the time it leaves the hose. However, if you pinch off the flow with your thumb, the water won't be able to convert its energy into kinetic form as it enters the hose. Most of the energy will remain as pressure potential energy. The water will move slowly through the hose and it will experience relatively little energy loss to frictional effects. Most of the energy will remain by the time the water reaches your thumb. Then, as the water flows past your thumb to the outside air, its pressure will drop suddenly and its energy will become kinetic energy. The water will spray out at very high speed.

129. Why is viscosity important in motor oil for today's high revving engines?
If the oil in your car is has too little viscosity, it will easily flow out of the gaps between surfaces and will not lubricate them well. Those surfaces will experience sliding friction and wear. If the oil has too much viscosity, it will waste the engine's energy by opposing motion and turning work into thermal energy. Modern motor oils have carefully adjusted viscosities that balance the two problems. Since temperature affects viscosity (e.g., hot molasses has less viscosity than cold molasses), motor oils add chemicals that stabilize their viscosities over wide temperature ranges.

130. For aerosol sprays such as Lysol, are they essentially creating "dustlike" particles that float in the air?
Yes, except that the word "float" isn't what you really mean. An aerosol is a suspension of fine solid or liquid particles in a gas. What holds those particles up against their downward weights isn't the buoyant force—these particles are much more dense than the gas that surrounds them. Instead, it's viscous drag. When the particles begin to fall downward through the gas, they experience such large upward viscous drag forces that they reach terminal velocity at only about 1 millimeter-per-second. The slightest breeze carries the particles with it so that they rarely have a chance to settle to the floor because of gravity. In an aerosol spray, the particles are carried forward by the gas emerging from the bottle and they hit the surfaces in front of the bottle.

131. How does a water aspirator pump work?
The water aspirator pump is essentially a pipe with a narrowing in it. As water flows through that narrowing, it speeds up and its pressure drops—it's exchanging its pressure energy for kinetic energy. A tiny opening in the side of the narrowing allows water or air to enter the high-speed flow. Since the pressure in that high-speed flow is very low, atmospheric pressure pushes fluids through the tiny opening and into the flow. The flow pumps fluids through the opening and into the water stream. If you connect a hose to the tiny opening, you can suck chemicals up the hose and into the water stream.

132. How does an airbrush work? Can you briefly explain it again.
In an airbrush, slow-moving but high-pressure air from a hose is allowed to pass through a very narrow channel. As the air enters this channel, it speeds up and its pressure drops—it has exchanged its pressure potential energy for kinetic energy. The channel is so narrow and the air moves so quickly through it that the pressure inside the channel drops below atmospheric pressure! There is a tiny pipe that attaches to this channel at right angles and that dips into a bottle of paint. As the pressure inside the channel falls below atmospheric pressure, the atmospheric pressure in the paint bottle pushes the paint toward the channel. The paint begins flowing into the channel and it collides with the high-speed stream of air. The paint is ripped into tiny droplets and these droplets travels through the channel along with the air. As the air emerges from the narrow channel, its pressure rises and it slows down, but it still moves fast enough to carry the paint droplets to the object that's being painted.

133. How does the fan in a vacuum cleaner boost the pressure back up so that the air flowing through the vacuum cleaner the air will go back into the room?
The fan is a rotating assembly of ramps. As the ramps move, they sweep the air from one side of the fan to the other and do work on that air. The air either accelerates as the fan blades spin past, or its pressure builds up. Either way, its total energy increases. The fan can take low-pressure air from one side and whisk it over to the other side where the pressure is higher. It can push air against the natural direction of flow (from high pressure to low pressure). It's essentially a pump for air.

134. Suppose that you fall out of a plane about 30 seconds after your parachute pack fell out. Is it really possible to catch up to your parachute pack and save yourself?
The answer depends on how high the plane was flying and just how much air resistance the pack experiences as it falls. After a few seconds of falling, an object reaches a terminal velocity—it stops accelerating downward. That's because the upward force that air resistance exerts on it grows stronger as its downward velocity increases. Eventually, the upward force it experiences exactly balances its downward weight and it has no net force on it—it doesn't accelerate. For a person, this terminal velocity ranges from about 100 mph to 200 mph, depending on the person's shape. Curling into a compact ball should allow you to reach a relatively high terminal velocity of 200 mph. Since the parachute pack is relatively light but has substantial surface area for the wind to push against, it probably has a lower terminal velocity of say, 100 mph. This arrangement would allow you to approach the pack at a relative velocity of 100 mph. In order to actually overtake the pack, you'll still need some time, so the higher the plane was when you started, the better your chances are. Since the pack has a 30 second head start and descends at 100 mph, it will be about 0.83 miles below you when you leave the plane. You'll catch up to it 30 seconds later, during which time you will have dropped a total of 1.67 miles. Thus in principle, you could catch the pack so long as the plane's altitude was more than about 1.67 miles. To allow time to put the pack on, for the parachute to open, and for your terminal velocity to then become low enough to avoid injury, you'd better have the plane at more than about 2.5 miles. Still, this doesn't sound like a fun experiment.

135. What is drag force?
A drag force is a force that opposes an object's motion through a fluid. Like sliding friction, drag always pushes the object in the direction opposite its motion though the fluid. Air resistance is really a drag force. You feel drag pushing you backward when you ride a bicycle fast. You also feel drag when you hold your hand out the window of a fast-moving car—it pushes your hand toward the back of the car and in the direction opposite your hand's motion through the air. If you were to fall downward, you would feel a drag force upward, in the direction opposite your motion through the air. And leaves experience a drag force when wind blows on them—pushing them downwind and in the direction opposite their motion through the air (they are moving upwind through the air, so it pushes them downwind). Incidentally, the object pushes back on the fluid with drag force, too, and this force on the fluid pushes the fluid in the direction opposite its motion past the object. This force tends to stop moving fluids and to turn their kinetic energies into thermal energy.

136. When you suspended the Ping-Pong ball in the stream of air from the pipe, why did the ball spin? The same thing happened to the two flat pieces of plastic that were held together when air flowed out between them.
The Ping-Pong ball spun because the viscous drag forces it experienced weren't equal on all sides. As we'll see shortly, there are a variety of different drag forces and they can act at different locations on an object. In the case of viscous drag, it acts locally at each point where air slides across the surface of the object. Since the airflow from the pipe wasn't perfectly uniform, the air swept past the ball faster in some places than it did in others. These differences in airspeed became most significant when the ball began to drift away from the airstream—the sudden increase in airspeed on the side of the ball nearest the center of the airstream is what created the low pressure that allowed the surrounding air to push the ball back toward the center of the airstream. But minor differences in airspeed also exerted unbalanced torques on the ball and caused it to spin. Similar flow imperfections between the two plates created differences in viscous drag and exerted torques on the two plates. That's why they began to spin around slightly.

137. Why does dust settle on the moving blades of a fan?
As the air flows across the blades of a fan, the dust particles in it occasionally pierce through the airflow and hit the blades. The same sort of process occurs when a bug hits the windshield of a car; the bug would normally follow the airflow but its inertia prevents it from moving out of the way quickly enough and it hit. Once a dust particle hits the fan blades, there isn't much to remove it. The air moves remarkably slowly right at the surface of the fan because that surface layer of air experiences lots of viscous drag. Even though the air is moving swiftly only a few millimeters away, the air right on the fan blade is almost stationary. Thus the dust can cling to the blade indefinitely.

138. Does the decreased density of the air in Denver make it easier to achieve turbulent flow at the boundary layer of a baseball and therefore make the ball fly farther?
Whew, this is a toughie. The air in Denver is less dense, so it tends to respond better to viscous forces. On that account, it would tend to be less turbulent. But it is also "thinner" and less viscous, so it would tend to be more turbulent. I think that those two effects essentially cancel, so that the ball experiences the same degree of turbulence at any altitude. However, the air in Denver has less pressure, so it exerts smaller forces on the ball than air at sea level. Thus, although the flow properties aren't affected by the increased altitude, the pressures involved are. The ball should certainly carry farther in Denver than at sea level. Imagine playing on the moon, where there's no air at all. The ball wouldn't experience any drag at all!

139. Does the design of balls (pentagons on soccer balls, lines on basketballs, panels on volleyballs, etc.) have a purpose or are they merely there for design?
In most cases they are simply design. However, they do affect the flow of air over the ball and will change its motion. The classic examples of balls with designs that matter are golf balls and baseballs. A golf ball has dimples because they dramatically change the airflow over the ball and allow it to travel much farther. A baseball's stitching also affects its flight from the pitcher to the mound and is very important to pitches like the knuckle ball and the spitball.

140. How does a boomerang work?
The correct way to throw a boomerang is overhand and, unlike a Frisbee, in a nearly vertical plane. (Usually the ideal angle is about 15° from vertical.) The boomerang is essentially a rotating airplane wing, and its shape produces lift using the Bernoulli effect in the same way an airplane wing does. But when it is thrown, notice that the top blade of the boomerang is moving faster through the air than the bottom blade, because of the rotation. This results in there being more lift on the top blade than on the bottom. From a right-handed thrower's perspective, there is a lift up and to the left, more so at the top than at the bottom. The upward lift is what keeps the boomerang in the air. You might think the leftward twist flips the boomerang over, but wait! The boomerang is also a flying gyroscope. Leaning the gyroscopic boomerang over results in its turning to the left, much the same way that leaning a moving bicycle leftward toward the horizontal causes the front wheel to turn and not fall over. (This is also why spinning tops start to slowly turn their axis of rotation when they lean, a process called "precession".) The boomerang doesn't flip over, but instead turns its axis of rotation around in a large horizontal circle, and it comes back to you.

After a moment's thought, you might wonder whether helicopters suffer the same effect. (How would a boomerang fly if thrown in a horizontal plane?) In fact, they do, and there is a tendency to pitch the helicopter upward (tip the nose up) precisely from this same effect, which the pilot instinctively corrects for.

(Thanks to Prof. Paul Draper, from the Physics Department of the University of Texas at Arlington, for writing this explanation.)

141. How does a Frisbee fly?
As you begin to move a Frisbee forward, the air in front of the Frisbee splits to flow either over the Frisbee or under it. Because of the Frisbee's shape and the angle at which it's held, the air that flows over the Frisbee has a longer distance to travel and arrives late at the back of the Frisbee. The air flowing under the Frisbee reaches the back first and initially flows upward, around the rear surface of the Frisbee. But once the Frisbee is moving fairly rapidly, this funny upward-flowing tail of air blows away from the back of the Frisbee. As it leaves, it draws the air flowing over the Frisbee with it and speeds that air up. As a result, the air over the Frisbee travels faster than the air under the Frisbee. But the airs above and below the Frisbee have the same amounts of total energy per gram. Since the faster moving air above the Frisbee has more kinetic energy than the slower moving air below the Frisbee, the air above the Frisbee must have less of some other form of energy than the air below the Frisbee. In fact, the air above the Frisbee has less pressure potential energy than the air below it—the air pressure above the Frisbee is less than that below the Frisbee. And since the pressure pushing on the bottom surface of the Frisbee is greater than the pressure pushing on the top surface of the Frisbee, there is a net upward pressure force on the Frisbee. This upward pressure force balances the downward weight of the Frisbee and keeps the Frisbee from falling.

142. How does fuzz on a tennis ball make it fly faster? It seems counterintuitive to me.
Yes, it is counterintuitive. The reason for the fuzz is that a swirling layer of air close to the ball makes a good buffer between the ball and the main airstream. As a result, the main airstream flows most of the way around the ball before it breaks away as a turbulent wake. Without the swirling layer of air on the ball's surface, the main airstream encounters backward-flowing air near the ball's surface and breaks away from the ball early, leaving a larger turbulent wake.

143. I am a little confused on how air pressure can drop as air flows through a small hole. I understand that speed and pressure are inversely proportional but aren't volume and pressure inversely proportional as well?
If you squeeze air into a small space, its pressure will rise (yes, pressure and volume of stationary air are inversely proportional). But flowing air is a different story. When an airstream passes through a narrow channel, it must speed up and when it does, its pressure drops. That's aerodynamics, not statics.

144. I understand how dimples on a golf ball reduce pressure drag, but wouldn't they also increase viscous drag? (i.e. a rougher surface experiences more "friction" from the air).
Yes, the dimpled golf ball probably does experience more viscous drag than a smooth golf ball. But viscous drag is only a small fraction of the total drag on the ball. Pressure drag is much more important (and much larger).

145. If you put a light plastic ball in the stream of air rushing upward from an open air hose, the ball will float in the airstream. How does this trick work?
In this trick, the airstream is rather narrow. When the ball is centered in the stream, air flows around all sides of the ball. But when the ball drifts off center, the airstream flows mainly on the side of the ball nearest the airstream. That air still has to flow around the sides of the ball and speeds up as it does. The air pressure there drops. The result is that the pressure drops on the side of the ball closest to the airstream and the ball is pushed back toward the airstream.

146. In a sealed car driving down the road, when would you have the lowest pressure outside the car: when a window was just a little open, or all the way open? Or would the overall pressure be constant once the window was opened at all?
The pressure outside the closed front windows of a moving car is lower than atmospheric pressure because the air flowing past the car is moving particularly fast as it arcs around the front portions of the car. When you open the front windows of the car slightly, you don't disturb this airflow very much, but you allow air from inside the car to flow outward toward the low-pressure air passing the windows. As a result, the air pressure inside the car drops below atmospheric pressure and you may feel your ears "pop." But if you open the windows wide, the air flowing around the car will probably be seriously disturbed and the low-pressure regions may vanish. As a result, the air pressure inside the car will probably be about atmospheric. However, there are times when the airflow past an open window becomes unstable and the moving air can actually fluctuate in direction, so that it's deflected in and out of the window. When that happens, the whole car begins to act like a giant whistle and you feel the air pressure inside it rise and fall rhythmically. This oscillation is irritating to your ears.

147. Is pressure drag the same thing as "air resistance"?
Yes. The air resistance you experience when you bicycle into the wind or hold your hand out the window of a car or jump from a plane with a parachute on is just pressure drag. In each of these cases, the air flowing around you slows in front of you (so that its pressure rises), speeds up on the sides of you (so that its pressure drops), and then becomes turbulent behind you (so that its pressure hovers near atmospheric pressure). With more pressure in front of you than behind you, you experience a net force in the downwind direction...the force of pressure drag.

148. My big square truck creates a lot of turbulence when it moves. Does my roof rack (a factory-installed one, close to the roof) actually improve aerodynamics, like fuzz on a tennis ball? (Also, what about the air dam at the back end?)
I'm sure that modern car designers consider aerodynamics when building a car or truck. They do structure the trailing edge of the car to minimize its turbulent wake. But I doubt that a roof rack helps much. It's probably too tall for the boundary layer on the car and extends into the free flowing stream beyond. As a result, it probably experiences its own pressure drag. The "fuzz" that trips the boundary layer has to be no taller than the boundary layer itself, otherwise it causes turbulence in the main airstream rather than preventing it. The same goes for the air dam.

149. Please explain what "lift" is.
Suppose that a horizontal wind is approaching a smooth, stationary ball from the right. The ball will experience a drag force that pushes it toward the left. We call it a drag force because it acts to slow the ball's motion through the air—in other words because it pushes the ball directly downwind. But if the ball isn't uniform or if the ball is spinning, it may experience a force that isn't directly downwind. If the ball experiences an aerodynamic force (a force due to the motion of the wind near its surface) that pushes it to the side, or that pushes it up or down, then it is experiencing a lift force. This lift force isn't necessary's just to the side—at right angles to the downwind direction.

150. How does a gas turbine engine (i.e., an aircraft engine) work?
A gas turbine uses energy stored in pressurized or rapidly moving gas to do work on a rotating mechanism. This rotary work can then be used to propel a vehicle or to generate electricity. Whether the gas is pressurized or rapidly moving doesn't really matter much. What is important is that the gas tends to flow from one region to another through a series of turbine blades. If the gas is pressurized, it is propelled through the blades by the unbalanced pressures (gases always accelerate toward lower pressure). If the gas is rapidly moving, it flows through the blades because of inertia.

As the gas flows through the turbine blades, it flows over and under each blade. The blades are shaped so that the gas goes faster over each blade than under each blade, and an imbalance of pressures results as a consequence of Bernoulli's effect. Each turbine blade acts like the wing of an airplane and experiences a lift force. This lift force pushes on each blade and twists the turbine around and around. The turbine blades effectively fly through the flowing gas stream and extract energy from it. The blades and turbine gain energy while the gas stream loses energy. The gas leaves the turbine at a lower pressure and/or speed than it had when it arrived.

151. If air in a turbofan engine bypasses the jet engine after going through the turbofan, why does the jet engine even exist in the system?
The turbofan engine has a giant fan at its inlet, with a much smaller turbojet engine behind it. That turbojet engine is what provides the mechanical work needed to turn the giant fan. About 5 to 10% of the air passing through the fan then passes into and through the turbojet behind it. The turbojet uses this air for its operations: compressing it, burning fuel in it, and then extracting most of that hot air's energy as rotational work. This rotational work is used to power the giant fan.

152. In a high bypass ratio turbofan engine, does the fan turn at the same RPM as the power turbine section — or is it geared down to run slower?
A turbofan engine uses a small turbojet engine to turn a giant fan and it is this fan that provides most of the engine's propulsion. The question asks whether the fan is turned directly by the turbojet engine or whether gears are use to allow the larger fan to spin more slowly than the smaller turbojet. This is an interesting question, particularly since many of the parts inside a jet engine are spinning almost as fast as they can tolerate without ripping themselves apart.

A turbofan engine contains two separate rotating assemblies or "spools," each of which is powered by hot exhaust gases flowing out of the combustion chamber through some turbine discs and each of which spins some compressor disc that push air toward the combustion chamber. The shorter of the two spools is hollow and the lower spool passes through its center.

The shorter spool, which spins at about 12,000 rpm, derives its power from high speed gas flowing through its turbine blades just after the combustion chamber and it powers a high pressure compressor just in front of the combustion chamber. The longer spool, which spins at about 4,000 rpm, derives its power from low-pressure gas flowing out of the high-pressure turbine and it powers both a low-pressure compressor in front of the high-pressure compressor and the actual turbofan blades. Overall, there is a rapidly turning hollow spool right around the combustion chamber and a more slowly spinning solid spool that extends both in front of and behind the high-speed spool. It's the low speed spool that spins the turbofan itself.

153. How can the total momentum still be zero when two objects are moving rapidly away from each other?
Momentum is a vector quantity, meaning that it has both an amount (a magnitude) and a direction. When two objects are moving rapidly away from one another, they each have momentums but those momentums are in opposite directions. When you add these momentums together to find the total momentum of the two objects, you must consider the directions of those individual momentums. If the two momentums are exactly equal in magnitude but opposite in direction, they will cancel when you add them together and the total momentum will be exactly zero.

154. How does a rocket engine work?
A rocket engine works by ejecting stored material. It pushes on this material to make the material accelerate and the material pushes back on the engine. If the force that the ejected material exerts on the engine is upward and greater than the rocket's weight, the rocket will accelerate upward.

Most rocket engines are chemical engines. They combine stored chemical fuels to produce hot, high-pressure gas. This gas is allowed to expand out of a narrow orifice—the throat of the engine's exhaust nozzle. Gases always accelerate toward lower pressure, so the high-pressure gas moves faster and faster as it rushes out of the nozzle. It reaches sonic velocity (the speed of sound) in the nozzle's throat and continues to move faster and faster as it flows out of the nozzle's widening bell. By the time the gas leave the engine completely, it's traveling several thousand meters per second. A liquid fuel rocket has an exhaust velocity of about 4,500 meters per second or about 3 miles per second. Accelerating the gas to this enormous speed takes a huge force—the engine pushes down hard on the gas. The gas pushes back and propels the rocket upward.

155. How does the use of sticks and fins stabilize rockets?
Sticks and fins both shift a rocket's center of aerodynamic pressure (center of drag) toward the tail of the rocket and behind the rocket's center of mass. As a result, the tail of the rocket normally remains at the rear during flight. The passing air twists the tail of the rocket until it's at the rear of the moving object.

156. How is it that gas moving very rapidly is unable to "communicate" with gas or surfaces in front of it?
When gas is moving slowly through a channel, it can respond to obstacles by flowing around them. For example, when the gas encounters a constriction in the channel, it speeds up to flow quickly through the narrowing and its pressure drops. But when the gas is moving very fast through the channel, it has trouble avoiding obstacles and behaves differently at a constriction. Instead of speeding up to flow smoothly through the narrowing, the gas collides with the walls of the constriction and is pressure rises. It just wasn't able to "sense" the presence of the constriction before it actually hit the constriction. When gas moves faster than the speed of sound in that gas, it can't anticipate changes in its environment and it doesn't follow Bernoulli's equation. That's why the nozzle of a rocket flares outward to handle the supersonic gas that emerges from the nozzle's throat. That high-speed gas experiences a pressure drop as it spreads out into the broad portion of the nozzle. The gas's density drops and its pressure goes down.

157. Since an object orbiting the earth is falling as it orbits, does it gradually get closer to the earth? Would it eventually reenter the earth's atmosphere and fall to the ground?-MG
If the orbiting object doesn't interact with anything but the earth, then the answer is: no, it will continue to orbit forever. That's because, although it is always falling and accelerating toward the earth, its sideways velocity continues to make it miss the earth. It just keeps on missing forever. Moreover, its total energy remains constant—the sum of its kinetic and gravitational potential energies. But if something removes some of its energy, it will gradually shift closer and closer to the earth and will reenter the atmosphere. That reentry occurs for low-lying satellites because they interact with the diffuse atoms in the extreme upper atmosphere. These satellites gradually lose energy and eventually come down in a blaze of frictionally heated material.

158. How is it that the torques cancel when you turn a bicycle?
During a turn, you lean the bicycle into the turn. For example, when you turn left, you lean the top of the bicycle toward the left. The result is that you (and the bicycle) experience two torques. First, the support force from the ground tries to rotate you one direction—it tries to make your head go left and your feet go right. Second, friction from the ground, which is making you and the bicycle accelerate toward the left as part of the turn, tries to rotate you in the opposite direction—it tries to make your head go right and your feet go left. These two torques will cancel one another if you are leaning just the right amount. As a result, the bicycle doesn't undergo angular acceleration and you don't tip over.

159. Is there a difference in the types of handles of a bike? On some bikes, there is the (upright) handlebar and on some the (drop) handlebar. Is there a purpose?
The shape of the handlebar determines your riding position. The upright position is generally more comfortable but, by sitting you upright, it increases the pressure drag you experience. Drop handlebars lower your body and make you more aerodynamic, but that position isn't as comfortable.

160. Please explain the advantage of air tires with less mass.
The rim of the wheel travels at a different speed from the rest of the bicycle. The top of the wheel heads forward faster than the bicycle, while the bottom of the wheel heads forward more slowly than the bicycle. But because kinetic energy is proportional to the square of speed, the increase in the top of the wheel's energy caused by its increased speed more than makes up for the decrease in the bottom of the wheel's energy caused by its reduced speed. The overall result is that the wheel rim has twice as much kinetic energy as it would have if it were simply sliding forward without turning. This fact is important because it means that you want as little mass in the wheel rim as possible. Every kilogram there counts double when you are trying to start up from rest. By putting air inside the tire, rather than rubber, you reduce the mass at the wheel rim and make the bicycle easier to start.

161. Why do the front brakes of a bike provide more braking power than the rear brakes, assuming both are applied with equal pressure?
When you apply brakes on a bicycle, you make it harder for the wheels to turn. The ground must then exert backward frictional forces on the wheels to keep them turning and it is these backward frictional forces that slow the bicycle's forward motion. But the forces that the ground exerts on the bottoms of the wheels also produces a torque on the bicycle about its center of mass—the whole bicycle has a tendency to begin rotating. Fortunately, the bicycle rarely actually rotates—if it did, you would fly forward over the front wheel of the bicycle. But this tendency to rotate during braking pushes the bicycle's front wheel downward onto the pavement and lifts the bicycle's back wheel upward off the pavement. The added pressure between the front wheel and the pavement improves traction there and makes the front wheel particularly effective for braking. The loss of pressure between the back wheel and the pavement reduces traction there and makes the back wheel particularly ineffective for braking. In fact, it's easy to begin fishtailing as the rear wheel loses traction completely.

162. Why is it easier for you to make sharp turns more quickly when your center of gravity is over the handle bar?
The force that causes you to turn is friction between the front wheel and the ground. When you turn left, friction pushes the front wheel left and you turn left. By putting all of your weight over the front wheel, you accomplish two things. First, you increase the maximum static frictional force between the ground and the front wheel. You push them together harder so that they are less likely to slide (skid). Second, you make it easier for that sideways friction force to accelerate you; the force acts closer to your body and more directly on you. There are fewer torques on the bicycle that might cause it to skid about on either the front or rear wheel.

163. How does a jackhammer work?
A jackhammer (or pneumatic hammer) uses compressed air to drive a metal piston up and down inside a cylinder. Each time the piston nears the top of the cylinder, it opens a valve that allows compressed air to flow above it and push it downward. Each time the piston reaches the bottom of the cylinder, it opens a valve that allows compressed air to flow below it and push it upward. Thus the compressed air makes the piston shuttle up and down very rapidly.

But while the piston rebounds gently from a cushion of air at the top of the cylinder, it collides suddenly with a metal bar at the bottom of the cylinder. That metal bar is the top end of the drill bit that the jackhammer uses to cut into pavement. Each time the piston moves downward, it pounds the drill bit a little farther into the pavement. The enormous force that pushes the bit into cement comes from the enormous force needed to stop the descending piston and to accelerate it upward. The drill bit pushes up on the piston very hard and the piston pushes down on the drill bit very hard. These two forces are equal and opposite, as they must be (Newton's third law of motion.) The piston ends up moving upward and the drill bit ends up moving downward.

164. If you pulled on the bottom of a multiple pulley while an object was hanging from the end of the multiple pulley's rope, would that object feel heavier than it really is?
Yes. If there were 5 segments in the multiple pulley, then you would have to pull down on the bottom of the multiple pulley with a force that was 5 times the magnitude of the object's weight in order to lift the object at constant velocity. But the object would also rise 5 times as fast as the end of the multiple pulley would descend.

165. What is the difference between a multiple pulley system in which the string you pull on comes down from the top pulley and the one in which the string you pull on comes up from the bottom pulley?
When the string you pull on comes down from the top pulley, it doesn't exert its tension on the thing being lifted so it doesn't count when add up the strings. But when the string you pull on comes up from the bottom pulley, that string is also helping to lift the object. That string does count. Thus if the multiple pulley has 5 segments going up and down between the two pulleys and one more segment going up to your hand, the total number of segments lifting the object is 6 and that object experiences an upward force equal to 6 times the tension in the string.

166. Why do many buses use air brakes instead of hydraulic brakes?
As you have noticed, buses, trucks, and trains often use air as the hydraulic fluid in their braking systems. That's because air is cheap and non-toxic, so that spilling it isn't a problem. While air's compressibility makes it a bit more complicated to work with than a liquid hydraulic fluid, it still works well in power braking systems.

167. With a pulley of 5 strings, why is each string experiencing 10 N of force and not 2 N apiece (when you pull on the string with 10 N of force)?
When you pull on the string with a 10 N force, you create 10 N of tension in that string. If there is less tension anywhere in the string, then that portion of the string will accelerate toward the side with more tension. That's why the tension in each string of a multiple pulley is 10 N when you pull on its loose end with a force of 10 N. The 5 strings are really just parts of the same string and that string has to have 10 N of tension in it.

168. Can a flame occur in space outside a spacecraft, where there is no oxygen? Can it burn or explode there?
For a piece of fuel to burn, it needs a source of oxygen. In open space, there is no oxygen and thus no way for fuel alone to burn. However, materials such as gunpowder that contain both a fuel and an oxidizing agent can burn in open space. In fact, because they don't rely on convection to bring new oxygen into the flame and to carry the burned gases away from the flame, such materials burn almost exactly the same way in space as they do on earth.

169. How does a convection oven work? How is it different from a regular oven?
In an electric convection oven, a fan circulates air rapidly through the cooking chamber. This rapid force circulation of air has two principal effects. First, it ensures that the temperatures throughout the oven are almost exactly equal. In a normal electric oven, the differences in temperatures that often occur lead to uneven cooking and require that you put the food in specific areas of those ovens to make sure that the food cooks properly. Since a convection oven has no temperature differences, you can put the food anywhere and you can fill the cooking chamber more completely with food. Second, a convection oven transfers heat more evenly to the food. By blowing hot air past the food, the oven prevents regions of colder air from building up near the surfaces of cool foods. Since the food in a convection oven is always in contact with hot air, it picks up heat faster and cooks faster. In a normal oven, heat is transferred to the food through normal convection (rising hot air and sinking cold air) and by radiation (particularly when the broiler is used). Both of these process are relatively slow and can be interrupted by over-filling the oven or blocking the line of sight between the hot filament and the colder food. In a convection oven, heat is transferred to the food mostly by forced convection (fan-driven hot air that circulates rapidly through the oven). This process is relatively fast and can't be interrupted by over-filling the oven (within reason) or blocking any line of sight between the hot filament and the food.

170. How does a steam heating system work?
A home steam heating system consists of a boiler, pipes, and radiators. The boiler is located in the basement and uses a burning fuel or electricity to heat water until it boils. Steam forms as the water boils and this steam accumulates above the liquid water. Steam isn't the mist that forms above a teapot—that's really just droplets of water. Steam itself is the clear gaseous form of water and it travels upward through the pipes to radiators in the rooms. Steam is actually a lighter-than-air gas and it's lifted upward by the same buoyant force that makes helium float. When the steam arrives inside the radiators, it begins to condense back into liquid water. As it does so, it releases an enormous amount of heat—the water molecules begin to stick to one another and they release chemical potential energy. After a short time, the temperature of the radiator rises until a balance is reached where the steam and the water are in equilibrium—typically about 100° Celsius, but dependent on the gas pressure inside the radiator. The hot radiator then heats the room. The water that forms as the steam condenses is carried by gravity back down the same pipe through which the steam arrived and returns to the boiler to be reheated.

171. If a flame always burns up, if you are in a weightless environment, how will the flame burn?
A flame should have serious problems in a weightless environment because it normally uses convection to carry burned gas away and to bring fresh air in. Since convection depends on gravity, there will be no tendency for the burned gas to leave and fresh air to replace it.

I talked with Kathryn Thornton, a former NASA astronaut who has actually performed combustion experiments in space and she described those experiments to me. In them, a drop of fuel was supported on a fiber and ignited. The flame front radiated outward from the fuel drop at ignition to form a spherical shell around the drop, which shrank slowly as it was consumed. Because convection requires gravity, there was no rising current of air to bring in new oxygen and to sweep away the burned gases. Instead, oxygen had to diffuse into the burning sphere and it did so quite slowly—the burns lasted for as much as 30 seconds on only a few cc's of fuel. Water vapor that formed during the combustion also had a tendency to diffuse into the fuel and dilute it so that it eventually stopped burning.

172. If lighter colors reflect more light, then why is it easier for a pale person to sunburn than someone with a darker skin tint?
The colors that you see are determined by the visible light absorbed by a surface. Thus, while the whitest skin reflects most visible light and appears white, it does absorb light that you can't see: ultraviolet light. This ultraviolet light is what damages the skin and causes sunburn. Darker skin absorbs most of the ultraviolet before it gets to sensitive skin cells while lighter skin lets that ultraviolet in far enough to cause injury.

173. In fast food restaurants, when they keep your hamburger warm under lights, is that heat from convection or radiation?
Radiation. Convection will take the heat up toward the ceiling rather than down toward the food. But radiation travels in straight lines, from the lamps to the burgers below them.

174. What is fire?
The fire of a burning candle begins with vaporized wax. Heat from the flame melts wax, which then flows up the wick because of its attraction to the fibers. The wax then becomes so hot that it turns into a gas and this gas mixes with air at the bottom of the flame. When the temperature becomes high enough, the wax molecules begin to decompose into fragments that react chemically with oxygen molecules. Water and carbon dioxide molecules are produced in the reaction and chemical potential energy is released as thermal energy. This thermal energy provides the candle's light and also the heat needed to sustain the combustion. The glow that the candle emits comes primarily from hot particles of carbon in the flame. These particles emit thermal radiation with a color spectrum that is characteristic of the flame's temperature.

175. Why doesn't glass have electrons to carry heat. What is glass made of?
Like everything in our world, glass does have electrons. Its atoms are built out of electrons. But those electrons are localized on the individual atoms or between them in such a way that they can't move easily. When you try to push these electrons through the glass, they won't go. Thus neither heat nor electricity flows easily through glass. In a metal, some of the electrons are mobile and can carry heat and electricity.

176. How does wearing a hat keep you warm (or cool)?
First, a hat serves as insulation against convective heat transfer. By trapping a layer of air near your skin, it slows the movement of air and heat away from your head. But it also acts as a barrier to thermal radiation. Just as a tree overhead radiates heat toward your skin and keeps you from losing heat to the dark night sky, the hat on your head radiates heat toward you and helps to keep you warm. On a sunny day, it keeps you cool by blocking the direct transfer of heat from the sun to your head.

177. If metal is a conductor of heat, why is it that aluminum foil will insulate food and reflect heat?
Aluminum may be a good conductor of heat, but its a terrible emitter or absorber of thermal radiation. When you wrap food in aluminum foil, you dramatically reduce that food's ability to lose heat via radiation if it's hotter than its surroundings or its ability to gain heat via radiation if it's colder than its surroundings. Aluminum foil doesn't have much effect on heat transferred to or from the food via conduction or convection because aluminum itself is a good conductor of heat.

178. At what point is it more efficient to leave a light on when leaving and the returning to a room?
Since turning an incandescent bulb on and off doesn't shorten the life of its filament significantly, you do well to turn it off whenever possible. The same isn't true of a fluorescent tube—turning it on ages its filaments significantly (due to sputtering processes) so you shouldn't turn a fluorescent lamp off if you plan to restart it in less than about 1 minute.

179. Can I produce light without using electric power?
Since light carries energy with it, something must provide that energy. However, the energy doesn't have to come from electric power. Since objects emit visible thermal radiation when they have temperatures above about 500 C, anything that heats an object to high temperatures will make light. But light can also be made without heat. There are many ways to convert electric energy into light without making anything hot (for example, a neon sign or a light-emitting diode). But you ask about making light with electricity. The next best choice is light-emitting chemical reactions, such as those used in light sticks (liquid-filled plastic sticks that you bend to activate and which then glow bright green for about 12 hours). However, such reactions don't produce all that much light and they consume the chemicals fairly quickly. If you are trying to produce large amounts of light without electric power, I'm afraid that you'll have to burn sometime. That's what people did before 1879 and the electric lamp.

180. How does a heat-seeking missile and a radar-homing missile work?
A heat-seeking missile studies the infrared light coming toward it from the sky in front of it. It uses a lens to form a real image of that light on an array of infrared sensors. If there is a hot object in front of the missile, that object will emit more infrared light than its surroundings and the missile's lens will form a bright image of the hot object on one of the infrared sensors. If the bright image falls on the central sensor, the missile will do nothing—it will flight straight ahead. But if the bright image falls on one of the side sensors, the missile will turn. It will turn by deflecting its rocket exhaust so that the missile begins to rotate in flight. As the missile rotates, the image of the hot object will move from one sensor to the next and it will eventually fall on the central sensor. At that point, the missile will stop turning and will flight straight ahead. Since the missile automatically turns to head toward the hot object, it will eventually fly right into the hot object and explode. A radar-seeking missile will do that same things, except that it will look for an object that is emitting lots of microwaves (radar), rather than lots of infrared light. A radar-guided missile is much more complicated, since it must first emit a burst of microwaves and then analyze the reflected microwaves to look for something to fly toward. Many laser-guided missiles are just like heat-seeking missiles except that they look for an object that is reflecting a laser beam. The people who fire the missile simply illuminate the target with a bright laser beam and the missile flies directly toward the laser spot on the target.

181. How does a regular lamp (light bulb) work?
A normal incandescent lamp contains a double-wound tungsten filament inside a gas-filled glass bulb. By "double-wound", I mean that a very fine wire is first wound into a long, thin spiral and then this spiral is again wound into a wider spiral. While the final filament looks about 1 or 2 centimeters long, it actually contains about 1 meter of fine tungsten wire. When the bulb is on, an electric current flows through the filament from one end to the other. The electrons making up this current carry energy, both in their motion and in the forces that they exert on one another. As they flow through the fine tungsten wire, these electrons collide with the tungsten atoms and transfer some of their energy to those tungsten atoms. The tungsten atoms and the filament become extremely hot, typically about 2500° Celsius. Tungsten wire is used because it tolerates these enormous temperatures without melting and because it resists sublimation longer than any other material. Sublimation is when atoms "evaporate" from the surface of a solid. The gas inside the bulb is there to slow sublimation and extend the life of the filament.

Once the filament is hot, it tends to transfer heat to its colder surroundings. While much of its heat leaves the filament via convection and conduction in the gas and glass bulb, a significant fraction of this heat leaves the filament via thermal radiation. For any object that is hotter than about 500° Celsius, some of this thermal radiation is visible light and for an object that is about 2500° Celsius, about 10% is visible light. The light that you see from the bulb is the visible portion of its thermal radiation. However, most of the filament's thermal radiation is invisible infrared light. While you can feel this infrared light warming your hand, you can't see it. Because only about 80% of the electric power delivered to the bulb becomes thermal radiation and only about 12% of that thermal radiation is visible, an incandescent light bulb is only about 10% energy efficient. Other types of lamps, including fluorescent and gas discharge lamps, are much more energy efficient.

182. Is there a better way to construct a light bulb? For instance, is there a way to prevent the surface of the bulb from heating so quickly and generating so much heat? Is glass the best cover?
Unfortunately, there is not much that can be done to increase the efficiency of an incandescent bulb. It emits light by creating a very hot filament. Some of the filament's heat is emitted as visible light but most ends up as hot air or infrared light (which you cannot see). There are tricks used to increase the bulb's visible light output slightly (e.g. heating the filament hotter as in a halogen bulb or reducing the heat transport in the bulb gas as in a krypton bulb), but mostly there is nothing that can be done. Glass is about the best material for a bulb: it's clear and a relatively poor conductor of heat.

183. On a three-way lamp, what are the switch settings for? Does it pump in more energy?
The lamp has four switch positions: off, filament 1 on, filament 2 on, and both filaments on. The bulb has three electrical connections to its filaments. One contact delivers electrical power to filament 1, another contact delivers electrical power to filament 2, and the third contact returns electricity from both filaments to the power plant. The switch carefully controls the flow of electricity to the two filaments so that at the low light setting, only the small filament is on, at the medium setting, only the large filament is on, and at the high setting, both filaments are on.

184. Which electric light bulb is best for the money, i.e. uses least electricity and has greatest light. I remember my high school physics teacher saying something like 50 watts -> 100 watts doesn't double the light, just eats electricity.
For a given type of light bulb, the higher wattage bulbs are more energy efficient. Each light bulb has some "overhead" of wasted power that goes into heating the supporting structure and glass envelope. The higher wattage bulbs produce a little more light per watt of power. But not all types of bulbs are equally efficient. Long life bulbs are the least energy efficient because they run cooler than normal bulbs. The filament lasts a long time, but wastes more power producing infrared light. Some "energy miser" bulbs aren't as good as normal bulbs. They may have lower wattages (typically 55 W instead of 60 W or 90 W instead of 100 W), but they actually produce significantly less light and thus consume more watts of power for each unit of light they produce. The most efficient incandescent bulbs are halogen lamps. These lamps, with their chemical recycling process, run substantially hotter than normal bulbs and produce more light per watt. They also last longer than normal light bulbs. They also produce whiter light (less red) and are just plain better bulbs than normal light bulbs. They cost more money up front, but it's worth it in most cases.

185. Why aren't you supposed to touch halogen bulbs with your bare hands?
When they're operating, halogen bulbs become extremely hot, so you certainly wouldn't want to touch them then. But even when a bulb is cool, touching it would deposit greases and salts from your skin onto its surface. The aluminosilicate glass used in the lamp's envelope would be weakened when these salts are baked into the glass during the lamp's operation and the greases would scorch and darken the bulb's surface.

186. Why do regular light bulbs have different effects on plants than fluorescent lights?
Regular (incandescent) light bulbs create light with a hot filament. This light is relatively reddish and contains very little blue, violet, or ultraviolet light. Since it comes from a hot, thermal source, this light covers all the wavelengths from infrared to the green and blue range of the spectrum continuously and smoothly, although its intensity peaks in the red and orange range of the spectrum. Fluorescent lights, on the other hand, create light through the fluorescence of atoms, molecules, and solids. The light is not created by hot materials so it contains certain regions of the spectrum, often including blue and violet light. Depending on the exact make-up of the fluorescent lamp, this light may include wavelengths that are particularly important to a plant's metabolic processes.

187. Why does a refrigerator light last so long?
The life of an incandescent bulb depends almost exclusively on how many hours its filament has been hot. Since the bulb in a refrigerator is only on for a few minutes a day, it lasts for many years.

188. A while ago, there was a fad to have T-shirts that changed colors with changes in temperature. How did those shirts work?
Those shirts probably use a microencapsulated chemical system that is temperature sensitive. In this system, tiny chemical bubbles are incorporated in a plastic material that can be used to form toys or household objects or even the writing on T-shirts. Inside each bubble are several chemicals, one of which melts at a temperature very near room temperature. When that chemical melts, it begins to interfere with the other chemicals so that they lose their colors. That way, part of the object's color disappears when it warms up. The plastic contains other normal pigments so that it doesn't become colorless when warm, but it does become more lightly colored.

189. How can temperatures be taken accurately and so quickly in the ear?
The fancy ear thermometers used in doctor's offices are almost certainly measuring the thermal radiation emerging from inside the ear. They probably use a thermopile detector that responds very quickly to the thermal radiation that reaches them. Since the thermal radiation emitted by a black object (or from within a deep cavity such as the ear) is characteristic of the object's temperature, a quick study of that thermal radiation is enough to determine the person's temperature.

190. How do certain mufflers provide more horsepower?
A muffler's job is to control the flow of exhaust from the cylinders to the outside air, so that the abrupt fluctuations in pressure created by the opening cylinders are smoothed away by the time the exhaust leaves the car's tail pipe. The pressure fluctuations create sound and, by smoothing them away, the muffler quiets the engine. But the easiest ways to smooth away the pressure fluctuations also impede the flow of exhaust from the cylinders. The result is that some exhaust is trapped in the cylinders and interferes with the operation of the engine. The car's gas mileage drops. A good muffler smoothes out exhaust pressure without impeding its flow and without reducing gas mileage.

191. How does a nitrous kit on a car make it go faster?
According to David Ingham, a nitrous kit is a system that injects nitrous oxide into the air intake. This technique was developed during WWII as a way to obtain short bursts of extra power from gasoline engines. Keith Spillman points out that the nitrous oxide is injected as a dense liquid so that it greatly increases the number of oxygen atoms inside the cylinder at the moment the fuel ignites. Since nitrous oxide breaks down into nitrogen and oxygen at high temperatures, it supports combustion and allows more fuel to burn during each engine cycle. The engine thus produces more power. The liquid nitrous oxide also provides an "intercooling" effect when it evaporates—it cools the gases in the cylinder prior to compression so that there is less possibility of knocking.

192. How does using better spark plugs make your car run more efficiently? Is it worth paying extra for those spark plugs? Would it improve your car's performance?
According to several readers of this web site, there is a difference between standard and high performance spark plugs. The high performance spark plugs produce a more intense spark and ignite the fuel and air mixture more reliably than standard plugs. That would indicate that igniting the fuel and air mixture at just the right moment isn't as straightforward as it seems. If the plugs don't fire reliably and don't light the gasoline at exactly the right moment every single time the cylinder is supposed to fire, the car's efficiency will suffer.

193. In modern car alarm systems, people can start the engine with a push of a button from a remote. How is this done?
This question has a long answer, because there's lots going on. First, there is a radio transmission from the key chain to the car when you push the button. That transmission is carefully encoded so that no one else can trigger your car (the car's receiver checks for the proper authorization code when it receives the radio transmission). I won't describe the transmission/reception process in detail, because that's a whole story in its self. The receiver than activates the car's electric system, which was cut off when the driver last turned off the car. The electric system is now ready to provide sparks at the proper moments when the engine turns. Finally, the receiver starts the engine turning by activating the starter motor. An electromagnetic solenoid (a coil of wire with a piece of iron inside) pushes the starter motor or a gear from the starter motor against the car's flywheel (a huge gear attached to the engine's crankshaft) and power is supplied to the starter motor. The motor begins turning and it turns the engine. The electric system provides sparks and the engine starts up.

194. I've heard about a car (I think some type of Ferrari) that has a clutch-less manual transmission.
According to Bryan Tiedemann, Ferarri makes a computer-shifted manual transmission. It begins with a standard manual transmission (gears, input/output shafts, synchronizers) that's similar to that of many "stick-shift" cars of today. However, instead of having a driver-controlled clutch and shift lever, a computer regulates the actual mechanical clutch movement and it also shifts gears via servos and motors. The driver uses a "shift paddle" on the steering wheel to shift, and the computer does the actual shifting. The automatically controlled manual is better than a normal automatic because manual transmissions give better performance than automatics and no energy is lost as heat in hydraulic couplings.

195. I've heard of people using moonshine as fuel for cars and pick up trucks. Is that possible and, if it is, how well does it work?
Yes, it's probably possible. Moonshine (and any distilled spirits) is a mixture of ethanol (ethyl alcohol) and water. Depending on how picky you are during the distilling process, the water content may be as low as 10% (you can't do better by distilling because 4.4% water and 95.6% ethanol form an azeotrope—a low boiling point mixture blend that can't be separated by distilling). Ethanol burns nicely and should make a pretty good fuel. Obviously, the less water the better, because water doesn't burn and may impede the combustion of ethanol. Ethanol is often included in gasoline to reduce exhaust emissions, but only at about the 10% level. Unfortunately, ethanol is also more corrosive than normal gasoline, so people worry about it damaging their engines.

196. What is the purpose of pistons in an engine?
The piston moves in an out of a cylinder, moving the air, fuel, and exhaust about and extracting work from the burned fuel and air. Without the piston, there would be no way to obtain energy from the gasoline.

197. What's the difference with a Mazda rotary engine?
The rotary engine was supposed to revolutionize automobiles when it was first introduced several decades ago. Instead of a piston and cylinder, it has a triangular shaped object that wobbles around the inside of a hollow chamber. This object traps a fuel and air mixture, compresses it, ignites it, extracts energy from it, and releases it into the outside air, just as a normal engine does. But it uses the wobbling motion of the triangle, rather than the reciprocating motion of the piston and cylinder. The rotary engine has fewer moving parts to wear out, but it evidently has other issues that have prevented its wide adoption.

198. Why do I need a choke?
When an engine is cold, it runs better with a rich mixture (more fuel, less air). Years ago, the choke pinched off the airflow to the cylinder (hence the name "choke") and was operated manually. Later it was operated automatically (often turning off too soon and causing the car to stall a few minutes after starting). In modern cars, there is no choke, just the computer controlling the fuel and air mixture on a moment-by-moment basis.

199. Why is it good to put premium gas in your car during the winter? If premium gas does not burn easily, does it also not freeze easily?
I'm not sure that it is better to put premium gas in your car during the winter. If you car operates properly on regular during the summer, then it should also operate properly on regular during the winter. Actually, summer gasoline and winter gasoline are slightly different. When it's cold outside, gasoline doesn't evaporate as easily so it needs to be reformulated to make it more volatile. During the winter, the gasoline manufacturers add more small molecules to the mixture so that it turns into a gas more easily. But they try to retain the same resistance to ignition in each of their gasoline grades. In any case, gasoline doesn't freeze at normal temperatures, so that's not a problem. Only water that condenses in your gas tank will freeze and can plug your gas line.

200. Does salt/sugar raise the boiling point or lower it? I thought you added salt to water so that it could form bubbles faster, but if the boiling point goes up isn't this kind of pointless?
When salt or sugar are dissolved in water, they raise the boiling temperature. It reduces the fraction of water molecules at the surface of the water, so that fewer leave each second. Because of this reduced leaving rate, the water has to get hotter before enough water molecules will leave each second to allow evaporation to occur inside the liquid (i.e. for the water to boil). As for the value of adding salt, sugar, or any other material to water to encourage boiling, that is a very different matter. Adding anything that can serve as a site for bubble formation will help the water to boil. Nucleating the tiny bubbles that eventually grow into the large bubbles we associate with boiling isn't easy. Often it occurs at a hot spot in the pot, or near a defect on the pot's inner surface. If there aren't any hot spots or defects, then adding sharp objects will aid bubble formation. That's why sprinkling sugar or salt into extremely hot water can help it boil. This boiling occurs before the sugar or salt dissolve. They are just acting as nucleation sites. You'd do just as well to add sand, which doesn't dissolve at all.

201. Is water, at say 35° F, more dense than water at 80° F?
Yes, water at 35° F is more dense than water at 80° F, so that the hotter water will float on the colder water. But water is special (and almost unique) in that it does expand slightly as it cools to its freezing temperature. Water's density reaches a maximum at 3.98° C (about 39° F) and then actually becomes less dense as you cool it toward 0° C (32° F). That means that 33° F water will float on 39° F water! This bizarre behavior allows ice and very cold water to float above slightly warmer water and keeps ponds from freezing solid. Without it, many animals would perish during the winter.

202. What is the function/purpose of water in your body? What happens if your body doesn't get sufficient water or gets too much water? What are the best sources of water?
Water plays so many roles in your body that I'll have to choose which one to describe. The role that strikes me as most important is water's capacity to dissolve chemicals and carry them about. Our bodies deal with thousands or perhaps millions of different molecules, each of which must migrate from place to place to perform its required tasks. For example, the molecules that are responsible for energy—carbohydrates, fats, oxygen, and carbon dioxide—must all move through us to keep us alive. Water acts as the carrier for all of these molecules. Many of the molecules dissolve easily in water—meaning that water molecules surround these molecules and carry them about one molecule at a time. Carbohydrates, carbon dioxide, salts, and many other organic compounds dissolve easily in water and are carried about by it. Other chemicals, particularly fats and oxygen, don't dissolve easily in water and need help. Hemoglobin in red blood cells is the molecular structure that carries oxygen through our blood. So my answer to the first question is that water is the principal solvent for all the chemicals in our body.

If your body gets too much or too little water, the dissolved chemicals become either too dilute or too concentrated. When you have too much water in your body, the water tends to flow out of you through any membranes that are permeable to water. Among other things, this process tends to make you excrete extra water as urine. When you drink too much water, it goes right through you. When you have too little water in your body, water tends to flow into you through any permeable membranes, or at least tends not to flow out of you. When you eat lots of salt and are overconcentrated, you don't excrete much water and feel extremely thirsty—your body is looking for more water.

Finally, the best sources of water are those that simply don't have many dissolved chemicals; or at least none that cause trouble for your body. That means that your water shouldn't have much lead or arsenic dissolved in it or any of a number of noxious organic chemicals. The purest waters are distilled water, rain water (assuming minimal air pollution), and water that has been chemically filtered (via ion exchange, reverse osmosis, and/or activated carbon). Spring and well waters tend to contain substantial amounts of dissolved calcium and magnesium salts, which make the water less pure but probably don't affect its healthfulness. One special case to look out for is water that was very hard but that has been passed through a water softener. The dissolved minerals that made the water hard will have been replaced by sodium compounds during the softening process and excessive sodium consumption may be a problem for some people.

203. Why does water boil quicker when you add salt?
Salt should actually delay boiling in water by raising the water's boiling temperature. The water simply won't boil until its temperature reaches a certain value and that requires heat. There isn't any trick that will shorten the heating time, except to add chemicals that boil at lower temperatures than water (e.g. alcohol). But sprinkling salt into very hot water may trigger boiling by providing nucleation sites on which the bubbles can begin to form.

204. How does a "water" clock work? What would be the simplest way to make a water clock that would maintain accuracy to say three minutes per hour?
The most common water clock, the clepsydra, has a reservoir that drips water into a cylindrical vessel. The height of the water in that vessel indicates the amount of time that has passed since the clock was started. The simplest version of the clepsydra just has a small hole in the bottom of the reservoir and doesn't take into account the decreasing drip rate that comes with the dropping water level in the reservoir and the decreasing water pressure at the hole. To make a more accurate clock, you should maintain a constant water level in the dripping reservoir. The simplest way to do this is to place an inverted bottle of water in the reservoir so that as the water level drops past the lip of the inverted bottle, air bubbles can enter the bottle and allow more water to flow out of the bottle and into the reservoir. If you keep the water level in the reservoir constant in this manner, you ought to be able to calibrate the clock to better than 3 minutes per hour. Use a carefully made graduated cylinder as the time-measuring cylinder and watch the water level gradually rise.

205. My grandmother used to have this watch she wound by shaking it. How is it possible?
Her watch was called an automatic watch and contained a heavy piece of metal at the end of a tipping arm. As her wrist moved during daily activity, the metal piece would swing back and forth, twisting the arm first in one direction and then in the other. The arm's twisting motion would wind the watch's mainspring, just as you can wind the mainspring of a normal wristwatch by rolling the knob back and forth in your fingers. On some automatic watches, you can feel and hear the weight swinging back and forth as you shake the watch.

206. Suspension bridges today can't oscillate like the Tacoma Bridge, can they?
Apparently not, because we've never seen or heard of one doing it. To prevent that sort of thing from happening, the bridge builders probably do two things. First, they damp all of the resonance in the bridge. By this, I mean that they introduce energy loss mechanisms that sap the energy out of all the resonant motions. For example, they could add plates that slide against one another as the bridge bends so that sliding friction will waste energy and spoil the resonant motion. Second, they make sure that there are no mechanisms for resonant energy transfer. The wind blowing on the Tacoma bridge gave it tiny pushes at just the right frequency. It oscillated the way a reed does in a musical instrument. These days, bridges are probably tested with computer modeling before they're built to make sure that they don't begin to oscillate when wind blows across them.

207. What are period, amplitude, and frequency?
Period is the time it takes for a resonant system to complete one cycle of its motion. For example, if a pendulum takes two seconds to swing over and back, then its period is two seconds. Amplitude is the maximum amount of motion a resonant system undergoes as it oscillates or vibrates (same thing). For example, if the pendulum swings one meter to the left of center and then one meter to the right of center, its amplitude of motion is one meter. Frequency is the number of cycles a resonant system completes in a certain amount of time. For example, if a pendulum swings over and back twice each second, then its frequency is two cycles-per-second or 2 hertz or 2 Hz.

208. What does the length of the string (in a pendulum) have to do with resonance?
When you lengthen the string or rod of a pendulum, you weaken the restoring force on the pendulum's weight. That weight must then drift farther from its equilibrium position to experience strong restoring forces. The result is that the pendulum swings more slowly through its cycles (its period increases and its frequency decreases). But no matter what the string's length, the pendulum will exhibit a resonance. The frequency at which this resonance occurs is all that changes.

209. Why are Rolex watches able to spin smoothly or what do jewelry inspectors look at to tell the difference between a fake and a real Rolex watch?
A real Rolex watch has a sweep second hand that appears to move steadily around the watch face. A fake, like most other watches, has a second hand that moves with a jerky motion, advancing a little bit each time the balance ring completes one half of a cycle. However, a reader has informed me that even a real Rolex moves its second hand in tiny steps—they're just very small. If you look closely, he writes, you'll see that the second hand makes 5 tiny steps each second. Evidently, the hand-advancing mechanism steps at a higher frequency (5 Hz) than in most other watches (1 Hz). These tiny steps are hard to see so the hand appears to move smoothly. I was relieved to hear this news because the balance ring mechanism is inherently jerky and it's hard to imagine a balance ring-based watch that avoid the jerkiness.

210. How can you make noise by running your finger (if it's wet) along the rim of a glass?
If you run your damp finger lightly along the rim of a crystal glass, it should begin vibrating. This trick involves a resonant transfer of energy in which your finger rhythmically pushes on the glass to make it vibrate more and more strongly. It takes a delicate touch. If you press to hard, you will prevent the glass from vibrating. If you press too lightly, you won't give it any energy. Your finger must stick and slip alternately, just as a bow does while sliding across a violin string.

211. What causes undertows?
When a wave breaks and then rushes up the beach, it leaves the water on the beach with excess gravitational potential energy. That's what's left of the wave's energy. The water accelerates back down the beach and returns to the sea. This returning flow of water tends to go under the sea's surface, probably because of the water's circular motion in waves. Remember that the water in a wave travels in a circle, always moving forward (in the direction of the wave's motion) when it's at its highest point and backward (away from the direction of the wave's motion) when it's at its lowest point. I suspect that the returning flow of water from the beach joins this backward moving low water. When this low-lying returning water flows past you, it tends to sweep you along with it, hence the name undertow.

212. What would happen if the moon instantly disappeared? (Tidal waves, earthquakes,...?)
The moon's gravity affects both the earth's path through space and the earth's shape. If the moon were to disappear, the earth's path would change but probably not enough to cause a noticeable difference. The earth and the moon normally orbit one another but the moon, which has much less mass than the earth, does most of the moving. Without the moon, the earth would just orbit smoothly around the sun. As for the earth's shape, the only part of the earth that responds noticeably to the moon's gravity is the water on its surface. The tides are caused mostly by the moon's gravity. Without the moon, the tides would be much smaller and caused only by the sun's gravity. Thus, in the long run, you would probably have trouble telling that the moon was gone without looking overhead—the earth's path wouldn't change much and you would have to look carefully to see the effect on the earth's oceans.

However, in the moments following the moon's disappearance, there might be some dramatic waves and a few stress-related earthquakes. The oceans and the earth's crust do experience substantial stresses due to the unevenness of the moon's gravity (it's stronger on the side of the earth nearest the moon than it is on the side of the earth farthest from the moon). But I doubt that the sudden change in stress caused by having the moon disappear would do more than temporarily flood a few coastal cities. One last effect worth noting is that the precession of the equinoxes, a 26,000 year process that shifts the earth's rotational axis in space and causes the stars that are overhead at night during a particular season to change gradually, is driven by the moon's gravity and would disappear if the moon were to disappear.

213. What's a rip tide?
According to the dictionaries, it's just a form of fast moving current, a rip current. Water returning either from the shore (after a wave breaks) or from a channel (as the result of the tide) can create a strong current that's difficult to stand still against.

214. Where would twisting waves be encountered (if you can't see them in water)?
They appear in rigid systems, such as beams or bridges. The Tacoma Narrows bridge failed because of a torsional (twisting) motion of its deck, driven by the wind. Before it failed, it was carrying torsional waves back and forth along its length. Torsional waves also appear in less spectacular engineering situations. When you lean on a loose tabletop, you actually send a torsional wave through it. However, it's so rigid that the wave is tiny and travels too quickly for you to see.

215. Why do waves get "choppy" when it is windy outside (a lot of consecutive choppy-whitewash waves)?
The wind pushes on wave crests. If the wind is relatively weak, it may add or subtract energy from the wave by doing work or negative work on it. But if the wind is too strong, it can blow the top off a crest. Choppy seas occur when the wind is so strong that it blows the surface water right out of wave crests and turns them white with foam.

216. How does sliding your feet on carpet give you a static charge?
Sliding friction tends to wipe electric charge from one surface to another. Which surface acquires positive charge and which negative charge depends on the chemical properties of those surfaces. You end up with one charge and the carpet with the opposite charge.

217. If one were to use an electrostatic precipitator in a house full of smokers, would the smell from the cigarettes disappear as well? Why or why not? Isn't the smell/odor contained in the molecules and the molecules are contained in the smoke particles, thus removing the odor from the room?
I'm not sure what fraction of the odor of cigarette smoke is associated with the particles of smoke. An electrostatic precipitator can certainly remove most of the particles and with them, at least a good fraction of the smell. But I suspect that some of the odor is in individual molecules that are less likely to be removed from the air. They are best removed by adsorbing them (sticking them) to a surface, such as the vast surface on granules of activated charcoal. Such granules have pores that allow the molecules to touch lots of internal surface and stick there.

218. If you rub a comb through your hair and hold it near a thin stream of water flowing from a faucet, the stream of water will deflect toward the comb. Why?
A stream of water can become charged when another charge comes near it. The negatively charged comb attracts positive charge onto the water stream and pushed negative charge off of it. As a result, the stream acquired a positive charge and the rest of the world, a negative charge. The stream deflects toward the oppositely charged comb.

219. How do photoconductors work?
When the atoms and molecules in a solid join together, some of their electrons may become shared between them. These electrons can travel about the solid as waves. Because they travel as waves, they can only follow paths that bring them back perfectly in phase with how they started out, like steady ripples on a pond. As a result, they can only follow certain paths and can only have certain energies. For complex and fundamental reasons, only two electrons can adopt any particular path, so the electrons take turns filling up all of these paths or "levels" from the lowest energy ones up. The electrons fill up these levels until there are no more electrons seeking a path. The behavior of the solid depends on the nature of the levels remaining after all of the electrons have found a path. The last few levels filled with electrons are called "valence levels" and the first few empty levels are called "conduction levels". If there are no more empty levels at energies near the last one filled, the material will behave as an insulator. The conduction levels are far higher in energy than the valence levels. If there are empty levels at energies near the last one filled, the material will behave as a conductor. The conduction levels and valence levels are right nearby. A photoconductor is of the former type: there are no conduction energy levels near the last one filled valence level so it is an insulator. But it becomes a conductor when exposed to light because the light can move the valence level electrons into empty conduction levels at much higher energies.

220. In the photocopying or xerographic process, what is the intensity, wavelength, and normal exposure time of the light that is emitted from lamps of these office machines? How does this light differ from sunlight?
The light sensing surface in a xerographic copier is a semiconductor or "photoconductor" film on a metal drum or belt. Light causes this film to convert from an insulator to a conductor of electricity, a change that is ultimately responsible for the formation of the copy image. However, the light particles ("photons") must each carry a certain amount of energy in order to cause that conversion. Since the photons of blue light carry more energy than those of red light, blue light tends to be more effective in the xerographic process than red light. In fact, far red and infrared light have no effect at all on the photoconductor film. However, considerable effort has been made over the years to make the photoconductor films used in xerographic copying very sensitive to all wavelengths of visible light. As a result, it doesn't take much light from even a normal lamp to produce a xerographic copy. Sophisticated copiers expose the original document to visible light from an incandescent lamp, a fluorescent lamp, or a xenon/krypton flashlamp and measure the light reflected by that document. They use this measurement to set the exposure time and/or the aperture of the lens that forms the image of the document on the photoconductor film. The light used in a copier doesn't contain as much ultraviolet light as sunlight, but otherwise the differences aren't very important to the xerographic process. As for the intensity and exposure times, you can see these for yourself when the machine operates. Just open the cover and watch the lamps or flashlamps in action.

221. How can currents and electromagnets encounter frictional effects without touching?
When you slide a strong magnet quickly above a metal surface, there is a friction-like magnetic drag effect. This effect occurs even when the two objects don't touch. The origin of this effect lies in the repulsions between the metal and magnet: it's strongest slightly in front of the moving magnet so the magnet encounters some difficulty heading forward. The reason why the magnetization of the metal is strongest slightly in front of the moving magnet is related to the loss of energy by current moving in the metal. The magnetization (of the metal surface) in front of the moving magnet is fresher than the magnetization behind it. The current responsible for the magnetization behind the magnet has been flowing for long enough to have lost energy. But the faster you move the magnet across the metal surface, the less time the currents in it have to lose energy and the less friction-like force the magnet experiences.

222. How does a magnet induce a metal to become attracted to the magnet? Does the metal become a magnet also?
A steady, motionless magnet can't induce a piece of normal metal (not iron, cobalt, or nickel) to become magnetic. Only a moving or changing magnet can do that. When a metal is exposed to a changing or moving magnet, it does become magnetic. That metal becomes a type of magnet; an electromagnet. The metal itself isn't really the magnet; the electric charges inside it are. These charges move in response to the changing or moving magnet nearby and they become magnetic, too. The effect is always repulsive, not attractive. The temporarily magnetic metal repels the magnet that is making it magnetic.

223. How does running current through a coil cause a magnetic field?
Electricity and magnetism are interrelated in a great many ways. At the very basic levels, they are manifestations of the same fundamental physical concepts. As a result, electricity can produce magnetism and magnetism can produce electricity. One way in which electricity can produce magnetism is for charged particles to move. When an electric current passes through a coil (or any wire, for that matter), it creates a magnetic field. The coil develops a north magnetic pole and a south magnetic pole. I can't really explain why because the answer is simply that moving charges create magnetic fields; that's the way our universe works and no one has ever seen otherwise.

224. If magnetic trains are to work, wouldn't friction on the bottom of the train create thermal energy which would destroy the magnetism of the train?
When a magnetically levitated train is operating properly, it doesn't touch the track and experiences no friction. In principle, it shouldn't get hot at all. The magnetic drag effect will warm the track slightly, but that won't matter to the train's magnets. Actually, the train's magnets will almost certainly be superconducting wire coils with currents flowing in them. That type of magnet doesn't depend on the magnetic order of permanent magnets. It's the magnetic order of permanent magnets that is destroyed by heat. An electromagnetic coil will stay magnetic as long as current flows through it, even if it's so hot that it's ready to melt.

225. If you have more volts is it more energy (like a stun gun—is it better to have one with more current or volts or both)?
Volts is a measure of energy per charge. Thus if you tell me how much charge you have and the voltage of that charge, I can tell you have much energy that charge contains. I simply multiply the voltage by the amount of charge. Current is a measure of how many charges are moving through a wire each second. If you tell me how much current a wire is carrying and for how long that current flows, I can tell you how much charge has gone by. I just multiply the current by the time. To figure out how much energy electricity delivers to something (such as a person zapped by a stun gun), I need to know the voltage, the current, and the time. If I multiply all three together, the product is the energy delivered. In a stun gun, the voltage is important because skin is insulating and it takes high voltage to push charge through skin and into a person's body. But current is also important because the more charge that passes by, the more energy it will carry. And time is important because the longer the current flows, the more energy it delivers. So all voltage and current are both important. I can't guess which one is most important.

226. What is the dangerous part of electricity: charge, current, voltage, or what?
Current is ultimately the killer. A current of about 30 milliamperes is potentially lethal when applied across your chest. But your body is relatively insulating, so sending that much current through your chest isn't easy. That's where voltage comes in. The higher the voltage on a wire, the more energy each charge on the wire has and the more likely that it will be able to pierce through your skin and travel through your body. Thus it's a combination of voltage and current that is dangerous. Current kills, but it needs voltage to propel it through your skin.

227. What is the difference between current and voltage?
Current measures the amount of (positive) charge passing a point each second. If many charges pass by in a short time, the current is large. If few charges pass by in a long time, the current is small. Voltage measures the energy per charge. If a small number of (positive) charges carry lots of energy with them (either in their motion as kinetic energy or as electrostatic potential energy), their voltage is high. If a large number of charges carry little energy with them, their voltage is low.

228. What is the difference between fields and charges (magnetic and electric)?
Electric charges themselves push and pull on one another via electrostatic forces. Magnetic poles push and pull on one another via magnetostatic forces. We can also think of the forces that various electric charges exert on one charge that you're hold as being caused by some property of the space at which that one charge is located. We call that property of space an electric field and say that the charge is being pushed on by the electric field. We could do the same with magnetic poles and a magnetic field. But these two fields are more than just a useful fiction. The fields themselves really do exist. You can see that whenever moving electric charge creates a magnetic field or when a moving magnetic pole creates an electric field. Light consists only of electric and magnetic fields.

229. What materials are magnets made of?
They are mostly iron, cobalt, or nickel, which are intrinsically magnetic metals. But to help them retain their magnetic alignments, permanent magnets have other elements in them, too. Iron is magnetic at the microscopic scale, but that magnetism is broken up into lots of tiny regions that all point in random directions. To make a whole piece of iron magnetic, something must help those tiny regions stay pointing in the same direction. The good permanent magnets have structures that keep all the tiny regions pointing in one direction.

230. How can a battery lose energy when it's not being used (like when it sits in a flashlight that's not turned on for months or years)?
The battery maintains a steady positive charge on its positive terminal and a negative charge on its negative terminal, month after month. These opposite charges attract one another and they do manage to get back together occasionally. They usually travel right through the battery itself, assisted by thermal energy. When that happens, the battery has to pump additional charge from the negative terminal to the positive terminal to make up for the lost charge and consumes a little more of its chemical potential energy. You can slow down this aging process by refrigerating the batteries. With less thermal energy available, the accidental movements of charge through the battery become less frequent.

231. How do collisions with tungsten atoms in the filament of a flashlight convert the current's electrostatic and kinetic energies into thermal energy?
When the electrons moving through the tungsten filament collide with the tungsten atoms, they do work on those tungsten atoms. Although the atoms are very massive and the electrons bounce off of them like Ping-Pong balls from bowling balls, the atoms do jiggle about after being struck. Bombarded by a steady stream of electrons, the atoms in the tungsten begin to vibrate harder and harder and soon become white hot. The electrons leave the tungsten filament with relatively little energy left-they use almost all of their kinetic and electrostatic potential energies to get through this gauntlet of tungsten atoms.

232. If you keep batteries in your car-where it gets really hot on a summer day-will the batteries "die" faster? (I got brand new batteries and have them in a flashlight in my car and they are almost dead, yet I never really used the flashlight but for a couple of minute.)
Yes. Thermal energy spoils everything and the hotter you heat an object, the more thermal energy it contains. Keeping batteries or photographic film cool preserves them against aging.

233. What exactly are fuses and why do people change them or blame them if something short circuits?
A fuse is a weak link inserted into a circuit to break the circuit if too much current flows through it. The electric resistance of the fuse is large so that the current deposits a fair amount of thermal energy into it as it passes through. When the current exceeds the designated amount, the fuse melts and burns out. A short circuit usually blows out the fuse because it causes an enormous increase in the current flowing through the circuit. When that happens in your house, you should be thankful for the fuse because it saved you from the fire that might occur if it weren't there. You sure don't want the wires in your wall to melt and burn out because they might take the whole building with them. A circuit breaker is just an electromagnetic variation on the fuse. As the current through the circuit break increases, an electromagnet inside the circuit breaker becomes stronger and stronger until it eventually flips a switch that opens the circuit.

234. What happens when a battery dies?
A battery uses its chemical potential energy to pump electric charges from its negative terminal to its positive terminal. Eventually it runs out of chemical potential energy. In an alkaline battery, the chemical potential energy is mostly contained in zinc powder and this powder oxidizes as the battery operates; in effect, it burns up in a very controlled manner. By the time the battery is dead, there just isn't much pure zinc metal left.

235. Can you explain power surges?
Sometimes lightning strikes a power line and deposits a large amount of charge on it. This charge has considerable electrostatic potential energy so its voltage is very large (a large positive voltage if the lightning carried positive charge, a large negative voltage if the lightning carried negative charge). A the charge flows outward along the wires, it raises the local voltages of the wires. This sudden, brief increase in the local voltages is what you mean by a power surge. Many devices (e.g. computers and televisions) can be damaged by such a surge in voltage. Even a light bulb can be damaged because the extra voltage pushes too much current through the filament and can burn it out.

236. How can we talk about positive particles flowing through wires when it is really negatively charged electrons?
The fiction of current being carried by positive charges really does work nicely. If a wire is carrying negatively charged electrons to the east, then the east end of the wire is becoming more and more negative and the west end is becoming more and more positive. The same would happen if that wire were carrying positively charged particles to the west. Even though these positively charged particles aren't really there, we can pretend that they are. By pretending that current is carried by positive particles, we don't have to worry about the arrival of a positive number of negatively charged electrons lowering the voltage of an object.

237. How does hydroelectric power work?
Hydroelectric power begins with water descending from an elevated reservoir, such as a lake in the mountains. While it's in the elevated reservoir, this water has stored energy—in the form of gravitational potential energy. As this water flows downward through a pipe, its gravitational potential energy becomes either kinetic energy or pressure potential energy or both. By the time the water arrives at the hydroelectric power plant, it is either traveling very quickly or has an enormous pressure or both. In the power plant, the water flows past the blades of a huge turbine and does work on those blades. The blades are shaped somewhat like airplane wings and they "fly" through the moving water. Since the blades are attached to a central hub, they cause this hub to rotate and allow it to turn the rotor of a huge electric generator. The rotor of this generator typically contains a giant electromagnet. The electromagnet turns within a collection of stationary wire coils and it induces electric currents in those coils. These electric currents carry power out of the generator to the homes or business that need it.

238. How does power get from the plant to my house? Where do the voltages go up and down?
The voltage is stepped up at the power plant so that a small current of very high voltage charges (high energy per charge) can carry enormous power across the countryside. When this current arrives at your city, its voltage is stepped down so that a medium current of medium high voltage charges can carry that same enormous power through your city. Finally, near your house, its voltage is again stepped down so that a large current of low voltage charges can carry this power into your house. Naturally, you do not use all of the power from the power plant yourself, so it is distributed among all of the buildings in the city.

239. How is AC converted in certain items to DC?
These devices use diodes, which are one-way devices for current. They only allow the current to flow a certain direction and block its flow the other way. With the help of some charge storage devices called capacitors, these diodes can stop the reversals of AC and turn it into DC. Those little black battery eliminators that you use for household electronic devices contain a transformer, a few diodes and a capacitor or two.

A step-up transformer has a secondary coil with many, many turns. As the current in the primary circuit flows back and forth, it creates a reversing electric field around the iron core of the transformer. This electric field pushes charges through the secondary coil so that it travels around and around the core. Each charge goes around many times, picking up more energy with each passage. By the time the charge leaves the transformer, it has lots of energy so its voltage is very high.

240. How is AC current (alternating current) made?
Usually with alternating current generators, which we will discuss next. It can also be made by electronic alternators, such as those found in the uninterruptible power supplies that provide backups for computers.

241. If current times voltage equals power, this makes it seem that high current times low voltage would equal low current times high voltage; but this is not true because of resistance. How is resistance taken into account in the current times voltage equal power equation?
Your first observation, that high current times low voltage would equal low current times high voltage is true; it means that electricity can deliver the same power in two different ways: as a large current of low energy charges or as a small current of high energy charges. That result is critical to the electrical power distribution system. The resistance problem is a side issue: it makes the delivery of power as a large current of low energy charges difficult. If you could get this current to peoples' houses without wasting its power, there would be no problem, but that delivery isn't easy. The wires waste lots of power when you try to deliver these large currents. So the electric power distribution system uses small currents of high-energy charges instead.

242. In what circumstances is a step-down transformer more advantageous than a step-up transformer and vice versa?
The transformer moves power from the primary circuit to the secondary circuit, almost without waste. The main reason for using a transformer is to change the relationship between voltage and current. Whenever you need a large current of low energy, low voltage charges, you probably want a step-down transformer. Whenever you need a small current of high energy, high voltage charges, you probably want a step-up transformer. I have already described the issues in power distribution, but transformers are used in many other devices. Step-down transformers are used to power small electronic devices instead of batteries (those little black boxes you plug into the wall socket contain transformers and some electronics to convert the resulting low voltage AC into low voltage DC). Step-up transformers are used in neon signs and bug-zappers.

243. Is it true that if you double the current through a wire then you double the voltage loss and if you halve the current then you halve the voltage loss?
Yes. When you try to push current through a wire, the voltage drop across that wire (i.e. the energy lost by each charge passing through that wire) is proportional to the number of charges flowing through that wire each second (i.e. the current through the wire). If you double the number of charges flowing through the wire each second, then each charge will lose twice as much energy (the voltage drop across the wire will double). If you halve the number of charges flowing through the wire each second, then each charge will lose half as much energy (the voltage drop across the wire will halve).

244. What are the relationships between Joules, Coulombs, Amperes, Volts, and Watts?
A Joule is a unit of energy; the capacity to do work. A Coulomb is a quantity of electric charge; equal to about 6,250,000,000,000,000,000 elementary charges. An Ampere is a measure of current; equal to the passage of 1 Coulomb of charge each second. A Volt is a measure of the energy carried by each charge; equal to 1 Joule of energy per Coulomb of charge. A Watt is a measure of power; equal to 1 Joule per second. A current of 1 Ampere at a voltage of 1 Volt carries a power of 1 Watt. That is because each Coulomb of charge carries 1 Joule of energy (1 Volt) and there is 1 Coulomb of charge moving by each second (1 Ampere). That makes for 1 Joule of energy flowing each second (1 Watt).

245. What causes large electric resistances?
Thin wires or wires made of poor conductors. Some metals are simply better at carrying current without wasting energy than other metals. It has to do with how often a charge bounces off of a metal atom and loses energy. Copper, Silver, and Aluminum are good conductors while stainless steel and lead are pour conductors. Metals tend to become better conductors as you cool them and worse as you heat them. Semiconductors such as carbon (graphite) are poor conductors but have the reverse temperature effect. At low temperature they are poor conductors but become good conductors at high temperature.

246. What does voltage rise mean?
When current flows through a battery or the secondary of a transformer, its receives power. Each charge leaves the battery with more energy than it had when it arrived. Since the energy of each charge has increased, the voltage (energy per charge) of the current has increased. Thus the current passing through the battery experiences a rise in voltage or a "voltage rise".

247. What is resistance?
Resistance is the measure of how much an object impedes the flow of electricity. The higher an object's resistance, the less current will flow through it when you expose it to a particular voltage drop. To use the water analogy, resistance resembles a constriction in a pipe. The narrower the pipe (higher the resistance), the harder it is to push water through that pipe. If you keep the water pressure constant (constant voltage drop) as you narrow the pipes (increase the resistance), then less water will flow (the current will drop).

248. What is the difference between current and voltage?
Current is the measure of how many charges are flowing through a wire each second. A 1-ampere current involves the movement of 1 Coulomb of charge (6,250,000,000,000,000,000 elementary charges) per second. Voltage is the measure of how much energy each charge has. A 1-volt charge carries 1 Joule of energy per Coulomb of charge. To use water in a pipe as an analogy, current measures the amount of water flowing through the pipe and voltage measures the pressure (or energy per liter) of that water.

249. What is the difference between single-phase and three-phase electric power?
In single-phase power, current flows to and from a device through a pair of wires. The direction of the current flow changes with time, reversing smoothly 120 times a second in the US or 100 times a second in Europe (60 or 50 full cycles of reversal, over and back, each second respectively). In its simplest form, one of the two wires is called "neutral" and its voltage is always close to 0 volts (meaning that it has essentially no net electric charge on it). The other wire is called "power" and its voltage fluctuates from positive to negative to positive many times a second (meaning that its net electric charge varies from positive to negative to positive). The difference in voltage between "neutral" and "power" propels current through the device.

In three-phase power, current flows to and from a device through a group of three wires. These three wires are often called "X", "Y", and "Z", and each one is a power wire with a voltage that fluctuates from positive to negative to positive many times a second. (A fourth wire, "neutral", with a voltage of approximately 0 volts, may also be used.) But while the voltages of the three power wires fluctuate up and down the same number of times each second, they do not reach their maximum or minimum voltages at the same time. They reach their peaks one after the next in an equally spaced sequence: first "X", then "Y", then "Z", and then "X" again and so on. Because these three wires or "phases" rarely have the same voltages, currents can and do flow between any pair of them. It is such current flows that power the devices that use three-phase electric power. The natural sequencing of the three phases is particularly useful for devices that perform rhythmic tasks. For example, three-phase electric motors often turn in near synchrony with the rising and falling voltages of the phases.

Another advantage of three-phase electric power is that there is never a time when all three phases are at the same voltage. In single-phase power, whenever the two phases have the same voltage there is temporarily no electric power available. That's why single-phase electric devices must store energy to carry them over those dry spells. However, in three-phase power, a device can always obtain power from at least one pair of phases.

250. What is the difference, if any, between appliances with a 2 prong plug and a 3 prong plug?
In the 2 prong system, current travels to the appliance through one prong and leaves through the other prong. The roles of the two prongs interchange every 120th of a second. In the 3-prong system, there is one extra prong and that connects the frame of the appliance to the ground (the earth). This extra connection is a safety feature. If a wire comes loose inside the appliance and touches the frame, the frame can deliver charge and current to you through your hand and you can deliver it to the ground through your feet or your other hand. The earth is very large and a large amount of charge can flow into it without repelling further charge. Moreover most electrical systems are actually connected to the ground at some point. So if current can travel out of the circuit feeding power to the appliance and travel through you and into the ground, it will. You'll get a shock. The ground connection (the extra prong) allows this extra current to flow to ground so easily that a huge current is drawn out of the power source, causing the fuse or circuit breaker in that power source to break the connection. When that power connection is broken, no power can flow to the appliance at all and you can't get a shock from it. Plastic appliances often omit the extra prong because they have nothing dangerous to touch on their exteriors.

251. What is the hum you hear when walking under large power lines?
The electric currents in those lines are reversing 120 times a second in the United States (60 full cycles of reversal, over and back, each second). That means that the electrostatic forces between the charges they carry and anything nearby reverse 120 time a second and the magnetic forces that they exert on one another when currents flow through them turn on and off as well. You hear all of the motions that are caused by the pulsating electric and magnetic forces.

252. What is the purpose of the iron core in a transformer?
The iron core of a transformer stores energy as power is being transferred from the primary circuit to the secondary circuit. This energy is stored as the magnetization of that iron. The transformer needs to store that energy for roughly one half cycle of the alternating current or about 1/120th of a second. The more iron there is in the transformer, the more energy it can store and the more power the transformer can transfer from the primary circuit to the secondary circuit. Without any iron, the energy must be stored directly in empty space, again as a magnetization. But space isn't as good at storing magnetic energy as iron is so the iron increases the power-handling capacity of a transformer. Without the iron, the transformer must operate at much higher frequencies of alternating current in order to transfer reasonable amounts of power.

253. What makes alternating current alternate?
The pump for alternating current (usually an electrical generator) creates electric fields that reverse their directions 120 times a second (60 full cycles of reversal, over and back, each second). This reversal pushes the current backward and forward through the wires connecting to this power source. The currents direction of flow alternates and so does its voltage.

254. When going from 12 volts to 240 volts, is the point that with higher voltage the power transfer proceeds with fewer particles?
Yes. If you use higher voltages, you can transfer the same amount of power with a small current of charged particles. The energy lost in the transmission through wires increases as the square of the amount of current through those wires so reducing that current is very important.

255. When you say that a transformer can change a small current with a high voltage into a large current with a low voltage, where do those extra charges come from?
A transformer involves two completely separate circuits: a primary circuit and a secondary circuit. Charges circulate within each circuit, but do not move from one circuit to the other. If the primary circuit of a transformer has a small current flowing through it and that current experiences a large voltage drop as it flows through the transformer's primary coil, then the primary circuit current is transferring power to the transformer and that power is equal to the product of the primary circuit current times the voltage drop. The transformer transfers this power to the current flowing in the secondary circuit, which is an entirely separate current. That current may be quite large, in which case each charge only receives a modest amount of energy as it passes through the secondary coil. As a result, the voltage rise across the secondary coil is relatively small. The power the transformer is transferring to the secondary circuit current is equal to the product of the secondary circuit current times the voltage rise.

256. Where does the exact reversal occur in an alternating current circuit (where does the energy diminish completely and then turn the opposite way)?
The reversal of the current in an alternating current (AC) circuit occurs everywhere in the circuit at once. The whole current gradually slows to a stop and then heads backward. At the moment it comes to a complete stop, the electric power company isn't supplying any power at all and the circuit isn't consuming any. Because the power delivery pulses on and off in this manner, devices that operate on AC power are designed to store energy between reversals. Motors store their energy as rotational motion. Stereos store energy as separated electric charge in devices called capacitors, or as magnetic fields in devices called inductors.

257. Why are there danger signs around high voltage equipment?
Your body is a relatively good conductor of electricity and it is easily damaged by currents flowing through it. Your body uses electricity to control its functions and an unexpected current of as little as a few hundredths of an ampere can interrupt those functions. In particular, your heart can stop beating properly. Fortunately, your skin is a pretty good insulator so it is hard to get any current to flow through you. But high voltages can push current so hard that it punctures your skin and begins to flow through you. While the current is actually what injures you, the high voltage is what breaks down your protective skin and allows that current to flow through you.

258. Why do north and south poles on magnets change back and forth?
Only electromagnets can change back and forth and then only when they are connected to a supply of alternating current. A permanent magnet, such as that used to hold notes to a refrigerator, has permanent poles that do not change. But an AC powered electromagnet, such as that found in a transformer, does have poles that change back and forth.

259. Why does a high voltage transformer make ozone?
High voltages involve large accumulations of like electric charges. These charges repel one another ferociously and can leap off into the air near sharp points and edges. They produce sparks and corona discharges. While these discharges are useful in some devices (e.g. copiers and air cleaners), they tend to transfer energy to air molecules and can break up those air molecules. When normal oxygen molecules (which each contain 2 oxygen atoms) break up, the resulting oxygen atoms can stick to other oxygen molecules to form ozone molecules (which each contain 3 oxygen atoms). That is why you can often smell ozone near electrical discharges, high voltage power lines, and after thunderstorms.

260. Why does less current flow through a longer wire?
Wires obey Ohm's law: the current flowing through them is proportional to the voltage drop across them. But the precise relationship depends on the wire's length. A short wire will carry a large current even when the voltage drop across it is small because that wire has a small electrical resistance; it does not impede the flow of electricity very much. But a long wire has a large electrical resistance and will only carry a large current if the voltage drop across it is large. If you do not change the source of electrical power (e.g. a battery) and replace short wires with long wires, those wires will not be able to carry as much current.

261. Why is direct current so much better than alternating current?
It depends on the situation. You cannot use a transformer with direct current, so in that sense, alternating current is better. But many electronic devices need direct current because they require a steady flow of charges that always head in the same direction. So there are times when you need DC and times when you need AC.

262. Are there any objects that use compressed air to create electricity?
Moving air is used to create electricity: wind-powered generators. Compressed air is usually created with electrical power, so using it to generate electricity would be inefficient. But wind-powered generators are a common sight in some parts of the country. The wind blows on the turbine blades, doing work on them and providing the mechanical power needed to turn a generator. The generator converts this mechanical work into electrical energy.

263. How can current alternate — why doesn't it cancel itself out.
Actually, it does cancel out on the average. When you plug a toaster into the AC power line and turn it on, current begins to flow back and forth through that toaster. At first it flows out one wire of the outlet, through the toaster, and returns into the other wire of the outlet. About 1/120th of a second later, the current has reversed direction and is now flowing out of the second wire of the outlet, through the toaster, and into the first wire. It continues flowing back and forth so that, on the average, it heads nowhere. But the toaster receives energy with every cycle of the current so that there is a net flow of power to the toaster even if there is no net flow of current through it.

264. How do diodes work?
Diodes are made of semiconductors, which are essentially the same as photoconductors. These materials normally have electrons filling all of the valence levels and empty conduction levels. The empty conduction levels are at energies well above those of the valence levels so that electrons cannot easily shift from a valence level to a conduction level, a shift that is necessary for the material to conduct electricity. Thus semiconductors are normally insulating. But when the semiconductor is mixed or "doped" with other atoms, it can become conducting. A doping that removes electrons from the valence levels and leaves some of those levels empty produces "p-type" semiconductor. A doping that adds electrons to the conduction levels produces "n-type" semiconductor. Both "n-type" and "p-type" semiconductors can conduct electricity. But when the two materials touch, the form a non-conducting "depletion" region, where all of the conduction electrons in the "n-type" material near the junction have wandered into the "p-type" material to fill the empty valence levels there. This p-n junction or diode can only carry current in one direction. If you add electrons to the "n-type" side of the junction, they will push into the depletion region and can cross over into the "p-type" side. Thus electrons can flow from the "n-type" side to the "p-type" side; current can flow from the "p-type" side to the "n-type" side. But if you add electrons to the "p-type" side, they fill in empty valence levels in that "p-type" material and make the depletion region even larger. The diode cannot conduct current from the "n-type" side to the "p-type" side. Thus the diode is a one-way device for current.

265. How do photocells work?
A photocell is just a diode that is specialized to turn light into separated electrical charge. When light hits the "n-type" side of this diode, it adds energy to the valence level electrons there and moves them to the empty conduction levels. These electrons may even have enough energy to leap across the p-n junction into the "p-type" material. Once they get there, they cannot return because of the depletion region and the one-way effect of the diode. Instead, they are collected by wires attached to the "p-type" material, flow out through some electrical circuit, and return to the "n-type" material through another set of wires.

266. I have an old car that has a generator instead of an alternator, so I assume it runs DC. What about newer cars? They still use a DC battery right? So what about the alternator? Doesn't that produce AC current? How does that work in a DC circuit?
Generators can produce either DC or AC power, depending on how they're arranged. A car generator was one that produced DC power. An alternator produces AC power. Since all cars operate on DC power (they use a battery, after all), the AC power is always converted to DC power. In modern cars, this is done with electronic devices, similar to those used in electronic equipment such as stereos and televisions. Converting DC to AC or vice versa is no big deal anymore. In the old days, it was harder and they used DC generators.

267. If you connect two direct current motors so that the current flowing through one also flows through the other, then turning one motor will cause the other motor to turn as well. If you reverse the direction of rotation, the other motor will also reverse its direction of rotation. Why does this happen?
DC motors turn in a direction that depends on the direction of that current. If you reverse the direction of current flowing through the motor, its direction reverses, too. When you use one DC motors as a generator, it produces DC current! The direction of that current depends on which way you turn the motor. Thus as you turn the first motor clockwise, it generates current in a particular direction through the circuit connecting the two motors and the second motor also turns clockwise. If you then reverse the first motor, the current in the circuit reverses and so does the second motor.

268. What happens to the current when it "stops"?
Current refers to moving charged particles. In most solids, the particles that do the moving are negatively charged electrons that move in the opposite direction from the way we say that current is flowing. These charged particles are the components of atoms and molecules, so they are always there inside a wire or the filament of a light bulb, even if they are not moving. Thus when the current "stops", these electrically charged particles simply stop moving. You can imagine a pipe full of water. The water can be flowing to the right or left (a current) or it can be standing still (no current). The water itself, like the charged particles, doesn't disappear when the flow stops.

269. When flashbulbs were used with cameras, was there a coil in the camera and a magnet, or how did they get it to light? Also, how are flashes used on cameras today different than flashbulbs?
Flashbulbs contain a wad of very fine magnesium wire that burns almost instantly in a gas of pure oxygen. The wire is ignited by a small piece of gunpowder-like primer material that is itself ignited by the camera. There are/were three techniques for igniting the primer: impact (a little lever smacked the side of a tube containing the primer and it burst into flame, just like a cap), electric current (a thin filament inside the bulb overheated when current ran through it), and spark (a spark jumped between two wires and ignited the primer). A camera that uses/used the current-ignited bulbs has a battery in it and taking a picture closes a circuit that then sends current through the bulb. A camera that uses/used the spark-ignited bulbs used a piezoelectric spark igniter, like the ones in outdoor gas grills. A camera that uses/used the impact-ignited bulbs just hit the primer itself. Modern cameras uses gas discharges to produce light. Since the flashlamp isn't burned up during a flash, it can be used many times.

270. Why does a moving magnet excite charges?
A moving magnet, which carries with it a magnetic field, creates an electric field. That's just the way our universe works. Changing magnetic fields create electric fields. Since an electric field exerts a force on any electrically charged particle, the charges in a wire are pushed around whenever a magnet moves past them.

271. Does the monorail at Disneyland and the metro in D.C. run on the idea of direct current motors? Since they reverse directions? Is it like plugging the train in backwards?
Those trains probably run on AC motors, because then they can use transformers to transfer power between circuits. Most likely, these trains use induction motors. To reverse the direction of the train, the engineer reverses the direction in which magnetic poles in the motors' stators circle the motors' rotors. When the poles reverse directions, the rotor has to reverse its direction, too, so that it chases those poles around in a circle.

272. How does a magnet manufacturer make a permanent magnet—how does it vary in strength?
The manufacturer assembles the magnet from hard magnetic materials. These materials are intrinsically magnetic (ferromagnetic) so that they have tiny magnetic domains inside. They are hard, meaning that these domains have great difficulty changing their magnetic orientations. As the final processing step, the finished magnets are exposed to an extremely strong magnetic field; so strong that it flips all of the domains into the desired direction. The domains become trapped in this new orientation and the magnet becomes permanently magnetized. Unless it is exposed to other very strong fields or excessive heat or shock, it will remain permanently magnetized indefinitely.

273. How does magnetism play a part in tapes to create sound?
The tape recorder first represents sound (pressure fluctuations in the air) as electric current and it then represents that current as magnetization of a tape. It magnetizes the tape to various depths to represent the different amounts of current and it uses the direction of the magnetization to represent which way the current should flow. During playback, the tape recorder measures just how deeply and in what direction the tape has been magnetized and uses that information to recreate the current and the sound.

274. I want to know what dB, BIAS, HX-Pro, Dolby A-B-C-S Noise Reduction, and 20-bit LAMBDA Super-Linear converter (from DENON) mean.
The term "dB" is a measure of power. It appears in many contexts, including sound power and audio signal power. Like the Richter scale used to measure the energy released by an earthquake, the dB scale is an exponential one—a sound that is 10 dB louder than another sound has 10 times as much sound power in it.

The term "BIAS" refers to a technique used to assist magnetic recording of weak audio signals. The magnetic particles on a tape's surface do not magnetize easily and need help when quiet sounds are being recorded. To provide that assistance, the tape recorder superimposes a strong, high-frequency "bias" signal on top of the weak audio signal. This inaudible bias signal allows the weak audio signal to influence the tape's magnetization. The characteristics of the bias signal must be adjusted to match the tape type.

The term "HX-Pro" probably refers to a variable bias techniques that prevents the bias signal from saturating the tape's magnetization and wasting some of the tape's dynamic range. (I'm less certain of this observation—please tell me if I am wrong and I'll fix this remark).

The term "Dolby Noise Reduction" refers to a collection of techniques for reducing high frequency noise on magnetic tapes. The higher the sound frequency, the smaller the patches of tape surface that are used to record each cycle of the sound. Since the recording occurs by magnetizing individual particles that are almost a micron long, the cycles of a high frequency sound do not use very many of the particles. A few miss-magnetized particles in each cycle can produce noticeable noise in the reproduced sound. To counter this noise, Dolby boost the volume of high frequency sounds during recording and then reduces their volume back to normal during playback. The noise caused by the particles is also reduced in volume and is less noticeable as a result. The different Dolby techniques refer to different filtering protocols, with C being an improvement over B, which was itself an improvement over A.

The term "20-bit LAMBDA Super-Linear converter" probably refers to a high performance Digital-to-Analog Converter (DAC). When a compact disc is played back, the audio signal must be converted from a stream of numbers into a smoothly varying electric current, which is then amplified and sent to a speaker. Turning each number into a current requires a DAC. The more carefully this DAC is built, the more perfectly the current passing through the speaker will represent the numbers on the disc and the recorded sound information. While most DACs work with only 16 bits, the one you mention provides 4 more bits of precision. However, the compact disc contains only 16 bits of sound information, so the 4 added bits must be created by some numerical analysis on the part of the compact disc player. This sort of signal processing may lead to reduction in noise during playback, but I wouldn't expect most people to be able to hear any difference.

275. Is magnetic flux another name for magnetic field?
The two quantities are related but they're not the same. If you think of a large magnet as made up of many tiny magnets all turned in the same direction, you can think of magnetic flux as strings that connect each tiny north pole to each tiny south pole. The large magnet effectively has many of the strings extending outward from its north pole and wrapping around to its south pole. The magnetic field at each location in space around the magnet is related to how many of these strings of magnetic flux pass through a small surface at that location. Near the poles of the magnet, the density of magnetic flux lines is high and so is the magnetic field. Far from the magnet, the density of magnetic flux lines is low and the magnetic field is weak.

276. Magnets can be demagnetized by heat. Is that true for permanent magnets or materials that have been magnetized?
It's true for both because permanent magnets are just a special material that has been magnetized. In fact, permanent magnets are often demagnetized more easily than other simpler materials. Anything that spoils the internal order of a material (heat or vibration) can demagnetize it.

277. When magnetic tape is put on top of another piece of magnetic tape, the tape on the bottom is demagnetized — its memory is erased. How are we then able to rewind and forward tape, scrolling the tape together, on a cassette tape with out damaging the magnetism?
The process of winding tape up on reels does damage its magnetism slightly. The adjacent layers of tape do interact with one another and they do cause the sound on one layer to appear on the adjacent layers. Fortunately, the effect is very subtle and takes a long time to appear so that the tape must remain tightly wound up for ages before you can hear the damage. Tapes don't age perfectly anyway because thermal energy slowly erases the magnetization, particularly in a hot environment.

278. Does an (audio) amplifier benefit from using matched pairs of power transistors?
A decade or two ago, it was important to match the power transistors used to control currents leaving an audio amplifier. If the transistor that controlled current flowing one direction through the speaker was significantly different from the transistor that controlled current flowing in the opposite direction, then the sound reproduction would be poor. That's because the current flows would be asymmetric and asymmetric currents lead to distorted sounds from the speaker. The most common measure of this sort of error is called "total harmonic distortion," an indication of how much power the amplifier puts into unwanted high frequency currents. Without carefully matched power transistors, an amplifier might put several percent of its power into these harmonic frequencies.

However, modern audio amplifiers generally use feedback techniques to correct for their own internal imperfections. They can compensate so well for mismatches in their components that total harmonic distortion has virtually disappeared from amplifiers. Amplifiers are still rated according to total harmonic distortion, but now it is rarely more than a few thousandths of a percent and depends more on the feedback techniques used than on the perfection of the power switching components. In short, the power transistors in modern amplifiers don't have to be matched well any more.

279. How does the coil in a microphone turn sound into electric current?
The coil in a microphone is attached to a movable surface that is pushed back and forth by the sound. Near the coil is a magnet so that, as the coil moves, the magnet induces electrical currents in it. Whenever a magnet moves past a coil of wire or a coil of wire moves past a magnet, a current is induced in that coil of wire.

280. How is sound made by a speaker?
As the current passing through the speaker's coil changes, the speaker cone moves back and forth toward or away from the speaker's permanent magnet. This moving cone pushes or pulls on the air, creating compressions and rarefactions that propagate through the air as sound.

281. Is the transistor in an audio amplifier used even during silent moments?
During the silent passages of the music, the amplifier does not vary the amount of current passing through the speaker so that the speaker doesn't move and doesn't produce sound. To conserve energy and to avoid heating up the speaker, a good amplifier doesn't send any current through the speaker during a quiet passage. Whether or not the amplifier actually consumes power during the quiet passage depends on the exact design of the amplifier. Some stereo experts claim that they can hear the differences between amplifiers the do or do not consume power with their output transistors during the quiet times and claim that the power wasting amplifiers sound better.

282. How do notebook computer monitors work?
These displays use liquid crystals, liquids that contain long chain or disk-shaped molecules. These molecules can be aligned by external electric fields or by their own interactions with one another to form very orderly arrays; hence the name "liquid crystals". The extent to which these molecules are oriented determines their optical properties. A notebook computer uses electric fields to orient or disorient the liquid crystals and control their optical properties. With some help from other optical devices, the notebook computer can make these liquid crystals block or unblock light to appear dark or light. Adding color filters allows them to produce colored images on their screens.

283. Is it possible to have memory in a computer monitor?
Yes. In fact, many modern monitors do have memory in them. However, this memory isn't used for the same information that's handled in the computer itself. Instead, the monitor's memory is used to control the monitor's behavior. Many sophisticated monitors are equipped with digital controllers that are almost full-fledged computers themselves. These controllers can adjust the size and position of the screen image and the manner in which that image is built. This work by the controller allows the monitor to respond properly when the computer changes the screen resolution or the refresh rate (the frequency with which the image you see is rebuilt). The controller requires memory to operate and it also needs to store data that it can expect to recover next time you turn the monitor on. On a sophisticated monitor, you adjust the image size by pushing buttons under the screen and the monitor uses special memory to record your button presses. When the monitor is turned on, it recalls its record of your adjustments and uses them to return the image size to what it was last time the monitor was on.

284. How can an antenna be short and still work as well as a long one?
The length of an antenna is very important. If the antenna is too short, the charges will reach its end too soon and the charge will not flow very smoothly back and forth in it. If the antenna is too long, the charges will not reach its end before it is time for them to reverse directions and some of the antenna will not be used (it will actually cause more trouble than help). Thus there is an ideal length for the antenna and this length depends on the frequency of the radio wave it is trying to create. But it is also possible to shorten an antenna by delaying the flow of charge to its ends. Adding a coil to the antenna (an inductor) will slow the flow of current through the antenna and make a short antenna behave like a longer antenna. Most portable AM radios use a coiled antenna that behaves as though it were much longer than its physical length. FM radios work best with antennas that are about 1 meter long.

285. How does the distance between the transmitting antenna and the receiving antenna affect the amount of current flowing between the two systems?
Actually, there is no current flowing between the two systems. Current flowing up and down the transmitting antenna causes current to flow up and down the receiving antenna, but there is no direct connection between the two and they do not share any current. That explains how an isolated radio can still receive music. But the amount of current flowing in the receiving antenna does depend on its distance from the transmitting antenna. When the two are very close, the charge in the receiving antenna responds directly to the charge moving on the transmitting antenna. As they move apart, this direct response quickly dwindles to virtually nothing. In its place, a new effect appears. The transmitting antenna creates radio waves that exist apart from the accelerating charges that created them. The strength of the radio wave diminishes in power roughly as the square of the distance from the transmitting antenna. The electric and magnetic fields diminish in power roughly in proportion to this distance. The current flowing in the receiving antenna also falls roughly in proportion to this distance.

286. How does turning the dial on your radio allow your radio to distinguish between stations? How does the receiver only recognize one frequency at a time?
When you turn the dial on your radio, you are adjusting the resonant frequency of its tank circuit (or some electronic equivalent). The tank circuit only responds to charge sloshing on the antenna when that charge is moving back and forth at the tank circuit's resonant frequency. When you tune the tank so that its resonant frequency is the same as the broadcast frequency of your favorite radio station, it only responds to charge moving up and down at that frequency. As a result, your radio detects signals from your favorite station but no others.

287. How good are store bought antennas and if they are better than factory issue, which ones are most advantageous?
Ultimately the only things that matter about an antenna are (1) how much charge it moves in response to the correct radio transmission and (2) how little charge it moves in response to the wrong radio transmissions. Most store bought antennas probably just boost the amount of moving charge by attaching an amplifier to an otherwise undistinguished antenna. While that trick will increase the amount of charge moving in response to the correct transmission, it will also increase the amount moving due to undesired transmissions. Almost everything electrical transmits radio waves and these may well interfere with your reception. For example, your neighbor's lawn mower may send out radio waves and introduce noise into your music. Just amplifying the antenna signal does nothing to eliminate that problem. Your best bet is to find a directional antenna; an antenna that responds most strongly to radio waves coming from a particular direction. TV antennas are typically directional, with many separate antenna elements. Satellite dishes are highly directional.

288. How is charge distributed to a tank circuit with the "correct" frequency?
The transmitting station has an electrical oscillator, an electronic system that experiences periodic reversals of current. This oscillator contains a tank circuit or some other clock-like system that acts as a timekeeper. With the help of its timekeeper, the oscillator causes the transmitting station to send current to the main antenna tank circuit at just the right moments to sustain and enhance the sloshing current there. The oscillator and the current sloshing in the tank circuit remain in perfect synchrony with one another. One of the best clock-like systems is a quartz crystal oscillator, like that in a typical wristwatch. In a quartz oscillator, a quartz crystal vibrates like the bar of a xylophone. In a watch, these vibrations are used to control a digital clock system so that it keeps accurate time. In a transmitter, these vibrations are used to control the distribution of current to the tank circuit at the antenna.

289. How is the charge moving in the waves related to what is actually played on the radio?
First, there isn't any charge moving in the waves themselves. The waves contain only electric and magnetic fields. These fields will push on any electric charges or magnetic poles they encounter, but they are not themselves electrically charges or magnetically poled. The amount of fields in a radio used for audio transmission depend on the station's transmitting power and on the encoding format for the music. In AM (Amplitude Modulation) encoding, the music is encoded as the strength of the radio waves. Each time the radio wave's strength goes up and down once, the speaker cone in your receiver goes forward and backward once. In FM (Frequency Modulation) encoding, the radio wave's strength remains steady but its precise frequency changes slightly. Each time the radio wave's frequency goes up and down once, the speaker cone in your receiver goes forward and backward once.

290. If electric and magnetic field are forever recreating one another - in radio waves - how do you change the sounds they produce?
Within each portion of the wave, the local electric and magnetic fields endlessly recreate one another. But this portion of the wave heads outward from the transmitting antenna at the speed of light and is soon far away from the earth. As the transmitter changes the amount of charge on the antenna or its frequency of motion up and down, it creates new portions of the wave that may differ from the portions sent out a minute ago, a second ago, or even a few millionths of a second ago. Thus the transmitter's changes very quickly pass outward to all of the receivers nearby. The farther you are from the transmitter, the longer it takes for the various patterns in the wave to reach you and your receiver. All of the music transmitted by radio stations in the 50's is still traveling outward because the patterns emitted back then continue to travel. They are now 40 or 50 light years away from the earth and are so widely dispersed across space that it would take a phenomenally sensitive receiver to detect them. But they are out there nonetheless. Many of the searches for extraterrestrial intelligence have focused on trying to detect this sort of radio transmission across the depths of space. If other peoples have invented radio, they are quite likely to have chosen AM or FM modulation as their encoding schemes, too.

291. Occasionally my receiver will pick up two stations at the same time, fading in and out and fighting to be heard. How is this possible?
In AM radio, the sound is encoded as the strength of the radio wave. If two transmitters are using the same frequency (or your receiver cannot distinguish between them due to its limited resolution), then it will responds to both of them at once. The sound that you hear will be the sum of them both, as though they were two musical instruments in the same room. In FM radio, the sound is encoded as the exact frequency of the radio wave. In this case, your receiver is likely to follow the strongest of the two stations and flip in between occasionally when their strengths change (due to weather or reflections from moving objects). Thus it is common for AM radio receivers to superpose two stations but not so common for FM radio receivers to do the same trick.

292. What is the difference between an electric and a magnetic field?
An electric field exerts forces on electric charges while a magnetic field exerts forces on magnetic poles. If you place a positive electric charge in an upward-pointing electric field, that electric charge will accelerate upward (in the direction of the electric field). But if you place a stationary north magnetic pole (if you could find one) in that same electric field, nothing will happen. An electric field exerts no force on a stationary magnetic pole. On the other hand, if you place a north magnetic pole in an upward-pointing magnetic field, that pole will accelerate upward (in the direction of the magnetic field). But if you place a stationary positive electric charge in that same magnetic field, nothing will happen. So electric fields act on stationary electric charges and magnetic fields act on stationary magnetic poles.

293. When a station transmits a signal, do all receiving antennae have the same reciprocal charge?
Yes. The transmitting antenna pushes huge amounts of charge up and down so that all of the receiving antennae respond primarily to it rather than to one another. However when many receiving antennae are very near one another, they can begin to cause trouble. In effect, each antenna draws a small amount of energy out of the radio wave. If there are too many nearby antennas, they will sap the radio wave's energy and each receiving antenna will get less than its normal amount. The other way to look at this effect is to realize that the receiving antennas actually retransmit the radio wave that they receive, but upside down. They weaken the wave as a result. If there are too many antennas around, they will reduce the wave to almost nothing.

294. Where does the charge on the antenna come from?
In the transmitting station, the moving charge is pumped back and forth between the ground and the antenna. The net charge in the vicinity of the station remains zero, but it is constantly being redistributed. Sometimes the antenna is positively charged and the ground is negatively charged and sometimes it's the reverse. In the receiving station, the same may be true. But there are also hand-held receivers that do not touch the ground. In that case, the receiver is still neutral, but charge is being pushed back and forth along the antenna and tank so that when the antenna is positively charged, the bottom of the tank circuit itself is negatively charged.

295. Why do radio waves travel better at night?
AM radio waves travel remarkably long distances near dusk because of the behavior of the earth's atmosphere. A layer in the upper atmosphere, the ionosphere, contains many electrically charged particles and it behaves like a poor electrical conductor. Its conductivity improves in the early evening. When low frequency radio waves encounter this conducting layer, it responds to them and reflects them just like a mirror reflects light. As a result, you can hear very distant radio stations as their waves bounce of the ionosphere. FM transmissions occur at high frequencies that are too fast for the ionosphere to reflect.

296. How can computer monitors and televisions have images burnt into them over time?
As the electron beam collides with the phosphor coating on the inside of the picture tube, it slowly damages that phosphor coating. Eventually the phosphors are burnt away and the inside surface of the picture tube stops being uniform. To avoid burning specific regions more than others, computers use screen savers that darken the images by turning down the electron beam and keep those images moving about randomly.

297. How can the magnets be manipulated in such a way that they can do this moving of the electron beam in such an incredibly small amount of time?
The electromagnets that control the beam are able to turn on and off very quickly. The only limit on the rate at which they can change the magnetic field comes from their inductance. They do resist changes in current passing through them. Fortunately, the television doesn't move the beam about randomly; it sweeps the beam smoothly. Thus the changes in the current through the electromagnetic coils are also smooth. The television has no trouble ramping the field through the horizontal sweep coils back and forth every 1/15,750th of a second.

298. How do high definition televisions differ from traditional ones?
High definition televisions have more individual spots of color and brightness than the traditional sets. They may also have a somewhat different aspect ratio (horizontal width vs. vertical height). Creating high definition picture tubes is not particularly difficult since they are now rather common on computers. However, transmitting the increased information needed to paint the picture on a high definition television is a serious problem. One approach is data compression, in which redundant information is eliminated from the signal so that only new information is sent to the television. To avoid making all of the present televisions obsolete, the new high definition television standards are supposed to be downward compatible with those televisions. Unfortunately, trying to serve both types of televisions with the same transmitted signal is going to be a difficult task.

299. How do projection or rear projection televisions work?
Inside the projection TV, there are three separate picture tubes that work very much like normal black and white picture tubes. One of these tubes creates an image of the red light in the television image, one creates an image of the green light, and the third creates an image of the blue light. In front of each tube, there is a color filter: red for the red tube, green for the green tube, and blue for the blue tube. There is also a projector lens that takes the light leaving the tube and filter and projects a clear image of that light on the screen in front of the projector. The light striking the screen looks exactly like the light leaving the surface of the picture tube. The three images (red, green, and blue) are carefully overlapped so that they mix and you perceive all colors.

300. How do the electrons know which spots on the screen to color darker and which to leave lighter?
The electrons simply deliver energy (their own kinetic energy) to the phosphors they hit. When they are hit by electrons, these phosphors emit light. They fluoresce. The picture determines which spots on the screen should be dark and which ones should be light by controlling the number of electrons that hit those spots; by controlling the current in the beam. When the current hitting a spot is low, that spot glows dimly. When the current hitting a spot is high, that spot glows brightly.

301. How do the magnets that redirect the electron beam in the picture tube move it to the exact point that it's supposed to?
The electromagnets that steer the electron beam are very carefully designed and constructed so that they steer the beam very accurately. They are coils of wire that are built on a form and then glued together so that they cannot move. There are some adjustments made electronically inside the television set to make sure that the beam follows a very start path as it sweeps across the screen. When you adjust the horizontal and vertical sizes of the picture, you are adjusting the currents flowing through these electromagnets.

302. How does a magnet change the picture on a television—does this hurt the TV?
When you hold a magnet up to the front of a television, you are introducing an additional magnetic field in the system. This field exerts forces on the moving electrons inside the tube and they are deflected. The picture is distorted. With a black and white television, no harm is done because there is nothing to magnetize inside the picture tube. But color television picture tubes contain metal shadow masks that can become permanently magnetic. The picture remains distorted, even after you remove the magnet. To clear up the "damage", you would have to demagnetize the picture tube. Although this is not a particularly difficult task, it requires a demagnetizing coil and is best done by a professional repairperson. The bottom line is, don't play with magnets near a color television set.

303. How does the horizontal sync signal work?
The brightness information comes to the television as a steady stream. While the television knows that this information should control the brightness of adjacent spots on the screen, from left to right, it needs to be told when each horizontal line begins and when each vertical sweep begins. It knows that a new line is coming when the brightness information contains a "blacker-than-black" level. This level seems to say that the electron gun should not only stop sending electrons at the screen, it should send less than no electrons at the screen! Actually, this level is an instruction to the television's electronics, telling the television to bring its electron beam back to the left side of the screen to begin a new horizontal line. A long "blacker-than-black" level is an instruction to the television to begin a new vertical scan down the screen.

304. How does the picture get to the TV itself? How does a radio wave make a picture?
The television can reconstruct an image from a series of brightness measurements. It takes these brightness measurement and uses them to control the electron beam as it sweeps across the screen of the picture tube. It paints the picture one dot at a time and then starts over when it has finished. Thus all that the radio wave has to send to the television is a series of brightness measurements and some synchronization information (when to start a horizontal scan and when to start a vertical scan). It uses an AM technique to send the brightness measurements on a radio wave. The transmitter's power varies up and down to indicate brightness just as an AM radio transmitter's power varies up and down to indicate which way to push the speaker cone.

305. How does the picture tube know where to push the electrons onto the right areas/dots?
The television and picture tube simply scans the electron beam across the screen, one horizontal row after the next as it moves "slowly" down the screen. When it gets to the bottom of the screen, the picture tube brings the beam back to the top of the screen and starts over again. While the TV is scanning the beam across the set, it uses the signal from the television station to control the intensity of the electron beam and those the brightness of the spots on the screen. It also watches for sync information to know when to begin new horizontal lines and vertical sweeps.

306. How does the television camera record the picture?
Like the television picture tube, the camera generates a signal that indicates the brightnesses of individual spots one at a time. It first measures the brightness of light reaching it from the upper left hand spot, then the spot to its immediate right and so on horizontally across the field of view. It then moves down to a low horizontal line and repeats this sweep. It eventually records the light levels from the entire scene in front of it and begins again. It detects this light using an optical system that forms an image of the scene on a light sensitive surface. This surface may be part of an imaging vacuum tube (sort of a reverse picture tube), or it may be a semiconductor device that resembles a vast array of tiny photocells.

307. If black is a high current from the television's radio receiver and white is a low current, why do you get a bright spot when you increase the flow of electrons at that instant. Isn't white a bright spot?
Yes, white is created by a strong flow of electrons. There are two separate circuits here. The current from the receiver section of the television isn't what is sent through the electron gun. Instead, that current controls the electron gun. When a large current arrives at the electron gun (actually the grid) from the receiver, the flow of electrons toward the screen is pinched off and a dark spot is created. When a small current arrives from the receiver, the electron beam remains intense and a bright spot is created.

308. If you stand between the two satellites, would you have light on you?
When two satellites beam their radio waves at you, you are exposed to both of those waves. A normal antenna would not be able to distinguish between them and it would be hard to receive the transmissions of one and not the other. But with a satellite dish, you can easily select the transmissions of one and exclude those of the other. The satellite dish is directional, meaning that it focuses and collects radio waves from a particular direction while ignoring those from other directions. With a satellite dish aimed at a particular satellite, you can receive only transmissions from that satellite.

309. What is one doing when changing the brightness, contrast, and color adjustments on a television?
The brightness control determines the maximum strength of the electron beam and thus the peak brightness of the phosphors on the screen. The contrast control determines the extent to which the electron beam current changes between bright regions and dim regions on the screen. If the contrast is high, then even a less-than-white spot in the image may produce full beam current and full brightness in the phosphors and a more-than-black spot in the image may be cast as full black (no beam at all). If the contrast is low, then almost the entire screen will be illuminated by a medium electron beam and the image have no full black or full white. The color adjustments control the relative intensities of the red, green, and blue guns. Because of the way color is encoded in the television signal, the traditional controls are hue and tint, which involve mixtures of red, green, and blue. All these controls involve adjustments to the voltages and currents in the electron guns (cathodes), grids, and anodes of the picture tube.

310. Are microwaves distributed unevenly in the oven? Why do manufacturers claim that microwaves with turntables are more effective than microwaves without turntables?
As the microwaves bounce around the inside of the cooking chamber, they tend to interfere with one another. There are usually regions in which the waves that follow various paths almost cancel one another and regions in which the waves reinforce one another. These regions don't cook food equally well. If the microwaves are canceled in one region, cooking will be slow there. If the microwaves reinforce one another in another region, cooking will be fast there. If you simply leave food in one place and try to cook it in the microwaves, the cooking will be uneven. However, if the food is rotated continuously, these good and bad cooking regions will be blurred away so that the food will all cook at about the same speed.

311. Are microwaves harmful to you? Is eating microwaved food harmful?
Microwaves can heat your body by adding thermal energy to the water molecules in you. This heating can be damaging if it's not controlled. Most of your body is protected from slow heating because your blood carries heat away from any local hot spots so that you warm evenly. However there are a few places that aren't cooled by your circulation and can heat up locally enough to denature the protein molecules and cause biological injury. The cornea of your eye is a good example. It can be heated and damaged because it's not cooled well. That's why you must be careful not to look into a strong beam of microwaves. As for microwaved food, the only effect of cooking with microwaves is hot food. There is no "radiation damage" or "radioactivity," as there might be with x-ray or gamma radiation. Some foods should not be cooked in a microwave only because the uneven heating may allow certain parts to become too hot. Those parts may burn you when you eat them or they may suffer thermal damage that diminishes their nutritional value.

312. Can microwaves be emitted to travel in one direction?
Yes. Like all electromagnetic waves, microwaves can be focused and concentrated in a particular direction. That is exactly what microwave dish antennas (e.g., satellite dishes) do. At the transmitter, they focus the microwaves emitted by a smaller antenna so that those microwaves travel as a parallel beam. At the receiver, they focus the parallel beam of microwaves onto a smaller antenna. You can think of the microwaves as very long wavelength light waves, so that anything you can do with light (e.g., focus it, form images with it, or bend it with optical devices), you can also do with microwaves. The only problem is that the optical elements you use for microwaves must be larger, because the microwaves have longer wavelengths.

313. How can microwaves heat something? Radio waves don't warm things very much.
The electric field of a microwave flips back and forth at just about the right frequency to have the largest effect on water molecules. The water molecules try to follow the reversing electric field and, in doing so, become hotter and hotter. Radio waves flip too slowly to have very much effect on water. Furthermore, the microwaves in an oven are far more intense than the radio waves that we're used to have around us so that common radio waves just don't do very much cooking.

314. How do metal rods short out the microwaves?
If you arrange a metal rod so that it's parallel to a microwave's electric field, the microwave will push electric charges up and down that rod. This moving charge will waste some of the microwave's energy by creating heat in the rod. But the main effect will be that the rod will reflect or scatter the microwave. The moving charge will emit its own microwave and this new microwave will interfere with the original one.

315. How does a microwave oven defrost foods? Doesn't it only work with water, not ice?
In any frozen food, there are some water molecules that are relatively free to turn about. These molecules may be at the surfaces of ice crystals or sitting on the surface of food particles. These water molecules can absorb microwaves and heat. However, the heating is very uneven because as soon as any water crystal absorbs enough heat to melt, the resulting liquid water will begin to absorb microwaves much more strongly. That is why defrosting must be done slowly. Then the microwave deposited heat will have time to flow through the food and melt it uniformly. Otherwise, you can end up with boiling hot spots mixed together with frozen icy spots.

316. How does the resonant cavity in the magnetron work?
When it's active, the magnetron's cavity has electric charge sloshing back and forth along its tines. The charge moves at a frequency determined by the shape and size of the cavity and these are carefully controlled so that the cavity's natural resonance frequency is 2.45 gigahertz. To keep the charge sloshing, the magnetron adds negative charge from a hot filament wire located in the center of the cavity. Electrons flowing off of this wire are steered toward the negative tines by a magnetic field. As a result, the charges continue to slosh back and forth indefinitely. A small wire connected inside the magnetron extracts some of the energy in the magnetron and converts it into microwaves outside the magnetron. This wire acts as an antenna. The antenna is located in the pipe that carries the microwaves to the cooking chamber.

317. I have a friend who refuses to stand in front of the microwave oven in his kitchen, because he feels the "nuclear waves" leak and will cause his sperm to deform (and he doesn't want ugly kids). Is this true? What about car phones? He heard they were bad, too!
Both microwave ovens and car phones emit electromagnetic radiation. But that radiation has relatively long wavelengths (about 12 cm in the case of microwave ovens and about 40 cm in the case of car phones) and is not at all like the electromagnetic waves emitted by nuclear processes. Nuclear electromagnetic radiation, usually called gamma rays, has extremely short wavelengths (less than 0.001 nanometer or about a millionth of the wavelength of visible light). All electromagnetic waves are emitted and absorbed as particles called photons. The energy in a photon is inversely proportional to its wavelength (in vacuum). Gamma rays, with their short wavelengths, have very energetic photons that can do lots of chemical damage to your tissue. But the longer wavelength radiation from microwave ovens and car phones comes as very low energy photons. These photons can't do chemical damage. The only thing those waves can do is heat things. Microwave ovens are carefully shielded so that they keep most of the microwaves inside. If those waves did emerge, they would simply warm your tissue up. This warming won't cause genetic damage but it could cook your tissue. There has been recent concern about low frequency electromagnetic fields causing subtle damage to tissue, but these have not be substantiated by scientific research and no physically reasonable scenarios for how such damage could occur have been offered.

318. I'd heard that if I cook in the microwave oven, there will be a possible formation of free radicals. Is it true? If yes, how? — Angela I.
It's doubtful that microwave cooking forms free radicals in food. The microwaves in a microwave oven cook by exerting torques on the water molecules and gradually increasing the water molecules' thermal energies through friction-like effects. There is never enough energy present in a single molecule at one time to shatter that molecule and form a free radical. While ultraviolet light, such as that found in sunlight, carries enough energy per photon (particle of light) to split a molecule and form a free radical, microwave radiation carries very little energy per photon. That's why microwave photons can't do chemical damage the way ultraviolet photons can. However, even if microwave radiation could form free radicals in food, that wouldn't necessarily cause you trouble when you eat that food. So much happens to the food before it enters your blood stream that a free radical probably won't survive. The more harmful free radicals are ones that are actually created inside your body, where they can immediately attack important molecules in your cells.

319. If a microwave does not melt ice, how does the "Defrost" setting on the microwave work?
I've already noted the issues of warming frozen food. However, the "defrost" setting is an interesting issue. If you've ever watched a microwave trying to defrost food, you've probably noticed that it heats the food briefly and then waits. It repeats this process many times. What it is doing is depositing energy (via the microwaves) into whatever water molecules are able to absorb microwaves. It then waits for this energy to flow as heat into the nearby food. Once the heat has been distributed rather evenly, the oven adds some more energy by turning the magnetron back on. This cycle of heating and waiting allows the food to defrost fairly evenly. Still, microwaves are likely to create hot and cold regions in the food so that some parts of the food will cook rather than defrost while some parts remain frozen.

320. If a radio station operated at 2.45 gigahertz, could you pick it up when your microwave was turned on and attached speakers?
If some radio station were to operate at 2.45 gigahertz, the main effect would be very poor reception of that channel on your radio. The oven isn't a transmitter for microwaves; it just makes them like crazy. Most of the microwaves never leave the cooking chamber and there are strict regulations on any leakage. But it would only take a few thousandths of a watt of leaking microwave power to cause trouble in your reception of the radio station. Your radio wouldn't be able to distinguish that station's transmission from microwaves leaking out of your oven. The radio would struggle to pick up the signal and you would probably hear lots of noise in the background.

321. In microwaves - you heat up food really fast. Is it true that microwaved food will cool down faster than oven heated food? Someone told me "if it heats fast, it will then cool fast."
No. Microwaves cook the food in a very different manner than normal thermal heating, but microwaved food has the same thermal energy that it would have if it had been warmed by more traditional methods. Microwaves heat food by exerting torques on the individual water molecules in the food. These molecules jiggle back and forth and sliding friction between them heats the food. This peculiar route to energy addition explains why frozen portions of the food don't heat well: the water molecules are rigidly oriented and can't jiggle back and forth in order to become hot. But despite the fancy heating scheme, the food retains no memory of how it was heated. Once it is uniformly hot, it cools at a rate that depends only on how heat is transported out of it. Microwaved food cools just as slowly as normally cooked food.

322. Inside the microwave oven, what is it that heats the food? How does the heat come out; where did it come from?
The food is heated by the microwaves themselves and these microwaves are piped into the cooking chamber from the magnetron. The magnetron has electric charge sloshing back and forth in its tines. A small antenna uses that sloshing charge to emit microwave radiation. The water molecules in the food absorb this microwave radiation and turn its energy into heat. The usual rules of heat transfer don't apply in the heating process—the energy arrives at the food as microwaves, not heat.

323. On the subject of defrosting frozen food in a microwave oven, you must refer to the old BTU formula which states "It takes one BTU to raise the temperature of 1 pound of water 1° (Fahrenheit), but when water is changing state from a solid (ice) to a liquid (water), it must absorb 144 BTUs (per pound)." - George R.
This observation accounts for much of difficulty with defrosting food in general and defrosting food in a microwave oven in particular. It often takes more heat to melt ice in the food than it does to actually cook the food once the ice has melted. Since ice doesn't absorb microwaves well, heating frozen foods in a microwave oven is a tricky business. Any region of food that melts early will absorb microwaves strongly and overheat while any region of food that remains frozen won't absorb microwaves well and won't receive the enormous amounts of heat it needs just to melt. The result is typically a food item with some frozen parts and some boiling hot parts. To avoid this problem, microwave oven defrost cycles let the food sit in between bursts of microwave heating. That way, there is time for heat to flow through the food and keep the internal temperatures relatively uniform. Parts of the food that heat well have time to transfer heat to parts that don't heat well and the whole item thaws and heats together.

324. What containers are not safe to use in a microwave? I am particularly concerned about Styrofoam containers as I use them to make TV dinners for my family. Is it OK to heat directly in these containers?
The two critical issues with containers in a microwave are (1) that they do not absorb or reflect microwaves and (2) that they tolerate high temperatures. Concerning the first issue, a container that absorbs microwaves will become extremely hot and may be damaged or destroyed. Most plastics (including Styrofoam) don't absorb microwaves and are fine. Glazed water-free ceramics and glasses are usually also fine, as long as they don't have any metallic trim. Metal dishes are a poor choice because they reflect microwaves and lead to uneven heating. Unglazed ceramics absorb water and will overheat.

Concerning the second issue, many plastics melt or soften below the temperature of boiling water. Polystyrene, the plastic from which Styrofoam is made, has a glass transition temperature of almost exactly 212° Fahrenheit (100° Celsius). That means that it will begin to soften at just about the temperature of boiling water. While pure water will boil without much problem in Styrofoam, water containing dissolved solids such as sugar or salt will boil at a higher temperature and may melt the Styrofoam. You'll know when this's not really a health issue, just a potential for a messy oven. I've only encountered the problem once myself, when a Polystyrene gravy separator melted in the microwave and let the gravy spill.

325. What exactly goes on when you're cooking a potato in the microwave and it explodes?
A microwave oven heats food by depositing energy in its water. If you cook the food long enough, that water can begin to boil. If the food has a hard outer shell (e.g. a potato or a corn kernel), the boiling water can create enough pressure in the food to make it explode. That is what pops the corn in microwave popcorn and why the potato explodes if you don't pierce it so that steam can escape.

326. What happens if you start the microwave oven with nothing inside?
The magnetron creates microwaves that travel into the cooking chamber and should be absorbed there. If there is no food (or rather no water-containing food), those microwaves will not be absorbed and will eventually find their way back to the magnetron. Eventually the magnetron will absorb as many microwaves as it emits. This situation is hard on the magnetron, which works best when it has very little radiation returning to it. That's why you should never run a microwave empty for more than a second or two.

327. Why do microwave ovens cook so rapidly?
When you put solid food (a potato, not soup) into a conventional oven, the heat flows slowly into the center of that food. This heat must work its way into the food via thermal conduction, in which adjacent atoms and molecules transfer their motional energies in a long bucket-brigade process. The last part of a potato to become hot is its center. However, in a microwave oven, the microwaves travel well into the solid food and deposit their energy everywhere. The potato cooks throughout at a relatively even rate. The actual amount of heat and energy involved in conventional and microwave cooking is about the same. However, the microwaves can heat the food throughout without having to wait for the slow process of conduction to carry it inward from the food's surface.

328. Why do some microwave ovens not seem to have a metal surface in the cooking area?
The cooking chamber of a microwave oven is always metallic. Even the glass door has a metal grid across it to keep the microwaves inside. This metal chamber may be coated with paint or plastic but it is there nonetheless. Without it, the microwaves would leak out and the oven would be hazardous and inefficient. It would cook objects throughout the kitchen.

329. You said an ice cube will not get hot in the microwave because the molecules won't "flip". If this is so, then why do frozen foods cook in the microwave?
As noted previously, the water molecules in frozen foods are not all bound up perfectly inside ice crystals. As long as there are a few relatively mobile water molecules, even frozen food will eventually absorb enough energy to melt. Once that happens, the food can cook easily. Of course, the melting process is frequently very non-uniform so that food comes out with hot and cold regions. In general, frozen food cooked in a microwave is not very satisfying.

330. As long as the sun is to our back, shouldn't the rainbow stay visible; instead of disappearing when we approach it?
If the sky were uniformly filled with water droplets and uniformly illuminated with sunlight, then you would always see the rainbow, no matter where you moved. However it would always appear out in the distance. The light that reaches your eyes as the rainbow comes from a broad range of distances, but it appears to come from pretty far away. As you walked toward this perceived rainbow, you would begin to see light from other raindrops, still farther away. You could never actually "reach" the rainbow. It would just move about with you; always appearing to be in the distance.

331. Can you see out of sunglasses which shade both horizontally and vertically polarized light?
No. Such sunglasses would absorb all light and would appear black. Polarizing sunglasses are designed to absorb only horizontally polarized light; the light associated with glare. There is no reason to absorb vertically polarized light.

332. Does a mirage operate under the same principle as the puddles on a road?
Not exactly. A puddle contains water, which reflects light directly. Light from the blue sky travels toward the puddle and illuminates it. As the light enters the water, with its higher refractive index, part of the light reflects. You see this light when you look at the surface of a puddle. But a mirage involves refraction (bending) of light. As light from the blue sky enters a regions of hot air near the surface, that light bends upward. You again see light from the sky, but bent upward by the air rather than being reflected upward by a surface of water. Since the two appear similar, you interpret the shimmering blue light of a mirage as coming from a pool of water. But it is just hot air.

333. Does air pollution contribute to the blueness of the sky (make it bluer)? Has the sky become more blue with the advent of technology (factories, machinery, etc.)?
Yes. Pollution does tend to make the sky bluer and the sunsets redder. However, pollution also imparts colors directly by absorbing certain wavelengths of light. The orange haze that hovers over cities is often caused by nitrogen oxides, which are simply orange in color and act like pigments to make everything appear orangish. However smoke and dust certainly change the look of the sky by increasing scattering. Natural disasters are even more effective: volcanic eruptions create the most beautiful sunsets of all by tossing vast amounts of dust into the air.

334. Does red or blue light bend more in glass?
Blue light almost always bends more than red light because blue light almost always travels more slowly through glass than does red light. This phenomenon is known as dispersion However, there are some glasses that exhibit anomalous dispersion, where red light travels faster and bends more than blue light. Anomalous dispersion only occurs when there is a resonant absorption of light in the glass, typically because of some impurity atoms or ions in the glass or because of some transition that occurs in the glass itself. While the resonance will only absorb light at one particular wavelength, it alters the propagation of light at nearby wavelengths. At wavelengths just shorter than the absorbed wavelength, light travels anomalously fast through the glass so that it bends less than light that is somewhat redder in color.

335. Does the rainbow go all the way to the ground?
Yes, it forms an arc that extends to the ground. However, any hills or valleys may obscure its visibility or its sunlight, so you often see it truncated or in shadow.

336. How come I never find the pot of gold at the base of the rainbow?
The people who invented that tale were well aware of the impossibility of reaching the rainbow itself. Knowing that the rainbow moves with you, they were free to promise anything about what lies at the base of the rainbow.

337. How do oil spills/spots (i.e. in parking lots and streets) create rainbows?
A thin layer of oil on water creates interference effects, just like those seen in a thin soap film. Sunlight reflects from both the top and the bottom of the oil layer and these two reflections can interfere with one another. If the blue/green wavelengths of light interfere destructively on their way to your eye, you will see the oil layer as red. If the green/red wavelengths of light interfere destructively, you will see the oil layer as blue. How you see the oil layer depends on its thickness and the angles of the light.

338. How do polarizing materials work?
The sheet polarizers that are used in sunglasses or in the demonstrations in class contain molecules that absorb electromagnetic waves of only one polarization. These molecules form long chains that interact with electromagnetic waves only when the electric fields push charge along the lengths of the molecules. In the polarizing sheets, the molecules are all oriented along the same direction so that they all absorb light of the same polarization. The other polarization of light passes through the sheets virtually unscathed. When unpolarized (randomly polarized) light enters one of these sheets, any waves that are polarized along the molecules are absorbed while any that are polarized across the molecules are permitted to pass. About half the light makes it through and that half is polarized across the molecules. If this remaining light is sent through a second polarizing sheet, turned 90° so that the molecules of the second sheet are aligned with the polarization of the light leaving the first sheet, then the remaining light will be absorbed in the second sheet and essentially no light will emerge from the pair of sheets. This arrangement, two polarizers turn 90° with respect to one another, is called "crossed polarizers". It is a useful arrangement for observing materials that rotate polarization by distorting the electric and magnetic fields. If a distorting material is placed between the two crossed polarizers, light from the first polarizer may be altered by the material and thus be able to pass through the second polarizer.

339. How do shadows form?
Light from the sun travels in straight lines (apart from some wave effects called diffraction, that are unimportant in this case). As sunlight passes objects, those objects absorb or scatter the sunlight, leaving regions of space that no longer contain any electromagnetic waves. Regions of space behind the objects contain no sunlight and do not appear illuminated. We perceive those dark, unilluminated regions as shadows.

340. How do window tints (for your car windows) work? Are they just polarized materials?
Some of them may be polarized materials, blocking horizontally polarized light, but most are simply absorbing materials that are embedded directly in the glass during its manufacture. Chemically tinted glass just darkens the sky be absorbing some of the light passing through the glass, regardless of polarization. It's not possible to chemically treat the glass to make it absorb only one polarization of light because that treatment would have to carefully align its molecules. In the plastic polarizing sheets, there is an alignment process (usually stretching in one direction) that lines up all the absorbing molecules.

341. How does light cancel in destructive interference?
When two identical waves (usually two halves of the same wave) arrive together out of phase, the electric field in one wave (or half-wave) is up at the same moment that the electric field of the other wave (or half-wave) is down. These two electric fields add together and create a total electric field that is neither up nor down. An electric charge at this location in space will experience no forces so there is no electric field (one wave pushes that charge up while the other wave pushes that charge down). With no electric field around, there is no light to be absorbed. If two waves coming toward you interfere destructively, you will see no light. You might worry about conservation of energy; where did the light and its energy go? It went somewhere else. Any time there is destructive interference at one point in space, there will always be some other point in space at which there is constructive interference. Thus when you look at a soap film and see no red light, you can be sure that the red light has gone somewhere else. In the case of the soap film, when you see no red light in the reflection from the film, that red light has been transmitted by the film and is visible on the opposite side of the film.

342. How does light create heat?
Actually, some light is heat. Heat is the energy that flows from one object to another because of a difference in their temperatures. The sun is hotter than you are so that it sends heat toward you. Sunlight is heat; it is the sun's heat being sent toward you as electromagnetic radiation. When it strikes the surface of your skin, this radiation is absorbed and becomes the more familiar form of heat: kinetic and potential energy in the atoms and molecules. From the surface of your skin, this heat flows inward to warm the rest of your body. Any material that absorbs light usually converts it to heat. The charged particles in that material move under the influence of the light's electric field and these moving charged particles transfer their energy here and there as heat.

343. How does suntan lotion work to prevent ultraviolet rays from damaging your skin?
Suntan lotion (or rather sunscreen) is a chemical whose molecules absorb ultraviolet light and turn its energy into heat. Like fluorescent compounds, these molecules absorb ultraviolet light strongly. But unlike fluorescent compounds, the sunscreen molecules do not reemit any light. They convert all of the ultraviolet light energy into heat, which does no damage to your skin.

344. If white color has a reflection close to one, what role does shininess or dullness play?
Just because two materials both reflect all of the light that strikes them doesn't mean that they look the same. When you send a flashlight beam at a white surface, you can see that reflected light from all directions. When you send the flashlight beam at a mirror surface, you can only see the reflected light from one particular angle. Both the white surface and the mirror surface reflect virtually all of the light that hits them. A shiny white surface is different from a dull white surface because a shiny white surface has a small amount of mirror character to it: you can see the whiteness from any direction but there is also a mirror aspect that you can only see from certain angles.

345. Is refraction the idea behind eyeglasses? If so, how?
Yes, refraction is used in eye glasses. By carefully sculpting the front and back surfaces of a sheet of glass or plastic, the light passing through that sheet can be bent in remarkable ways. We will look at image formation in the section on Cameras.

346. What causes a magnifying glass within a ray of sun to burn such a small, specific spot? Is it the shape of the glass?
The magnifying glass is a lens, a carefully shaped piece of glass that can refract sunlight to create an image. When you burn wood with a magnifying glass, you are creating an image of the sun on the wood. This tiny image, a circle that looks just like the sun itself, only much smaller, is so bright and contains so much thermal radiation that it overheats the wood that it strikes and causes that wood to burn.

347. What causes the colors in the aurora borealis?
These colors come from the atomic fluorescence of particles high above the earth's surface. As charged particles from the sun's "solar wind" spiral through the earth's magnetic field toward its poles, they collide with one another and with atoms in the earth's upper atmosphere. The energy of such collisions can excite the atoms involved and cause them to emit light.

348. What color is the sun as viewed from outside our atmosphere?
The sun appears bluer when viewed from outside our atmosphere. The earth's atmosphere scatters a substantial fraction of the violet and ultraviolet light in sunlight, leaving a reddened sun disk to our view. Without that scattering, the sun's disk will appear to contain more blue and ultraviolet light.

349. What is black light and how does it work?
Black light is ultraviolet light. You cannot see it so a room illuminated only by ultraviolet light appears dark or "black". However any fluorescent materials in the room (e.g. brighteners in your clothes) will absorb the ultraviolet light and reemit it as visible light. That is why things with fluorescent pigments on them glow when illuminate by black light.

350. What is Brewster's angle?
When light reflects from a horizontal surface at an angle, the reflected light tends to be polarized horizontally. At a specific angle, Brewster's angle, the light is completely horizontally polarized because any vertically polarized light that hits the surface at this angle is allowed to enter the surface without reflection. Since reflections from horizontal surfaces are mostly horizontally polarized, glare is mostly horizontally polarized. Polarizing sunglasses deliberately block horizontally polarized light to reduce glare.

351. What makes the clouds white - or having colors at sunset and why is the sky gray on a cloudy day?
The water droplets in clouds are quite large; large enough to be good antennas for all colors of light. As light passes by those droplets, some of it scatters (is absorbed by the antenna/water droplets and is reemitted by the antenna/water droplets). Since there is no color preference in this scattering from large droplets, the scattered light has the same color as the light that illuminated the cloud. In the daytime, the sunlight is white so the clouds appear white. But at sunrise or sunset, the sun's light is mostly red (the blue light has been scattered away by the atmosphere before it reached the clouds) so the clouds appear red, too. If the clouds are very thick, they may absorb enough light (or scatter enough upward into space) to appear gray rather than white. Another way to see why the clouds are white is to realized that light reflects from every surface of the water droplets. As the light works its way through the random maze of droplets, it reflects here and there and eventually finds itself traveling in millions of random directions. When you look at a cloud, you see light coming toward you from countless droplets, traveling in countless different directions. You interpret this type of light, having the sun's spectrum of wavelengths but coming uniformly from a broad swath of space, as being white. These two views of how light travels in a cloud (absorption and reemission from droplets or reflections from droplet surfaces) turn out to be exactly equivalent to one another. They are not different physical phenomena, but rather two different ways to describe the same physical phenomena.

352. When I look up at the sky on a clear day, there is the sun, then a surrounding circle of white-blue light covering maybe half the sky, encircled by deep blue down to the horizon, followed by a white layer at the horizon itself. Please explain these zones.
The ring that you see surrounding the sun is probably the 22° halo caused by refraction from ice crystals in the upper atmosphere. These tiny ice crystals are hexagonal prisms and they deflect the light that passes through them to form a ring of light around the sun. Because the particles are large enough to bend all the colors of light equally, the ring appears white—or blue-white when superimposed on the blue sky. The deep blue of the surrounding sky is caused by Rayleigh scattering of the sunlight passing through it. In this process, small groups of air molecules and tiny dust particles deflect sunlight toward your eye. Since they deflect short wavelength light (blue light) more effectively than long wavelength light (red light), they give the sky a bluish glow. Finally, the white appearance of the horizon is probably light scattered toward your eyes by surface haze. Relatively large particles in the air scatter sunlight in all directions so that you see a white glow from the air near the ground.

A wonderful reference for some of these ideas is "Rainbows, Halos, and Glories" by Robert Greenler.

353. Why are tanning beds not good for you; also there are some new ones recently that claim that they are safer than others (have no B rays)? Are they about the same as the sun itself or how much worse for you?
Tanning beds emit ultraviolet light in order to trigger your skin's tanning response. This ultraviolet light can and does cause chemical damage to your skin. Like all light, ultraviolet light is absorbed and emitted as particles. The energy in each light particle depends on its wavelength and, since ultraviolet light has short wavelengths, ultraviolet light particles carry lots of energy. They carry enough energy to rearrange the molecules that absorb them. If those molecules are part of the genetic information of a cell, the cell may die or, worse yet, may become cancerous. The shorter the wavelength of the ultraviolet light, the more energetic its particles and the more damage it can do. Tanning beds walk a narrow line between inducing tanning and causing significant damage. Leather skin is one end result of too much chemical damage. Tanning beds that emit relatively long wavelength ultraviolet are probably less harmful than those that emit shorter wavelength ultraviolet (these wavelength ranges are sometimes designated by letters A, B, and C...I think that A is the longest wavelength and least harmful). Still, you skin's tanning response is a defense against chemical damage and is probably not worth trying to trigger with light. Recent research seems to have found chemicals that trigger tanning. These chemicals mimic light-damaged molecules in your skin. Your skin senses these molecules and responds by tanning. If these chemicals work, you'll soon be able to develop a true tan without exposure to light.

354. Why are there sunspots?
The sun is a ball of incandescent gas. That gas moves about, flowing up and down as well as across the sun's surface. This movement keeps the sun's temperature roughly uniform but there are occasionally imperfections; regions of the sun's surface that get out of balance with the rest of the sun. When you cook a thick soup on the stove, there will also be regions of the surface that are cooler than others.

355. Why can water appear brown, blue (as in the ocean), and clear (as in a glass of water)?
Brown water contains colored contaminants that provide the color. Brown is the typical end result for a random mixture of pigments. The blue ocean is caused mostly by the sky. Since the ocean reflects some of the light from the sky, it appears blue. Pure water is almost completely colorless. Thus a glass of water has no color (unless you illuminate it with colored light). But if you look at a white light through many meters of water, that light will become slightly colored. Water absorbs a very small amount of visible light and you will see only what is not absorbed. I'm not sure what color pure water has. It may appear slightly green.

356. Why do dark clothes absorb heat more than light clothes?
Dark fabrics or surfaces are very good at absorbing and emitting light. That is why they are dark. They must contain electric charges that move fairly easily (making them good antennas) and these charges must be good at exchanging energy with the surrounding material as heat. When light strikes these charges, the charges begin to move and absorb the light's energy. This energy flows into the material as heat. Since the light is absorbed, the material appears dark (no light is reflected back toward you). But the material will also emit light very effectively when hot. If you heat a black object up, heat will flow into the charges, which will begin to move and will emit light. Thus black objects are good at both absorbing and emitting light.

357. Why do different sunglasses appear darker than others?
Polarizing sunglasses block half the light (stopping horizontally polarized light and passing only vertically polarized light). But sunglasses of all types contain chemicals that absorb light of both polarizations. The darkness of the sunglasses depends on which chemicals are used and how much of those chemicals they contain. Some sunglasses are also coated with thin metallic layers that reflect a fraction of the light that strikes them. These semi-transparent mirrors can change the transmission of the sunglasses dramatically so that those sunglasses may transmit 50% of the light or 0.01% of the light. The manufacturer can choose.

358. Why do fine mists of water create rainbows?
Fine mists of water are basically spherical water droplets in air and these can produce rainbows in exactly the same manner as raindrops do in natural rainbows.

359. Why do sunspots affect radio and TV reception?
Although I do not really know very much about the connection between sunspots and radio reception, I believe that the problem lies in with the solar wind. The solar wind is a steady stream of electrically charged particles that is responsible for the aurora, among other things. Since charged particles that interact with the earth's magnetic field accelerate, they emit radio waves. These waves should cause reception problems on earth. If anyone reading this knows otherwise or has more information, please let me know.

360. Why do you sometimes see a circular rainbow surrounding a light?
It is most often caused by the bending of light by mist around the light or by flaws in the optical components through which you are viewing the light. Whenever light passes through a clear material, its path bends. In most cases, you only notice that the light is distorted by its passage through the material. But different colors (wavelengths) of light bend by slightly different amounts so that the colors of light sometimes appear to come from slightly different directions. That's the origin of the rainbow you see.

361. Why do you think you see water on a road ahead of you when it's not really there?
On a sunny day, heat from the pavement can create a layer of very hot air at the surface of the road. Since hot air is less dense than cold air, its index of refraction is slightly less than that of cold air, too. As light from the sky enters this layer of low-index air, that light is bent. Light from the sky far out in front of you is turned upward so that you see the sky "reflected" from the road's surface (actually bent upward by the air above the road's surface). You interpret this sky light as coming from a pool of water on the road. But as you approach the road and look down at it, you see that the road is dry and black.

362. Why does purple bend more in a prism than, say, red?
Purple (or violet) light travels slower in most materials than does red light. That occurs because violet light is higher in frequency than red light and gives the charged particles that it jiggles about less time to move up and down. With very little time to move, these charged particles barely notice that they are parts of atoms and molecules and respond easily to the passing electromagnetic wave. But when red light pushes and pulls on charged particles, there is more time for them to find the limits of their freedom. These charged particles are not able to move so easily when pushed on by a passing wave of red light so they do not interact with that passing wave as well as with one of violet light. Thus red light passes by with less effect and it behaves more like it would in empty space. Violet light, which interacts relatively strongly with the atoms it passes, slows down more than red light. Since red light travels more quickly than violet light, it bends less in passing through a prism. Violet light slows down more and bends more than red light.

363. Why doesn't light go through the other side of a water droplet, refracting as it goes through, rather than reflecting back?
Actually, 96% of the light hitting the "other side of a water droplet" does pass out of the droplet. What you see in the rainbow is the 4% that reflects back from the far side of the water droplet. If all of the light reflected, the rainbow would be much brighter.

364. Why is a blue flame hotter than a red flame?
The colors of flames can be deceiving because they involve emissions from particular atoms (which impart their own characteristic colors to the light they emit). However, a blue-hot object such as a star is hotter than a red-hot object such as a glowing coal in the fireplace.

365. Why is it any worse to observe a solar eclipse rather than a normal glimpse at the sun?
The problem with looking at the sun during a solar eclipse is not that it is somehow brighter than normal but rather that (1) you tend to stare at it and (2) the size of its bright region is reduced so that it doesn't hurt as much to stare at it. It's hard to stare at the full sun because it feels uncomfortable but looking at a tiny part of the sun may not feel bad enough to make you avert your eyes. Nonetheless, that tiny part of the sun can cook your retina and cause permanent damage.

366. Why is it that after swimming in a heavily chlorinated pool, you can see the spectrum around lights?
Your eye works very hard to keep all of the different wavelengths of light together so that they can form sharp images on your retina without any color errors. If you look at a white light bulb, all of the different colors from that bulb must arrive together on your retina or else you will see colors where they shouldn't be. Keeping these colors together is no small task and is one of the biggest problems encountered by lens makers for cameras and telescopes. The chlorine in a pool evidently upsets your eye's ability to control these color errors. However, I'm not sure what goes wrong or why chlorine causes this problem.

367. Why isn't the sky bright blue when the sun is red?
During the day, the sky is blue because the air and dust in the air scatter mainly blue light toward your eyes. They also scatter some red light, but the blue light dominates. But at sunset, things change. The setting sun approaches the earth's atmosphere at a very shallow angle so that it must travel many kilometers through the air before reaching your eyes. During this long trip, most of the blue light is scattered away and the sun appears very red. If the path is long enough, the blue light is scattered away many kilometers to your west so that there isn't much of it left. When this occurs, even the sky around you appears somewhat reddish because there just isn't any more blue to scatter. The missing blue light is visible to people living 50 or 100 kilometers to the west as their blue sky.

368. Why, if white doesn't absorb heat, do I get very hot when I wear a white shirt?
A white shirt doesn't absorb visible light (or at least very much visible light), but it may absorb lots of infrared light. Since much of the sun's light and heat are in the form of invisible infrared light, that infrared absorption can be very important. There are many materials that appear white to your eye that do absorb strongly in the infrared and thus get very hot in sunlight.

369. Are flood lights incandescent or fluorescent? Why are they so bright?
Most modern commercial and industrial floodlights are fluorescent lamps. Fluorescent lamps are so much more energy efficient than incandescent lamps that they quickly pay for their higher cost by saving electricity. Fluorescent lamps also last much longer than incandescent lamps, particularly if they are left on for long periods of time. Fluorescent lamps age most during their start-up cycles. Even around the house, fluorescent floodlights are becoming popular. Fluorescent lamps using about 150 W of power are as bright as incandescent lamps using 500 W. Both are bright, but one is much more energy efficient.

370. As a kid, we'd shake streetlights. They'd get real bright and then explode. Then we'd run away. Why'd they get brighter and explode?
I'll have to guess at this one. If the lamps you are talking about are mercury vapor, then they contain a reservoir or droplet of liquid mercury. If shaking these lamps would cause the mercury to flow out of the cooler reservoir and into hotter regions of the bulb, the mercury would boil and raise the pressure inside the lamp. The current passing through the lamp would increase and the bulb would get very bright. It would also get hotter and hotter, so its pressure would rise still further. Eventually the pressure would become so high that the bulb would explode.

371. Can you get a tan from an ultraviolet light bulb?
Yes. Tanning appears to be your skin's response to chemical damaged caused by ultraviolet (high energy) light. Each photon of ultraviolet let carries enough energy to break a chemical bond in the molecules that make up your skin. Exposure to this light slowly rearranges the chemicals in your tissue. Some of the byproducts of this chemical rearrangement trigger a color change in your skin, a change we call "tanning". Any source of ultraviolet light will cause this sequence of events and produce a tanning response. However, the different wavelengths of light have somewhat different effects on your skin. Long wavelength ultraviolet (between about 300 and 400 nanometers) seems to cause the least injury to cells while evoking the strongest tanning response. Short wavelength ultraviolet (between about 200 and 300 nanometers) does more injury to skin cells and causes more burning and cell death than tanning. However, all of these wavelengths have enough energy to damage DNA and other genetic information molecules so that all ultraviolet sources can cause cancer.

372. Do fluorescent light fixtures emit magnetic fields? If so, would they be intense enough to affect diskette magnetic media?
While fluorescent light fixtures do emit magnetic fields, those fields are far too weak to affect magnetic media. Any electric current produces a magnetic field, even the current flowing through the gas inside a fluorescent tube. However, that field is so weak that it would be difficult to detect. Nearby iron or steel could respond to that weak magnetic field and intensify it, but the field would still be only barely noticeable. The only strongly magnetic component in a fluorescent fixture is its ballast coil. The ballast serves to stabilize the electric discharge in the lamp and relies on a magnetic field to store energy. However, the ballast is carefully shielded and most of its magnetic field remains inside it.

As for affecting diskette magnetic media, that's extraordinarily unlikely. Even if you held a diskette against the ballast, I doubt it would cause any trouble. Modern magnetic recording media have such high coercivities (resistances to magnetization/demagnetizations) that they are only affected by extremely intense fields.

373. Do neon lights have glass that is not colored, but has phosphors that emit a particular color?
A true neon light tube has completely clear (no color, no phosphor) glass surrounding a thin gas of neon atoms. When current runs through that gas, the neon atoms emit red light. In "neon tubes" that emit colors other than red (green, pink, orange, yellow, etc.), there is a layer of phosphor on the inside surface of the glass and mercury vapor inside the tube. These fluorescent tubes probably don't contain any neon at all. You can see the light coming from the phosphor coating. In a true neon tube, you can see the light coming from the gas itself, well inside the glass tube.

374. Does the size of the bulb affect its intensity?
The intensity of a normal fluorescent light bulb is determined by how many times each second (1) a mercury atom can absorb energy in a collision and emit a photon of ultraviolet light and (2) a phosphor particle can absorb a photon of ultraviolet light and emit a photon of visible light. The first rate depends on how much current and electrical power can flow through the tube, which in turn depends on (A) the geometry of the tube and (B) the density of mercury vapor inside. As for (A), the long, thin tube seems to be the best geometry choice for a low voltage (120V) tube, producing a certain amount of ultraviolet light per cubic centimeter of volume. The longer or fatter the tube, the more electrical power it will require and the more ultraviolet light it will produce. As for (B), at room temperature, the density of mercury vapor is just about right. In very cold weather, the density drops quite low and the bulb becomes dim (thus fluorescents are not recommended for outdoor use in cold climates). Finally, the second rate (conversion to visible light) depends on the coating of phosphors on the inside of the tube. A tube that is too fat will send too much ultraviolet light at the phosphors and they will become inefficient. So a long thin tube is a good choice again. Each region of tube surface converts the light from a relatively small volume of mercury gas. Overall, the intensity of the bulb scales roughly with the volume of the tube. Big tubes emit more light than little tubes. One of the challenges facing fluorescent lamp manufacturers is in making small tubes emit lots of light. To replace an incandescent lamp with a miniaturized fluorescent, that miniaturized fluorescent must emit lots of light for its size. They're getting better every year, but they aren't bright enough yet.

375. How do "forbidden transitions" become less forbidden as pressure builds?
For an atom to determine that it cannot make a particular transition (that its electron cannot move from one particular orbital to another), it must first "test the water". The atom effectively tries to make particular transition but finds that this transition is not possible. However, if the atom experiences a collision during the test period, the atom may "accidentally" undergo the forbidden transition. It is as though the atom was prevented from canceling the experiment.

376. How do phosphors change the light from ultraviolet to visible?
They absorb the light and light energy by transferring electrons from low energy valence levels to high-energy conduction levels. These electrons wander about inside the phosphors briefly, losing energy as heat, and then fall back down to empty valence levels. Since they have lost some of their energy to heat, the light that they emit has less energy than the light they absorbed. Incoming ultraviolet light is converted to outgoing visible light.

377. How does a fluorescent light work?
A fluorescent lamp consists of a gas-filled glass tube with an electrode at each end. This lamp emits light when a current of electrons passes through it from one electrode to the other and excites mercury atoms in the tube's vapor. The electrons are able to leave the electrodes because those electrodes are heated to high temperatures and an electric field, powered by the electric company, propels them through the tube. However, the light that the mercury atoms emit is actually in the ultraviolet, where it can't be seen. To convert this ultraviolet light to visible light, the inside surface of the glass tube is coated with a fluorescent powder. When this fluorescent powder is exposed to ultraviolet light, it absorbs the light energy and reemits some of it as visible light, a process called "fluorescence." The missing light energy is converted to thermal energy, making the tube slightly hot. By carefully selecting the fluorescent powders (called "phosphors"), the manufacturer of the light can tailor the light's coloration. The most common phosphor mixtures these days are warm white, cool white, deluxe warm white, and deluxe cool white.

The only other significant component of the fluorescent lamp is its ballast. This device is needed to control the current flow through the tube. Gas discharges such as the one that occurs inside the lamp are notoriously unstable—they're hard to start and, once they do start, tend to become too intense. To regulate the discharge, the ballast controls the amount of current flowing through the tube. In most older lamps, this control is done by an electromagnetic device called an inductor. An inductor opposes current changes and keeps a relatively constant current flowing through the tube (although that current does stop and reverse directions each time the power line current reverses directions — 120 times a second or 60 full cycles, over and back, in the United States). Some modern fluorescent lamps use electronic ballasts—sophisticated electronic controls that regulate current with the help of transistor-like components.

378. How does an ultraviolet ("black light") fluorescent tube work?
Some ultraviolet fluorescent tubes are simply the mercury discharge tubes (as in a normal fluorescent tube) but without any phosphor coating on the inside of the tube and with a quartz glass tube that transmits 254 nanometer light. In such a bulb, the 254-nanometer light emitted by mercury vapor in a discharge is emitted directly from the tube without being converted into visible light. A filter somewhere in the system absorbs the small amount of visible light emitted by a low-pressure mercury discharge. For the longer wavelength black light used in most applications, other gases that emit lots of 300-400 nanometer light are used. Again, these tubes have no phosphor coatings to convert the ultraviolet light into visible light. One other way to make longer wavelength black light is to use a mercury discharge but to coat the inside of the tube with a phosphor that fluoresces ultraviolet light between 300 and 400 nanometer.

379. How does radiation trapping work?
Each atom has certain wavelengths of light that it is particularly capable of absorbing and emitting. For mercury, that special wavelength is about 254 nanometer (ultraviolet). For sodium, it is about 590 nanometer (orange-yellow). If you send a photon of the right 590 nanometer light at a sodium atom, there is a good chance that that atom will absorb it, hold it for a few billionths of a second, and then reemit it. The newly reemitted light will probably not be traveling in the same direction as before. Now if you have a dense gas of sodium vapor and send in your special photon of light, that photon will find itself bouncing from one sodium atom to another, like the metal ball in a huge pinball game. The photon will eventually emerge from the gas, but not before it has traveled a very long distance and spent a long time in the gas. It was "trapped" in the sodium vapor. This radiation trapping makes it hard for high-pressure gas discharges to emit their special wavelengths because those wavelengths of light become trapped in the gas.

380. Is a neon light actually a mercury/phosphor tube?
Most "neon" lamps are mercury lamps with a colored phosphor coating on the inside. However the true neon lamp (that special red glow) is really neon gas glowing directly. Take a close look at an advertising lamp that contains a variety of colors. The mercury/phosphor ones will seem to emit light from their frosted glass walls. You are seeing the phosphors glowing. But the real neon lamp will emit light from its inside. The glass will be clear and you will see the glow originate in the gas itself.

381. Is having a black light in your room dangerous?
It depends on how bright the light is an how long you are exposed to it. If it is simply a normal lamp, coated with some filter that absorbs all the visible light, then it is no worse than having the visible light around. It will be a very dim ultraviolet light. However, if it is a serious ultraviolet lamp, emitting several watts or even tens of watts of ultraviolet light, then it is not a great toy. Long wavelength UV is less dangerous than short wavelength UV, but neither is great. Sunlight itself contains a far amount of both long and short ultraviolet. Fortunately for us, the small amount of ozone gas in the earth's upper atmosphere absorbs much of the short wavelength UV. But long exposure to sunlight is dangerous, too.

382. I've heard (from observations recorded in an office environment) that fluorescent light bulbs "emit" their energy at a certain frequency. If this frequency is at or below the rate at which our eyes blink/scan, this will cause eye fatigue and other health "problems." What would be the best light system for the office environment?
Fluorescent light bulbs flicker rapidly because they operate directly from the alternating current in the power line. The light that you see is emitted by a coating of phosphors on the inside surface of the glass tube. These phosphors receive power as ultraviolet light and emit a good fraction of that power as visible light. The ultraviolet light comes from an electric discharge that takes place in the mercury vapor inside the tube. Since this electric discharge only functions while current is passing through the tube, it stops each time the current in the power line reverses. Thus, with each reversal of the power line, the discharge ceases, the ultraviolet light disappears, and the phosphors stop emitting visible light. So the tube flickers on and off. However, the alternating current in the United States reverses 120 times a second in order to complete 60 full cycles each second. The fluorescent lamps flicker 120 times a second. Even the very best computer monitors don't refresh their images that frequently because our eyes just don't respond to such rapid fluctuations in light intensity. In short, you can't see this flicker with your eyes. If you get eye fatigue from fluorescent lamps, it's the color or intensity of the light that's bothering you, not the flicker. It's just too fast to affect you.

383. We have some problems with a "fluorescent lamp igniter", the device that turns on the lamp. I would like to know what is necessary for the fluorescent lamp to turn on?
A fluorescent lamp produces light as the result of an electric discharge that takes place inside the lamp tube. Electrons, emitted from hot filaments at each end of the tube, are pulled through the tube by electric fields and collide violently with mercury atoms inside the tube. These mercury atoms then emit ultraviolet light, which is converted to the visible light you see by the phosphor coating inside the glass tube.

To emit the electrons needed to sustain the discharge, the filaments at each end of the fluorescent tube must be heated. In the "preheat" style of fluorescent lamp, these filaments are heated red-hot for a few seconds by sending current directly through them. There are two pins at each end of the tube and current is sent to the filament through one pin and extracted through the other pin. Once the filaments are hot enough, the lamp turns off this current flow and tries to send current through the tube itself. If the discharge starts, the discharge is able to keep the filaments hot enough to emit electrons continuously. But if the discharge fails to start, the filaments are heated some more to try to release enough electrons to initiate the discharge.

The "igniter's" job is to preheat the filaments for a few seconds and then to test the main discharge. If you see no red glow from the filaments at each end of the tube or you see no attempt by the igniter to start the main discharge, then the igniter should be replaced. It could also be that the tube itself is bad—that its filaments have burned out. If you see only one end of the tube glowing red or you see the igniter trying repeatedly to start the discharge, the tube is probably bad. I'd suggest replacing both the igniter and the tube and seeing if that fixes the problem. The only other component of the lamp, other than wiring, is the ballast—the device that controls the amount of current flowing through the discharge. It, too, could be bad.

384. What actually causes a fluorescent bulb to burn out?
The electrodes age, particularly during start up. The endless bombardment of charged particles gradually chips or "sputters" material off of the electrodes until they are no longer able to sustain a steady discharge. The final blow usually occurs when the heater filament breaks and the lamp cannot be started at all.

385. What happens when a fluorescent lamp flickers during start-up but doesn't fully light?
Sustaining the discharge in a gas lamp requires the steady production of charged particles. Even if a lamp contains many negatively charged electrons and positively charged ions, these particles will quickly migrate to the electrodes once electric fields are present in the tube. If they don't produce more charged particles as they fly across the tube, these charged particles will quickly disappear and the discharge will stop. It takes a critical number of charged particles in the tube to ensure a steady production of new charged particles. Thus the tube may not always start, even if it has a brief flicker of light.

386. What is the correct way to dispose of fluorescent lamps? Do they really have mercury inside them? Is the powder that covers the inside of them dangerous? Is there a simple way to get rid of a burned fluorescent lamp without pollution? - Augusto
While there is mercury in a fluorescent lamp, the amount of mercury is relatively small. There are only about 0.5 milligrams of mercury in each kilogram of lamp, or 0.5 parts per million. In fact, because fluorescent lamps use so much less energy than incandescent lamps, they actually reduce the amount of mercury introduced into our environment. That's because fossil fuels contain mercury and burning fossil fuels to obtain energy releases substantial amounts of mercury into the environment. If you replace your incandescent lamps with fluorescent lamps, the power company will burn less fuel and release less mercury. That's one reason to switch to fluorescent lamps, even if you must simply throw those lamps away when they burn out. Nonetheless, there are programs to recycle the mercury in fluorescent lamps. Last year, the University of Virginia recycled 31 miles of fluorescent lamps. They distilled the mercury out of the white phosphor powder on the inner walls of the tubes. Once the mercury has been removed from that powder, the powder is not hazardous. The university also recycled the glass. One last note: the mercury is an essential component of the fluorescent lamp—mercury atoms inside the tube are what create ultraviolet light that is then converted to visible light by the white phosphor powder that covers the inside of the tube.

387. When the temperature is sub-zero (e.g., -40°), is it necessary to heat the electrodes or the gas or both for the tube to light? What is the optimum tube temperature with respect to efficiency?
Fluorescent lamps do not operate well in extreme cold. Below about 15° C (59° F), the density of mercury atoms in the tube's vapor is too low to produce efficient light. While the tube also contains inert gases that allow it to start at almost any temperature, the scarcity of mercury atoms leads to a reduced light output. In any case, the electrodes must be heated to make them emit electrons to sustain the discharge.

The optimal internal temperature for a fluorescent lamp is about 60° C (140° F). The tube reaches this internal temperature when its outside is about 40° C (104° F). When the surrounding temperature exceeds 40° C, the tube begins to waste energy again because the density of mercury atoms in the vapor becomes too large.

388. Where does the extra energy go after ultraviolet light goes through the phosphor coating? Is it lost as heat?
Yes. The extra energy is converted into heat by the phosphors. Their electrons absorb the light energy, convert some of that energy into heat, and then reemit the light. Since the new light contains less energy per particle (per photon) than the old light, it appears as visible rather than ultraviolet light.

389. Why do fluorescent emissions of light not produce more heat?
When an atom is excited by a collision and then emits energy as light, it converts most of the collision energy into light. Thus the gas in a fluorescent lamp experiences many collisions but emits most of the collision energy as light. The gas becomes slightly hot, but not nearly as hot as the filament of an incandescent bulb. The electrical energy arrives at the fluorescent bulb as a current of charged particles and most of this energy leaves the bulb as light, without ever becoming heat. However the electrical energy arriving at an incandescent bulb becomes heat first and then becomes light. The conversion of electrical energy to heat dramatically reduces the bulb's ability to emit visible light efficiently.

390. Why do fluorescent tubes explode if broken (is it the compression of the gas)?
Fluorescent tubes operate at very low pressure; roughly 1/1000th of an atmosphere. They do not explode when broken; they implode. The atmospheric pressure surrounding the tube crushes it as soon as it begins to crack. The tube shape of a typical fluorescent tube is chosen because it can withstand the enormous compressive forces of the atmosphere better than most other shapes.

391. Why do many fluorescent lamps blink before they come on?
The lamp first heats the filaments in its electrodes red hot so that they begin to emit electrons and then tries to start a discharge across the lamp. If there are not enough electrons leaving the electrodes to sustain a steady discharge, the lamp will blink briefly but will not stay on. The lamp will try again; first heating its filaments and then trying to start the discharge. The lamp may blink several times before the discharge becomes strong enough to keep the electrodes hot and sustain the discharge.

392. Why do mercury lamps without phosphors emit visible light at high pressure? What are the "forbidden" transitions?
At low pressure, a mercury lamp emits mostly 254-nanometer ultraviolet light. That light is created when an electron in the mercury atom goes from its lowest excited orbital to its ground (normal) orbital. The other wavelengths of light emitted by the low-pressure lamp are weak and widely spaced in wavelength. An electron must sent into a very highly excited orbital in order to emit one of these other wavelengths. But at high pressure, mercury atoms have trouble sending their favorite 254 nanometer light out of the lamp. Whenever one of the atoms emits a particle of 254-nanometer light (moving its electron from the first excited orbital to the ground orbital), another nearby atom absorbs that particle of light (moving its electron from the ground orbital to the first excited orbital). As a result the 254-nanometer light cannot escape from the lamp; it becomes trapped in the mercury gas! Instead, the atoms begin to send their energy out of the lamp by concentrating on radiative transitions between highly excited orbitals and that lowest excited orbital. These wavelengths become more common in the light emission from the lamp as its pressure rises. But some radiative transitions that are forbidden at low pressure (that cannot occur because an electron is not able to move from one particular excited orbital to another particular excited orbital) become allowed at high pressure. Collisions break many of the rules that govern atomic behavior, allowing otherwise forbidden events to occur. In the case of the mercury lamp, collisions at high pressure permit the mercury atoms to emit wavelengths of light that they cannot emit a low pressure when collisions are rare.

393. Why does a fluorescent bulb sometimes appear blue, especially right before it burns out?
I'm not aware of any tendency to change colors as it begins to burn out, but many fluorescent bulbs are relatively blue in color. The phosphor coatings used to convert the mercury vapor's ultraviolet emission into visible light don't create pure white. Instead, they create a mixture of different colors that is a close approximation to white light. There are a number of different phosphor mixtures, each with its own characteristic spectrum of light: cool white, deluxe cool white, warm white, deluxe warm white, and others. The cool white bulbs are most energy efficient but emit relatively bluish light. This light gives the bulbs a cold, medicinal look. The warm white bulbs are less energy efficient, but more pleasant to the eye.

394. How does laser surgery work?
Lasers are used in medicine in a variety of ways. In surgery, lasers are used mostly as intense sources of heat. They deposit large amounts of power into small areas, vaporizing and "cooking" tissue. Because they produce very local heating, there is no much bleeding from a cut made with a laser scalpel. In some eye surgery, intense pulsed lasers are used and take advantage of the peculiar effects that happen at very high intensities. The most important of these effects is the creation of free charged particles, which reflect and absorb the laser beam. Because it creates free charged particles when it encounters a surface, an intense pulsed laser beam only penetrates a few microns into a surface. The charged particles that it creates prevent it from traveling deeper, even in a clear material. In eyes, that allows surgeons to remove outer layers of tissue without damaging inner layers or the retina beyond.

395. Is all light other than lasers incoherent?
Yes, in the sense that the only way to create coherent light is through the use of laser amplification. While it is possible to create coherent radio waves by synchronizing the motion of many charged particles, it is extremely difficult to synchronize the charged particles that emit visible light. (The one exception to this statement is a free electron laser, an exotic device that uses the beam of electrons from a particle accelerator to produce coherent light.) In general, you must use stimulated emission if you want to create coherent light.

396. What do mirrors do for lasers?
Mirrors help to create laser beams by sending light back and forth through the laser medium. They also reflect laser beams and are used to redirect laser light.

397. What is an interference pattern in lasers?
When the wave of light emitted by a laser can follow more than one path to a target, the waves taking the different paths may "interfere" with one another. If the electric field in the wave taking one path is in phase with (always pointing in the same direction as) the wave taking another path, then the two waves will help one another and they will push together on charges in the target. The amount of light reaching the target will be particularly strong. However, if the two waves arrive out of phase with one another (always pointing in opposite directions), then they will cancel one another and the amount of light reaching the target will be particularly weak. Usually a pattern of bright and dark regions appears on an extended target as the waves following different paths alternately interfere constructively (helping one another) and destructively (canceling one another).

398. What kinds of lasers are used at laser demonstrations? Why and how do they get different colors? How do you see the actual beams?
Most of the visible lasers used in light shows are gas lasers: tubes with gas discharges in them that are arranged to produce laser light. The most common gases used in these tubes are argon and krypton. Argon lasers produce green and blue light very nicely, while krypton lasers are best for intense red light. The colors come from the structures of the atoms themselves; the energies of their various electron orbitals. To see the beams, something must scatter the light. If the lasers are intense enough, Rayleigh scattering from the air is enough to make the beams visible. However, a little mist added to the air helps a lot.

399. Why are lasers harmful to your eyes?
You eyes treat the laser light as though it came from a very distant object with a very small size. As a result, your eyes focus all of the laser light to a single tiny spot on your retina because that is where light from a tiny, distant object should go. However, there is a lot of power in the laser light and when all of that power lands on only a few cells at the surface of your retina, it cooks those cells. Its very similar to what happens when you hold a magnifying glass in sunlight and create a white hot spot on a piece of wood. With powerful lasers, damage can be done to your retina very quickly.

400. Why does the laser not create a beam of light that you can see as it travels through the air to its destination (like a flashlight)?
You can only see light travel across a room if something in the air scatters that light toward you. If there is dust, smoke, or mist in the air, you will see that light pass through it. You will see a flashlight beam scattered by these particles and you will also see a laser beam. In that respect, the two kinds of light are very similar. Some laser beams are so intense that the Rayleigh scattering (the scattering that creates the blue sky) is strong enough to make the beams visible even in perfectly dust-free air. The beams shown in class are not that strong and would only be visible if something in the air scattered their light toward your eyes.

401. Why is a semi-transparent mirror better than metal and how does it work?
Metal mirrors usually absorb about 5% of the light that strikes them. Thus a fully reflective metal mirror, with a thick layer of aluminum, silver, gold, or some other metal, will typically only reflect about 95% of the light. A partially reflective metal mirror, with a very thin layer of metal, might reflect 50% of the light, transmit 45% of the light, and absorb 5%. That 5% absorption is terrible in a laser because the metal layer will heat up and fall apart. Instead, dielectric (insulator) mirrors are created. These mirrors used layer after layer of perfectly clear insulators (usually metal oxides and metal fluorides) to reflect light. Each time light moves from one of these layers to the next, its speed changes and part of it reflects. The thicknesses of the layers are carefully controlled so that the desired wavelengths are reflected in just the right amounts. Since the layers absorb no light, any light that is not reflected is transmitted. A dielectric mirror might reflect 50% of the light, transmit 50% of the light, and absorb 0%. Since they absorb no light, dielectric mirrors do not heat up in use and work well with even very high-powered lasers.

402. Wouldn't a laser in laser surgery cut straight through the organ being worked on?
Laser light, like any other light, only travels so far in a material that absorbs it. In surgery, the wavelength of light is chosen so that it is absorbed near enough to the source that it doesn't damage tissue far from the source. The laser vaporizes nearby material but doesn't burn holes through people. If the surgeon paused for a long time, the hole being cut would gradually get deeper. But normally, the depth of the cut isn't very great.

403. Does your pupil opening and closing have anything to do with it focusing on a more distant object?
The size of your pupil does not depend on the distance to an object. It depends only on how bright the scene in front of you is. But the size of your pupil does affect your ability to focus. When it is relatively dark and your pupil is wide open, the whole lens of your eye is involved in light gathering. Focusing becomes very critical and you have very little depth of focus. Moreover, if your lens isn't perfect, you will see things as blurry. But when it is bright out and your pupil is small, you are only using the center portion of your lens and everything is in focus. That's why it is harder to focus at night than during the day. When you squint, you are artificially shrinking the effective diameter of the lens in your eye and increasing your depth of focus. Unfortunately, you are also reducing the amount of light that reaches your eye. If you look through a pinhole in a sheet of paper, you will find everything in focus, although it will appear very dim.

404. How does a video camera work?
There are many parts to this question, so I'll deal with only two: how the camera forms an image of the scene in front of the camera on its imaging chip and how the camera obtains a video signal from that imaging chip. The first part involves a converging lens—one that bends rays of light toward one another. As the light from a particular spot in the scene passes through the camera's lens, the lens slows the light down. Because the lens' surfaces are curved, this slowing process causes the light rays to bend so that they tip toward one another. These rays continue toward one another after they leave the lens and they all meet at a single point on the surface of the camera's imaging chip. That point on the chip thus receives all the light from only one spot in the scene. Likewise, every point on the imaging chip receives light from one and only one spot in the scene. The lens is forming what is called a "real image"—a pattern of light in space (or on a surface) that is an exact copy of the scene from which the light originated. You can form a real image of a scene on a sheet of paper with the help of a simple magnifying glass. The actual camera lens often contains a number of individual glass or plastic elements, which allow it to bend all colors of light evenly and to adjust the size and brightness of the real image that it forms on the imaging chip.

The second part of this question revolves around the imaging chip. In this chip, known as a "charge-coupled device," the arriving light particles or "photons" causes electric charge to be transferred into a narrow channel of semiconductor—that is a material that can conduct electricity in a controllable manner. Each photon contains a tiny amount of energy and this energy is enough to move the electric charge into the channel. The imaging chip has row after row of these light-sensitive channels so that the pattern of light striking the chip creates a pattern of charge in its channels. To obtain a video image from these channels, the camera uses an electronic technique to shift the charge through the channels. The camera thus reads the electric charge point-by-point, row-by-row until it has examined the pattern of charge (and thus the pattern of light) on the whole imaging chip. This reading process is just what is needed to build a video signal, since a television also builds its image point-by-point, row-by-row. To obtain a color image, the imaging chip is covered with a tiny pattern of colored filters so that each point on its surface is only sensitive to a certain primary color of light: either red, green, or blue. This sort of color sensitivity mimics that of our own eyes—our retinas respond only to red, green, or blue light, but we see mixtures of those three colors as a much richer collection of colors.

405. How does the camera know (measure) what the distance is to the object?
Modern cameras use a variety of techniques to find the distance to objects. Some cameras bounce sound off of the objects and time how long it takes for the echo to return. Others observe the central portion of the image (presumably the object) from two vantage points simultaneous and then adjust the angles at which those two observations are made until the images overlap. This rangefinder technique is the one you use to sense distance with your eyes. You view the object through each eye and adjust the angles of view until the two images overlap (in your brain). At that point, you can tell how far away the object is by how crossed or uncrossed your eyes are. A rangefinder camera has two small viewing windows and lenses to look at the object, just as you have two eyes to look at the object. Finally, some cameras don't really measure the distance to the object but instead adjust the lens until it forms the sharpest possible image. A sharp image has the highest possible contrast while an out-of-focus image will have relatively low contrast. The cameras adjust the lens until the light striking a sensor exhibits maximal contrast (brightest bright spots and darkest dark spots).

406. Is the eye similar to a camera?
Yes, your eye is exactly like a camera, except that the real image forms on your light sensitive retina rather than on a sheet of film. The lens bends light to a focus on the retina. If you are nearsighted and can only see nearby objects clearly, then your lens is too strong and bends light too much. Light from a distant object focuses before reaching your retina. If you are farsighted and can only see distant objects clearly, then your lens is too weak and bends light too little. Light from a nearby object doesn't reach a focus by the time it strikes your retina. It would focus beyond your retina, if it could continue on through space.

407. Why are there various types of film (speed, purposes, etc.)?
The different speeds of film have to do with how light sensitive the film emulsion is. A portion of the surface of a high-speed film will register exposure to light when only a few particles of light (photons) reach it. In contrast, a low speed film requires more photons per square millimeter to undergo the chemical changes of exposure.

While high speed film can take pictures with less light than low speed film, there is a trade-off. High-speed films are grainier and have less resolution than low speed films. Thus photographs that you would like to enlarge should be taken with relatively slow film.

408. Why do camera flashes make eyes red and why do two flashes correct this problem?
The retinas of your eyes appear reddish when you look at them with white light. The red eye problem occurs because light from the flash passes through the lens of your eye, strikes the retina (which allows you to see the flash), and reflects back toward the camera. This reflection is mostly red light and it is directed very strongly back toward the camera. The camera captures this red reflection very effectively and so eyes appear red. The double flash is meant to get the irises of your eyes to contract (as they do whenever your eyes are exposed to bright light or you are startled or excited). The first flash causes your irises to contract so that less light from the second flash can pass into and out of your eyes. Unfortunately, this trick doesn't work all that well.

409. Why do people in flash pictures have "red eye"? How do cameras try to solve that problem?
When light from the flash illuminates people's eyes, that light focuses onto small spots on their retinas. Most of the light is absorbed, by a small amount of red light reflects. Because the lens focused light from the flash onto a particular spot on the retina, the returning light is focused directly back toward the flash. The camera records this returning red light and eyes appear bright red. To reduce the effect, some flashes emit an early pulse of light. People's pupils shrink in response to this light and allow less light to go into and out of their eyes. Professional photographers often mount their flashes a foot or more from the lens so that the back-reflected red light that returns toward the flash misses the lens.

410. Why is film ruined when it is exposed to light?
Photographic film chemically records information about the light that it has absorbed. Normally, this light was projected on it by a lens and formed a clear, sharp pattern of the scene in front of the camera. However, if light strikes the film uniformly, the information recorded on the film will have nothing to do with an image. The entire sheet of film will record intense exposure to light and will have no structure on its chemical record.

411. Diffraction: I would have thought that the waves wouldn't go through the screen because the wave was too long to recognize the holes in it. How did the light go through the screen?
When I sent laser light through a fine screen, it formed an interesting diffraction pattern on a distant wall. The holes in the screen were small, but not nearly as small as a wavelength of light. The light had no trouble going through these holes, but it did suffer diffraction effects. Because the wave passed through many separate holes, these waves interfered with one another and created the complicated pattern on the wall.

412. What happened to the Hubble mirror?
The mirror of the Hubble space telescope was ground with the aid of a flawed measuring device. Although the mirror was perfectly ground, it was given the wrong curvature and thus did not form a clear image at its focus. Light from one star that hit different points on the mirror did not converge to a single point on the imaging chip. To correct for this problem, the astronauts inserted a corrective optic into the path of the light. This refractive lens compensates for the incorrect convergence of the light so that it reaches a single point on the imaging chip. However, because it is a refractive optic, it cannot pass all wavelengths of light. Any light that is absorbed by the refractive optic is no longer measurable with the telescope.

413. What is the difference between object distance and focal length?
The object distance is simply a measure of the distance between the object and the lens. The image distance is a measure of the distance between the lens and the image that it forms. A positive number for the image distance means that a real image forms. A negative number for the image distance means that a virtual image forms (on the same side of the lens as the object). The image distance depends on both the object distance and the focal length of the lens. The focal length of the lens is a characteristic of the lens itself and doesn't change as the object and image distances change. The focal length is equal to the image distance when the object is very, very distant (e.g. a star). A positive focal length lens (a converging lens) forms a real image of the star at a distance from the lens equal to its focal length. A negative focal length lens (a diverging lens) forms a virtual image of the star at a distance from the lens equal to its focal length.

414. What is the difference between real images and virtual images?
A real image is a pattern of light in space that you can touch or put a piece of paper in. When you insert the paper in this light pattern, it appears just like the scene that created it, although it is typically flipped upside-down. A virtual image is an image that you cannot touch. As you look into the optic that creates this virtual image, you can see the image as though it were a pattern of light in space, but that pattern of light is located on the opposite side of the optic, where you cannot touch it. Subsequent optical devices (including the lens of your eye) can study this virtual image and form new images of it, but you can't put a piece of film in the virtual image itself.

415. What would happen if a magnifying glass is set at the end of a telescope? How would the stars appear?
You could place the magnifying glass at one of two spots: at the entrance to the telescope or at the eyepiece of the telescope. If you put it at the entrance, it would bend the light before it had a chance to reach the main optic for the telescope. The effect would be to increase the light bending ability of the main optic and reduce the lens's focal length. This change would make it difficult to focus the telescope on distant objects, such as stars. The images of these distant objects would form too close to the main optic and you would have trouble observing them through the telescope's eyepiece. But very nearby objects form real images farther from the main optic. The magnifying glass would help the main optic form real images of very nearby objects. It would act as a close-up lens. That is what close-up lens attachments for cameras or even cheap reading glasses do: they help the camera lens or your eye form an image of very nearby objects. On the other hand, a magnifying glass held over the eyepiece of a telescope would increase the power of the telescope. You would have to adjust the focus of the telescope because the added magnifying glass will reduce the effective focal length of the eyepiece. The new super eyepiece will have to be placed closer to the real image formed by the main optic of the telescope. When it is place properly, it will give you a very highly magnified view of that real image, so you will see a highly magnified view of the stars.

416. When you look through the outer side of your eyeglasses, sometimes out of the corner of your eye, you can see a light star like all the light came together and lit up. Why do we see this? Where does it come from? Is it reflected light?
I'm not sure what effect you are observing. I do not see it myself. However, different types of lenses behave differently, so my nearsighted correction may not behave the same way yours does. There is certainly a reflection problem in some glasses. Although the main beams of light passing through the lens are handled well, internal reflections or reflections from behind the lens are not handled properly. They can form strange patterns of light on your retina, such as the light star you mention.

417. When you take an eye test at the doctor's office, they use many lenses to find your prescription. Are these lenses cut differently so that to your eye, some objects (on the Snellen chart) look like virtual image and others are real?
The lenses that they place over your eye create virtual images that are closer or farther from your eyes than the object itself. Some lenses are converging (bending rays of light together) and these "magnifying" lenses form virtual images that are located farther away from you than the object itself. If you are farsighted (seeing distant objects well) you will be able to see a nearby object well through such glasses because you will see that object as more distant. Other lenses are diverging (bending rays of light apart) and these "demagnifying" lenses form virtual images that are located nearer to you than the object itself. If you are nearsighted (seeing nearby objects well) you will be able to see a distant object well through such glasses because you will see that object as nearer. If you vision is particularly poor, the lenses you need may not form virtual images at all but will still correct your vision. In that case, you do better to think of these lenses as joining together with the lens of your eye to form a single, image-forming lens. That combined lens works to form a real image on your retina. The other complication with eyeglasses is cylindrical correction (correction for astigmatism). Some people have lenses in their eyes that are not symmetrical and focus light differently up and down or left and right. A water glass is a cylindrical lens, focusing light horizontally but not vertically. To compensate for this cylindrical character, some eyeglasses have the opposite cylindrical character cut into them and rotated into the proper position.

418. Why a spoon will allow one to "appear" up-side down on one side and right-side up on the other side?
A spoon forms an inverted real image of you when you look into the concave (hollow) side. This real image is located a few centimeters in front of the spoon, where you can touch it with your finger or insert a small piece of paper into it. Try it, you will see the pattern of light appear on the paper sliver. A spoon forms a right side up virtual image of you when you look into the convex (bowed outward) side. This virtual image is located a few centimeters behind the spoon. You appear very small because it is a small virtual image that you are looking at. You cannot touch this virtual image.

419. Why do virtual images often look far away?
A virtual image is always located behind the optic (lens or mirror) that creates it. Thus when you look into a magnifying glass, eyepiece, or a make-up mirror, you see light that appears to come from beyond the optic that creates the image. You can't touch the virtual image or put your hand in the pattern of light that you seem to see. The virtual image can appear to come from just behind the optic or from a great distance behind that optic. It depends on how things are arranged. As you lift a magnifying glass off the surface of a newspaper, the virtual image of the newspaper starts just behind the glass and slowly moves back away from the glass. As the distance between the magnifying glass and newspaper approach the magnifying glass's focal length, the virtual image moves away to an infinite distance behind the glass. After that, there is no longer a virtual image at all. Instead, a real image begins to appear on the other side of the magnifying glass.

420. Why is it that images are right side up (instead of upside-down) when looking through a magnifying glass?
In forming a real image, a camera lens behaves symmetrically, taking light reaching it from above its central axis and projecting that light onto a spot below its central axis. But in forming a virtual image, a magnifying lens merely redirects the light subtly to have it appear to come from a point nearer or farther than the original object. You still see the object as it was (right-side up) but moved toward you or away from you.

421. Although I have heard that CD players are on average better at reproducing sound, I have also heard that the best sound quality can still be had from high end phonographs. To what extent is this true?
The digitization process does introduce some distortions into the sound signal, including aliasing (confusion about high frequencies) and quantization error (round-off errors in recording the softest sounds). However, these distortions should be so small or at such high frequencies that they should be inaudible. Still, there are always some audiophiles who can hear (or claim to hear) these imperfections.

422. Do you know anything about a special kind of digital tape that could replace the CD?
Digital audiotapes have been around for a few years. These tapes store sound as digital information on a tape. Because of the digital recording and playback, the reproduction is almost perfect. The digital process involves an enormous amount of information each second; too much to be recorded in the conventional method used in cassette tapes. Instead, I think that a helical technique is used, in which information is written as diagonal stripes across the length of the passing tape. By writing a closely spaced series of these stripes, the DAT (digital audio tape) player uses much more of the tape's surface than a standard cassette and stores much more information on that surface. I doubt that DAT tapes will replace CD's because CD's are so easy to mass-produce. DAT tapes must be recorded one at a time.

423. How are the binary numbers represented in the ridges of the CD?
In principle, the binary numbers could be written as the presence or absence of ridges (i.e. a 1000 nanometer long ridge could be a 1 while a 1000 nanometer long flat area could be a 0). However, this technique has technical problems. The main problem is that the number "0" would be a long flat region (16 adjacent flat regions would be one 16000 nanometer flat region). If the flat region became too long, the CD wouldn't be able to follow the track any more. So an encoding scheme is used to make sure that ridges and flat areas are never too long. They use a length-encoding scheme, where ridges of different lengths correspond to a short group of binary bits. Furthermore, a very extensive error correcting arrangement makes sure that the music can be read even if a great many bits are unreadable. About 25% of the CD's surface is dedicated to this error correcting information.

424. How does a laser diode work?
A laser diode resembles a light emitting diode, in which electrons flowing across a p-n junction (in a diode) find themselves in conduction levels of the p semiconductor, with lots of excess energy. These excited electrons give up their excess energy by emitting light and they drop down into empty valence levels with much less energy. In a laser diode, the region in which this energy release occurs is a very narrow channel with mirrored ends. Instead of emitting their light spontaneously, the electrons experience stimulated emission. Light bounces back and forth between the ends of the channel and is amplified as it passes new excited electrons. Because all of the light produced by a laser diode emerges from one end of this very narrow channel, it experiences severe diffraction and spreads out into a wide, cone-shaped beam. To convert this cone of light into a narrow beam, a converging lens is usually attached to the diode laser's housing and this lens bends the beam into a fine pencil of light. Most laser diodes operate in the red or infrared portion of the spectrum, although some laser diodes that emit blue light have recently been developed.

425. How does alternating current affect the laser? Does it make the laser reverse?
A diode laser will only emit light (lase) when current flows through it in the proper direction. It is, after all, a diode and only conducts current in one direction. But small fluctuations in current do affect the light emission. If you run a modest current through a laser diode, so that it emits a steady stream of light, and then begin to modulate that current up and down slightly, the light emitted by the laser will modulate up and down slightly, too. In this manner, you can send sound or other information over a laser beam. This technique is useful as a private means of communicating over long distances. Only someone who can "see" the blinking laser beam can detect the information that it contains.

426. How does the player (laser) read the whole disk at once?
It doesn't. It reads only a tiny portion of the disk at any given moment. The disk spins and the reading system slowly works its way from the center of the disk towards its edge, following a spiral path around the disk.

427. To what extent are laser disc players similar to CD players?
Optically, they are identical. However, the laser disc player uses an analog recording technique (non-digital) to recreate the video signal. I think that the lengths of ridges (or perhaps pits) in the aluminum surface are used to control the analog signal strength.

428. What does a cleaning CD do?
The optical system of the CD player must be very clean. If the final lens has dust on it, the photodiode will not see the full range of light and dark patches that it expects. A cleaning CD presumably cleans this final lens, although I'm not sure how. In principle, the whole CD player should be pretty resistant to dust problems because the laser beams are large except when they focus on the CD itself.

429. What is the deal with the new mini disc players?
I only know how the prerecorded mini disc players work: they work a lot like CD players. However, they use a much smaller disk, made possible by intelligent data reduction. Instead of using 16 bits to represent each current measurement, the mini disk uses a variable number of bits. The recording equipment determines how many bits are needed to represent the sound accurately and eliminates unnecessary (or inaudible) details in the current measurements. The optical systems in mini disc players are the same as CD players.

430. Why can't CD's be recorded onto other CD's?
Most of the CD's you encounter are prerecorded. These CD's were mass-produced from a master, using plastic molding techniques, followed by metal deposition and painting. Recordable CD's, which are used now in CD-ROM applications, are written by an intense laser beam, which alters the reflectivity of the CD spot by spot to create a disk that behaves just like a prerecorded CD. However, once a CD has been "written", it cannot be cleaned for rewriting. At present, recordable CD's can only be written once. There are some new optical and magneto-optical techniques around that allow erasure, but I don't think these techniques have appeared in CD's yet.

431. Why can't I record songs directly onto CD's, like I can onto a tape?
To record CD's, you need a much more powerful laser and a blank recordable CD. Both of these items cost lots of money. Reading a CD does not alter the CD but writing it does. You need more laser power and a special CD disk. If you tried to record a normal CD, you would not be able to restructure its aluminum layer. You would not "erase" the old material on it and would not "write" new material onto it.

432. Why do CD's skip?
CD players must position their optical system very precisely, relative to the spinning disk itself. It uses very sophisticated electromechanical devices to keep it in place. But if you jar a player violently enough, it will lose its position and the audio may suffer. Most modern CD players save a short amount of information so that they are reading ahead of where they are playing. Even if they lose the track for a few hundredths of a second, they have enough music saved up that they can keep playing continuously. But if the upset is severe enough, they will run out of saved music and will go silent for a moment or two.

433. Why do some CD players sound better than others even if the CD is seriously scratched on the bottom half?
At this point, there should be very little difference between CD players that are playing perfect CD's. They all create almost distortionless reproductions of the original sound. However, different players use different tracking techniques and optical systems and thus have different abilities to recover from imperfections in the CD.

434. Why do you need to separate the different polarizations of light?
Any light wave can be described in terms of horizontally and/or vertically polarized light. For most things, these two polarizations are unimportant. But when light reflects from surfaces or passes through certain materials, these polarizations become important. The charges in surfaces and materials do not always respond equally to the two polarizations of light. The two polarizations may even travel through very different paths (e.g. in the polarization beam splitter).

435. Will light going in two directions in the same space create destructive interference?
In general, the answer is no—there won't be large regions of space in which the two light waves cancel one another. That's because, while the electric fields from the two waves do add to one another at each moment, those fields go in and out of phase with one another very rapidly as the waves pass and the end result is that they do not interfere with one another over broad expanses. However, there can be points or surfaces in space at which the electric fields from the waves at least partially cancel for extended periods of time and at which there is destructive interference. These points and surfaces are often observed in experiments with single frequency laser beams.

436. How do steak knives differ in structure from the "super" cut-through-anything non-damageable knives?
A good knife is distinguished both by its cutting edge and the backbone that supports that edge. The ideal knife has a very hard cutting edge (one that never undergoes plastic deformation and thus never becomes dull) and a very tough backbone (one that can absorb an enormous amount of energy before breaking). The backbone can experience plastic deformation when necessary, in order to absorb energy. Cheap steak knives are made of only one steel: a moderately hard and moderately tough material. They gradually dull because of plastic deformation in their edges but they never break because their backbone is flexible. A great knife is made of several steels, which can be formed by proper heat treatment of a single piece of metal: a very hard edge and a very tough backbone. It never gets dull because its cutting edge never yields and it never breaks because it bends before breaking.

437. How is the strength of a clipping device such as Caribeener, hook, or chain link calculated? I think it is measured in kilo-newtons. What elements are taken into consideration when that strength is measured?
One of the most critical measures of a clip-ping device is the maximum tension that it can tolerate without failing. I would expect a tester to measure that failure tension by putting the clipping device in a simulated working environment and exposing it to greater and greater tension until it fails. For example, a chain link would be put between two sturdy hooks and then the hooks would be pulled apart until the chain link broke or deformed permanently. Since tension is a force, it's natural to measure it in newtons or kilo-newtons (1000 newtons). (There are 4.4482 newtons in 1 pound of force.) But what constitutes failure is complicated since anything that is exposed to tension deforms somewhat. However, if the tension is less than a certain threshold, the deformation will be purely elastic—meaning that the device will return to its original shape once the tension is released. But if the tension exceeds that threshold, the deformation will be plastic—meaning that it will be permanent and the device will not return to its original shape once the tension is released. I would expect the rated strength of a clipping device to be a reasonable fraction (probably about 50%) of the tension required to cause plastic de-formation of that device.

438. Is it true that striking two hammers together will release little splinters? If so, why?
The head of a hammer is made of very tough steel. Depending on the type of hammer, that head may even hardened tool steel. In that case, the head will not yield, except to the most incredible forces. It will instead deform elastically and then return to its original shape. However, if you smack two hardened hammerheads against one another, the forces that they exert on one another may become so great that the heads will shatter. The symptom will probably not be the release of a few tiny splinters but rather large chunks of hard steel flying off in all directions.

439. What makes stainless steel stainless?
Stainless steel resists corrosion because one of the metals (iron, nickel, and chromium) or one of their oxides is bound to be stable in almost any chemical environment. Corrosion stops at the grain boundaries around the stable materials so that they form a protective layer above the other materials beneath them. — Thanks to David Ingham for this answer

440. Why is silver used so often for tableware?
Silver is used in tableware because of its whitish luster and preciousness. It is not really the most practical metal for cutlery. It tarnishes slowly by reacting with sulfur pollutants, which are present in the air in trace amounts. Pure silver is also very soft because it allows slippage to occur easily. To harden tableware, silver is alloyed with about 5% copper. The resulting material is much harder than either of the pure metals. Jewelry silver has even more copper; up to about 20%.

441. Why is stainless steel a sterile material? Why is it used for surgical tools and to pierce ears?
Stainless steel is not inherently sterile but it can be made sterile and its lack of corrosion provides no hidden cavities that might harbor germs. A stainless steel surface can be made relatively flat and it will remain that way indefinitely. In contrast, a rusting steel surface has a complicated surface that is constantly changing. That surface is harder to keep clean than a flat stainless surface. Although stainless steel seems ideal for medical purposes, it is not hypoallergenic. Many people react badly to nickel, which is present it high quantities in surgical stainless. It also turns some people's skin green.

442. I thought that glass could move, that is supposed to be a check for antique glass (It has flowed downward). Does glass move over hundreds of years?
People used to think so. They thought that glass was simply a very, very viscous liquid. However, it now appears that something happens below the glass transition temperature that stops all flow. In effect, the viscosity goes to infinity. While it might be a liquid in principle, it simply doesn't flow, even in terms of geological time scales. Antique glasses have non-uniform thicknesses because of how they were made. The earlier techniques involved stretching blown glass and tended to make sheets with irregular thicknesses. Antique glass exhibits these irregularities.

443. How do covalent bonds work?
When two atoms form a covalent bond, their total energy is reduced by their proximity. It thus takes energy to separate them. If that energy isn't available, they will cling to one another indefinitely. The two ways in which they lower their total energy by being close are (1) electrostatic attraction and repulsion and (2) lower kinetic energy. Two atoms experience both attractive and repulsive forces as they approach one another. Their positively charged nuclei repel one another, their negatively charged electrons repel one another, but their nuclei attract their electrons. The nuclei never get very close and the electrons manage to stay relatively far apart, too. The dominant effect is an attraction between the electrons and the two nuclei. The result is a net attraction. The nearby atoms are pulled toward one another by these electric forces. The lower kinetic energy comes about because of quantum effects. The electrons travel about the nuclei as waves. When the atoms are far apart, the electrons must orbit their individual atoms. Because they are then confined to small domains, they must have short wavelengths. These waves must be short enough to fit properly into their small confines. Short wavelength objects have high kinetic energies (e.g. short wavelength light is x-rays and gamma rays). But when the atoms are touching, the electrons can spread out between both atoms. Their wavelengths increase and their kinetic energies diminish. These two effects (lowered electrostatic potential energy and lowered kinetic energy) reduce the total energy when the two atoms touch. The result is the covalent bond.

444. How does glue get objects to stick to it? Do molecules in the objects bind with molecules in the glue?
Ideally, the glue would form strong covalent bonds with the material and then form countless strong bridges from one object to another. Unfortunately, getting the glue to form such strong bonds with a surface is rarely possible. Instead, the glue forms weaker hydrogen bonds or van der Waals with the surface and is not so firmly attached. The glue's polymer molecules may also extend into the surface, in cracks and fissures to form a more sturdy attachment. Clearly, surface preparation can help the gluing process. Glue will bind more effectively to a porous, rough surface than to a very smooth, impermeable one.

445. How does the process of retreading a tire work?
Since a tire cannot be melted, it can't simply be reformed into a new tire. Moreover, it contains lots of belting materials that would have to be removed and reinstalled in the new tire. So the only recycling technique available for tires is to replace the tread itself. They shave away the outside of the tire to remove any remaining tread (working carefully, so as not to damage the belts), and glue a new layer of unvulcanized rubber onto the outside of the tire. The tire is then placed in a mold and heated. This heating causes a chemical reaction known as vulcanization to occur in the new tread rubber. This vulcanization bonds all the rubber molecules together and also binds them to the original tire. If done correctly, the entire tire, old and new, becomes a single gigantic molecule and the chances of losing the tread while driving should be minimal. Furthermore, the mold forms a tread on the surface of the new rubber so that the tire is structurally very much like a new tire. However, poor retreading work or accumulated damage due to many retreading operations can produce a weak tire and allow the tread to tear away from the tire body. This separation usually occurs while the tire is spinning rapidly and the tension forces within the tire are maximized. Such separation accounts for the huge strips of tread material you often see on highways.

446. If a racquetball is one long strand of molecules, if you made a cut in the ball, wouldn't the whole ball fall apart?
A racquetball is made of vulcanized rubber. Rubber consists of countless molecules, each one of which is principally a long chain of carbon atoms, decorated with hydrogen and other atoms. It resembles of bowl of tiny spaghetti strands though each rubber molecule is much, much longer than it is thick. But simple rubber melts rather easily and becomes gooey when warm. To make it more durable, it must be vulcanized. During vulcanization, the individual rubber molecules are cross-linked to form a permanent network of coupled strands. They can't move relative to one another, which is why the racquetball can't melt. It can only burn when you heat it. So the whole racquetball is one giant molecule. If you cut it in half, you are slicing the molecule in half. It doesn't crumble, it just has many of its bonds broken. That's not a problem because bonds break and remake all the time in the molecules around us.

447. If nothing sticks to Teflon, then how does Teflon stick to a pan?
Working with Teflon is difficult in any case. The molecular chains are extremely long, typically 100,000 carbon atoms long. It does not melt easily (it is used for high temperature applications) and is a very viscous liquid even when it does melt. Teflon is attached to surfaces by sintering it from a powder. At a high enough temperature, the molecular chains begin to move about somewhat so that they bind together into a continuous material. They also enter pores and crevices in the surface and becomes wedged inside when it cools. With enough of its chains extending into the pan surface, the whole Teflon sheet is permanently attached to the pan.

448. If rubber cannot melt, how is it molded (vulcanized?) into tires, o-rings, gaskets, and such? You answered this later in the lecture; sulfur is added to the rubber and then the things are molded, right?
Yes. Vulcanization is done with the object in its final form. The plastic is assembled while it is still thermoplastic; without the cross-links that render it unmeltable. It is then vulcanized into a single giant molecule; a thermoset. This vulcanization may be done with sulfur, as in car tires, or it may be some other reaction. In silicone rubber (e.g. bathtub chalking), the vulcanization occurs spontaneously in air. The polydimethyl siloxane molecules are treated at their ends so that they vulcanize in air, releasing acetic acid (the vinegar smell). The resulting thermoset silicone rubber is one giant molecule and cannot melt any more.

449. Is it true that milk stored in plastic is not as healthy as milk in cardboard containers due to radiation?
Probably. HDPE (high density polyethylene) allows blue and ultraviolet light to strike the milk, degrading some of its nutrient molecules. It isn't radiation from the plastic but rather the sunlight that the plastic doesn't keep out of the milk. Adding an absorbing chemical to the plastic would help, but it would create an amber plastic (like amber medicine bottles; which are colored for this same reasons). If we could get used to having amber plastic, we would probably be better off. However, people seem to tolerate amber orange juice jugs but not amber milk jugs.

450. What chemical reactions cause the basic atoms to form different molecules and, therefore, different polymers?
Covalent bonds are very strong and very directional (meaning that they tend to arrange the atoms at specific angles with respect to one another). Once a molecule has formed, the covalent bonds usually prevent it from rearranging at all but the highest temperatures. Much of the field of organic chemistry is devoted to the problems of controlling the formation of covalent bonds. Very subtle reactions are used to replace one atom with another or with a specific group of atoms. The only real control that the organic chemist has is energetics, dynamics, and statistics. By energetics, I mean that objects tend to follow paths that reduce their potential energies as quickly as possible so that molecules will undergo reactions that reduce the overall potential energies as quickly as possible. If you chose the right chemicals, you can use this energetic control to determine the final molecules. By dynamics, I mean that the reaction pathways are also influenced by issues of motion (inertia, momentum, etc.) so that some energetically favorable reactions may not form because inertia and momentum makes it hard for them to occur. By statistics, I mean that reactions that increase the order of the molecules tend to be rather rare. Nature is always becoming more disordered so that a reaction that brings more order to the universe is unlikely to occur. When you mix chemicals together, they are unlikely to react to form a complete Faberge Egg, complete with a miniature winter scene inside. These different reaction issues can be used together or separately to manipulate atoms into a specific molecule. Usually some of the molecules produced in a synthesis are imperfect and must be separated from the desired molecules. So most organic synthesis projects involve many reaction and purification steps.

451. What is plastic explosives made of?
I don't know for sure, but I suspect that they are plasticized materials (polymer molecules and softening chemicals) in which either the polymer molecule or the plasticizer or both are explosives. Actually, I just looked it up and found that it is based on RDX (a nitrated form of hexamethylenetetramine). The RDX is mixed with oils, waxes, and plasticizers to make a stiff putty. That being the case, it isn't really based on polymer molecules so that the name "plastic" refers more to its ability to assume different shapes at will.

452. Why do some glues dry faster than others?
Some glues literally "dry," since they contain a plasticizer chemical that evaporates to leave a firmer plastic. Other glues polymerize directly during the gluing process. For the glues that dry by evaporating plasticizer, the choice of plasticizer is critical. Water leaves relatively slowly compared to volatile organic solvents such as toluene or acetone. That is why water-based white glue dries more slowly than organic-based plastic cement. But the glues that polymerize during the gluing process (they "cure" rather than "dry") have a broad range of speeds. Some of those glues polymerize very rapidly (e.g. superglues and 3-minute epoxies) and some go much slower (normal epoxies). In general, slower glues produce stronger materials because they contain long polymer molecules. The fast curing glues form too many short polymer molecules and are not as tough.

453. Why is it so expensive to recycle plastic?
Different plastics are handled differently for recycling. Thermosets, such as rubber in tires, cannot be melted and cannot be recycled. Only thermoplastics can be melted for true reuse. There are 6 common thermoplastics that are recycled. These are numbered 1 through 6 on their bottoms. Objects made from one of these plastics can be collected together, melted, and then reformed into new useful objects. Unfortunately, the melted and reformed plastic isn't as pure as the original. The plastics manufacturers would rather clean up petroleum into petrochemicals and then make pure plastics than start with plastic objects, clean them, and reuse them. Because the recycler can't control what was in the plastic objects, these objects cannot be used for critical applications such as food containers or plumbing. Thus most recycled plastic is used for less profitable applications. If the recycler could be absolutely sure that the plastic hadn't been contaminated, some of it could be reused very easily. Plastic milk jugs could be reformed into plastic milk jugs over and over again.

454. By firing neutrons into a nucleus to change an atom, can you make gold using other cheaper metals?
Yes. However, trying to build gold with nuclear reactions is an expensive way to make the precious metal. Furthermore, you would probably end up with radioactive gold because at least some of the nuclei you made would have the wrong numbers of neutrons in them and would be unstable.

455. Can (or has) the nucleus be seen through microscopes?
Not exactly. A microscope "sees" an object by sending waves at that object and then looking at the waves it reflects or transmits. For example, a common light microscope sends light waves at an object and allows you to observe the transmitted or reflected light.

Unfortunately, light waves can't resolve details smaller than about 1/2 their wavelength. With a light microscope, the smallest objects you can make out are about 1/4 of a micron wide. To see still smaller objects, you must use something with a shorter wavelength than light. Because of quantum physics, even seemingly particulate objects such as electrons have a wave character and a wavelength, and fast moving electrons have much shorter wavelengths than light. Electron microscopes can resolve details down to about 1/2 the electron wavelength (in principle) and that brings their resolution down toward the level of individual atoms.

But to see a nucleus, which is much smaller than an atom, you need particles with even smaller wavelengths than are available in electron microscopes. The electrons in particle accelerators have such small wavelengths that they can resolve features as tiny as nuclei. However, the particles making up nuclei are always moving so that the "images" formed by accelerators are blurry. Nonetheless, it's possible to learn much about the structure of nuclei from these accelerator experiments. In fact, people now look at features even smaller than nuclei. They are presently looking at the individual nucleons (protons and neutrons) that make up nuclei and even at the quarks that make up those nucleons.

456. How do you do work on an atom so that the nuclear force can overcome the repulsive force.
Building large nuclei is a curiosity in modern science, not a sensible scheme for synthesizing elements. Most of the heavy elements in our world were made during a supernova explosion sometime before the formation of our present solar system about 5 billion years ago. During the supernova explosion, the temperatures became so high that nuclei of all sorts crashed into one another violently and many heavy nuclei were created. The work needed to build these giant nuclei came from the heat of this horrendous explosion.

457. How do you keep the nuclear bomb stable until you're ready to use it? (For example, on the way to Hiroshima)
The nuclear material will only explode if it is assembled to the point of critical mass. If that assembly is done slowly, the material will overheat and melt, perhaps causing a minor explosion but creating more of a radiation hazard than a nuclear detonation. Only if the assembly is done rapidly to well over the critical mass will the bomb explode. To keep a nuclear weapon "safe", the bomb makers ensure that the assembly cannot occur prematurely. They probably remove the triggers for the high explosives or block the paths through which the nuclear material must move. In most cases, even an accidental triggering of the high explosives use in assembly wouldn't cause the bomb to explode because all of the high explosives must be triggered at the same time for the assembly to work properly. If only part of the explosives ignited, the bomb would fizzle (very loudly).

458. If we were ever to have a nuclear war, would we have to live underground?
The long-term effects of nuclear war would come primarily from the release of radioactive isotopes into our environment. Large nuclei, such as that of uranium 235, have many more neutrons than protons. These neutrons "dilute" the repelling protons and made these large nuclei less unstable. But once a large nucleus shatters into fragments of medium size, these fragments acquire electrons and become "normal" atoms with medium sized nuclei. Unfortunately, these medium sized nuclei need fewer neutrons than they wind up with and they are generally unstable. While they resemble normal atoms chemically, they contain unstable cores and eventually decay. The decays release energy and this energy can do chemical damage to surrounding material. If the atom has been incorporated into a biological system (e.g. a person), it can do chemical damage to that biological system, perhaps causing cancer or genetic damage. To avoid this insidious damage, people would have to stay away from the fallout chemicals. That would be a difficult task, even underground.

459. What is the difference between the nuclear bomb and the H-bomb?
The fission bomb (uranium or plutonium bomb) derives its energy from the shattering of large nuclei; those in uranium or plutonium. The H-bomb (hydrogen, thermonuclear, or fusion bomb) derives most of its energy from the fusion or coalescence of small nuclei; those in hydrogen. The H-bomb releases more energy per kilogram than the fission bombs and can be made larger than the fission bombs. However, triggering a hydrogen bomb requires the enormous temperatures of a fission bomb.

460. When a plane drops a nuclear bomb, what sets the detonation process into effect?
The altitude at which the bomb explodes affects its results. Near or on the ground, the blast would cause incredible local damage, but less long-range damage. Above the ground, the blast would cause less local damage, but more long-range damage. So the bomb-makers build altitude sensing equipment into the bomb; probably a pressure sensing or radar-based altimeter. When the bomb has determined that it is at the right height, it triggers. High explosives assemble the critical mass as quickly as possible (typically by crushing the central sphere with carefully shaped high explosive charges). Once the fissionable material exceeds its critical mass, the chain reaction starts and the bomb explodes.

461. Why can't you make nuclear weapons with any old element?
Only a few elements/isotopes are fissionable, meaning that only a few elements/isotopes have nuclei that shatter when struck by a neutron. Moreover, only a few of this fissionable nuclei release more neutrons than they take to fission. Of naturally occurring isotopes, only Uranium 235 is suitable for nuclear weapons. Plutonium 239 is also suitable, but it must be made artificially in a nuclear reactor.

462. You said that in the Three Mile Island Incident, it overheated due to the lack of cold water. How did that happen? Isn't that a huge oversight?
The loss of cooling water was unexpected and was caused by a pump failure. The broken pump was actually part of the power-generating loop, not the reactor core-cooling loop. When everything was working properly, water flowing through a loop that included the reactor core transferred heat to water flowing through the generating plant loop. But when the generating plant loop shut down, the reactor core loop had nowhere to deposit its heat and the water in it boiled. Backup cooling water evidently did not exist, did not work, or was not sufficient to keep the reactor core from over heating. I don't know whether it was poor design or poor maintenance that caused this disaster.

463. How do the "user-friendly" MRI machines work vs. the old catacomb type? (the opened vs. closed types)
The shape of the MRI machine is dictated primarily by the strong magnetic field it uses to record information about protons in a person's tissues. This field needs to be very uniform over a large region of space and the simplest way of producing such a uniform field is with a huge coil of current-carrying wire. The person would go inside the coil, in the uniform field and other parts of the MRI machine would record the information. While the coil could be dressed up to look more like a tubular hole than a coil of wire, it was still very confining. Newer MRI machines use two smaller coils of current carrying wire, one above the other, to create a uniform field for imaging. This arrangement is trickier because the two coils must be shaped very carefully to ensure that the field is appropriately uniform. Moreover, most MRI machines use superconducting wires in these coils to achieve very high magnetic fields. Since superconducting coils must be cooled to very low temperatures, they require liquid helium coolants and sophisticated thermal insulation. While the single coil magnets required only a single refrigerator and insulating chamber, those with two coil magnets required two refrigerators and insulating chambers. That increases the expense of the magnet and its operation, but produces a more open imaging region.

464. If bones "stop" electrons, then why do we see a skeletal image on an X-ray? Would we get a negative image?
The bones cast shadows on the film; wherever there is bone, few X-rays strike the film. When the film is developed, it turns black wherever X-rays hit it. Thus the areas that were shadowed by the bone appear white.

465. On an X-ray result picture, why is the film in the background blue? Is this the only way it will show up? If so why?
The X-ray image itself is formed by tiny black silver particles, just as in a normal black and white photographic negative. If those particles were supported by a clear plastic sheet, then the X-ray should appear either clear or black and have no color. The blue you are referring to must be caused either by a colored pigment in the plastic X-ray film sheet or by a colored light used to illuminate the X-ray. I suspect the later. Fluorescent lamps tend to be bluish and the ones used to view X-rays are probably particular blue. It probably increases the apparent contrast in the image so that small variations in density become visible.

466. What exactly does the bone do with the X-rays that the skin doesn't?
The skin's atoms are too small to experience the photoelectric effect with X-rays. Most X-rays go right through skin and soft tissue. However calcium atoms are large enough to experience the photoelectric effect and thus absorb many of the X-rays. Bones cast a shadow on film, which is how an image of your bones is formed.

467. How do black holes work?
As you assemble more and more mass together in a small volume, the gravity there becomes stronger and stronger. At first, it becomes more and more difficult to throw a ball upward hard enough to make it sail away from the mass into space. Eventually, you need a cannon to get the ball to leave. And by the time you get enough mass together, the gravity becomes so strong that light itself begins to have trouble escaping. Light falls in gravity, just like anything else. But it travels so fast that you barely notice it falling. However when the gravity becomes strong enough, light falls enough to cause some weird effects. A black hole forms when the gravity is so strong the even light is unable to escape from the mass.

468. How do compasses work?
A compass contains a magnetized needle, with a north pole at one end and a south pole at the other. Since opposite magnetic poles attract one another, the north pole of the compass is attracted toward any south poles it can find and the south pole of the compass is attracted toward any north poles it can find. The earth happens to have a strong south magnetic pole near its north geographical pole and a north magnetic pole near its south pole. As a result, compass needles turn (the experience torques) until their north magnetic pole ends are pointed northward (toward the south magnetic pole located there).

469. How do electronic water softeners, where a coil of wire is wrapped around the incoming water pipe, work?
I've never heard of such a water softener, but I can voice some skepticism about it anyway. Hard water is water that contains substantial amounts of dissolved calcium, magnesium, and iron. These elements form multiply charged ions in solution and these multiply charged ions tend to bind with soap and detergent molecules to form an insoluble scum. To soften the water, you must remove those ions. A conventional water softener does this by replacing them with sodium ions. The active part of a conventional water softener is an ion exchange resin that releases sodium ions as it binds up the calcium, magnesium, and iron ions. Eventually the resin runs out of sodium and it must be regenerated by flushing it with strong salt water. This regenerating process flushes the calcium, magnesium, and iron ions out of the resin and puts the sodium ions back into it. As for the electronic water softener, where does it put the calcium, magnesium, and iron ions and what does it replace them with? It can't make those ions disappear and, if it were to extract them without replacing them, it would leave the water electrically charged. So I'm skeptical that any device that doesn't chemically treat the water directly can soften the water.

470. I read in an article about batteries about a Reverse Coulomb Counter. What is it?
Although I've never heard of such a device myself, I can guess what it means. A coulomb is a standard unit of electric charge. Since a battery is a pump for electric charge, measuring the number of coulombs that have flowed through a battery is a way to determine what fraction of that battery's storage capacity has been used. (It's analogous to measuring how many grams of sand have flowed through the neck of an egg timer or how many liters of water have flowed out of a water tower.) When a battery is being recharged, measuring the number of coulombs that have flowed in the reverse direction through the battery is a way to determine how much recharging has occurred. Thus, I suspect that a "reverse coulomb counter" is a device that monitors the flow of charge backward through a battery as it is being recharged. This backward flow of charge should be almost exactly proportional to the extent of recharging.

471. In high school, we said that an object on the ground has zero gravitational energy, while an object above the ground has some. But if a hole opened up in the floor, the object on the ground would fall - so it must have SOME potential energy, right? At the center of the earth, would you have no gravitational potential energy? If not, why - doesn't the sun still pull on you?
You've brought up an interesting subject. Many quantities in physics are only well defined relative to some reference point. For example, your velocity is only defined relative to some reference frame; typically the earth's rest frame. Viewed from a different reference frame, your velocity will be different. The same holds for gravitational potential energy. When you choose to define the object's gravitational potential energy on the floor as zero, you are setting the scale with which to work. For altitudes above the floor, the object's gravitational potential energy is positive, but for altitudes below the floor, that energy is negative. As the ball falls into the hole, its gravitational energy becomes more and more negative and its kinetic energy increases. To avoid working with these annoying negative potential energies, you should choose to set the gravitational potential energy to zero at the lowest point you'll ever have to deal with; for example, the center of the earth. But the center of the earth isn't really the limit of gravitational potential energy. The object could release even more gravitational potential energy by falling into the center of the sun. It could release still more by falling into the center of a giant star. Fortunately, there is a genuine limit. If you were to lower the object slowly into a black hole, the object would release absolutely all of its gravitational potential energy. In fact, it would release energy equal to its mass times the speed of light squared (the famous E=mc2 equation of Einstein). The object would actually cease to exist, having been converted entirely into energy (the work done on you as you lower the object, presumably at the end of a very sturdy rope). This effect sets a real value of zero for the gravitational potential energy of an object: the point at which the object ceases to exist altogether. Final note: if you drop something into a black hole, it doesn't vanish the same way, because its gravitational potential energy becomes kinetic energy as it enters the black hole. The black hole retains that energy and grows slightly larger as a result. When you lower the object on a rope, you retain its energy and it doesn't remain with the black hole. The black hole doesn't change as it "consumes" the object.

472. Is there a relationship between the black hole and the point of origin of the universe?
Yes and no. Both involve lots of mass in a very small space. A black hole is a very strange region of space-time, where time runs slowly and the gravity is extraordinarily intense. Around the black hole, everything is swept inward through the hole's surface. But (as best I understand it) the early universe didn't necessarily have strong gravity. With mass uniformly distributed in the tiny, compact universe, an object felt gravity pulling it equally in all directions. There was as much mass to the left of the object as to its right. Thus the object would have been roughly weightless. With no gravity to make things lump together into galaxies, stars, and planets, there was no reason for those celestial objects to form. Why they did form is one of the great questions of modern cosmology. As for the universe's character at the very moment of creation, I don't think that anyone has a clear picture of what was happening. The very nature of space-time was probably all messed up and the theories needed to understand it don't yet exist.

473. What information is now available about magnetic fields and free radicals in our bodies?
Free radicals are molecular fragments with unpaired electrons. The organic molecules in our bodies are normally held together by covalent bonds, an arrangement in which a pair of electrons orbits between and around two atoms in a manner that reduces the total energy of the atoms and thus binds the two atoms together. When only one electron is orbiting an atom by itself, it is chemically aggressive and tends to attack other molecules. That electron is also magnetic and is influenced a tiny bit by surrounding magnetic fields. My guess is that the magnetic fields you normally encounter, whether they are due to the earth's magnetic field, or to nearby power lines, or even to strong magnets such as those used in magnetic resonance imaging, have very little influence over the chemistry of free radicals in your body. Free radicals are themselves a health issue, but I don't think that magnetic fields make free radicals any more or less hazardous. If I learn more about this issue, I'll add it here.

474. What is DTMF and how can I measure the pulses on a rotary phone?
DTMF is short for "Dual Tone MultiFrequency" and refers to the pair of tones that a telephone uses to send dialing information to the telephone switching system. Each time you press one of the buttons on the telephone, it emits two tones simultaneously. A decoder at the other end recognizes these two tones and determines what button you pushed. One tone is associated with the button's row and one tone with the button's column. Since there are four rows of buttons, there are 4 possible row tones and since there are three columns of buttons, there are 3 possible column tones. A fourth column of buttons, A through D, and a fourth column tone are part of the specifications for DTMF but do not appear in normal telephones. Naturally, all 8 tones are different and the web has countless pages that discuss these tones (touch here for an example)

As for measuring the pulses on a rotary phone, you can do this if you can study the telephone's electric impedance (or resistance). As the dial switch turns, it briefly hangs up the telephone repeatedly. The number of hangups is equal to the number you are dialing (although dialing "0" causes it to hang up 10 times). You can actually dial by hanging up the telephone rhythmically and rapidly several times. If you click the hang-up button 5 times rapidly, you will dial a "5". To detect that this hanging up is happening electronically, measure the telephone's impedance—the impedance rises dramatically during each hang-up. If there is a constant current passing through the telephone, the voltage across its two wires will rise. If there is a constant voltage reaching the telephone, the current passing through it will drop. The telephone company detects this repeated change in impedance and determines what number you dialed.

475. When astronomers study sunspots they occasionally notice that there only seems to be one magnetic pole. But I thought that monopoles didn't exist that we know of. What's going on?
While a sunspot may have only one magnetic pole associated with it, there is sure to be an equal but opposite pole somewhere else in the sun. Probably it's located deep inside the sun or somewhere else on the sun's surface. Like one end of a long bar magnet, the sunspot looks like a single pole, but it's really connected to an equal but opposite pole.

476. If you apply the brakes while making, say, a left turn on a motorcycle, the motorcycle will tend to "stand up." That is, it will tend to fight the lean you make into the turn. Why?
When you turn left, you are accelerating toward the left and your velocity is changing toward the left. This leftward acceleration requires a leftward force and that force is supplied by friction between the ground and the motorcycle's wheels—the ground pushes the wheels toward the left. However, this leftward force on the wheels also exerts a torque (a twist) on the motorcycle about it's own natural point of rotation—its center of mass. As the ground pushes the wheels toward the left, the motorcycle tends to begin rotating. In this rotation, the wheels begin moving toward the left and the driver's head begins moving toward the right—the motorcycle "stands up"! Actually, if you lean far enough to the left as you turn, an opposing torque due to the upward force that the road exerts on the wheels will balance the first torque and your motorcycle will experience no net torque—it won't stand up at all. On a high-speed turn, you must lean quite a bit to avoid the "standing up" problem, which is why motorcycle racers practically touch the ground as they turn.

477. Does everything (all matter) emit radiation? What about if something is at absolute zero? What about if it's inside a black hole? Does a black hole emit radiation? Are Hawking particles emitted by the black hole or are they spontaneously created? If a black hole causes particles to be created, is that the same as the black hole emitting them?
To begin with, matter always emits radiation. That's because, at any temperature above absolute zero, the electrically charge particles in matter are in thermal motion and they accelerate frequently. Any time an electrically charged particle accelerates, it emits electromagnetic radiation. If you could cool matter to absolute zero, the thermal motion would vanish and the matter wouldn't emit radiation. However, absolute zero is an unreachable destination—it can't be achieved—so everything experiences thermal motion and emits radiation.

The issue of radiation emitted by a black hole is another story. For decades, people thought of a black hole as perfectly black—it absorbed radiation perfectly but emitted none itself. However, Stephen Hawking showed that a black hole does emit radiation and that it behaves like a normal blackbody: an object that emits thermal radiation characteristic of its temperature. The temperature of a black hole is inversely proportional to its mass. For black holes of any reasonable size, this temperature is so extraordinarily low that the black hole emits very little Hawking radiation.

This radiation originates in the vicinity of the event horizon, the surface inside which the black hole's gravity finally becomes strong enough to prevent even light from escaping. At that surface, quantum fluctuations in which particles are temporarily created and destroyed can occasionally lead to the creation of a particle that escapes the black hole forever. In effect, two particles are created simultaneously, one of which falls into the black hole and is lost and the other of which escapes forever. The particle that falls into the black hole actually decreases the mass of the black hole, and the missing mass escapes with the other particle. As for whether the black hole causes this emission or is actually doing the emission, there is no difference. The only feature that the black hole has (other than electric charge and angular momentum) is its event horizon (actually a characteristic of its mass). If the event horizon is causing the particles to be created, then the black hole itself is at work creating those particles.

478. How can I differentiate between daylight and incandescent light?
Actually daylight is a form of incandescent light. Incandescent light is the thermal radiation emitted by a hot object such as the filament of a light bulb or the surface of the sun. But the spectrum of incandescent light emitted by an object depends on its temperature. Since the filament of an incandescent light bulb has a temperature of only about 2500° C, its light is much redder than the light emitted by the 6000° C sun. That's why photographs taken indoors with incandescent lighting turn out so orange—the light just isn't white, it's orange-red. So you can differentiate between sunlight and the light from an incandescent bulb by comparing the spectrums. Look for the relative intensities of red, green, and blue lights. Sunlight will have much more blue in it than light from an incandescent bulb.

479. Can magnetic energy be used to power a vehicle?
When you talk about "magnetic energy," you are referring to magnetic potential energy. A potential energy is energy stored in the forces between objects. In the case of magnetic potential energy, that energy is stored in the forces between magnetic poles. But there is only so much potential energy in any given collection of objects. Potential energy is released by allowing the forces between objects to push the objects around and once it is used up, there isn't any more available. That's because energy is a conserved quantity—something that can't be created or destroyed and that can only be transferred between objects or changed from one form to another. While you can store energy in a collection of magnets, that potential energy is limited by how much was put in in the first place. So to answer to your question: yes, magnetic energy can be used to power a vehicle, but not indefinitely. The only practical magnetic energy storage proposals I'm aware of are ones that suggest using huge superconducting magnets to store electric power. While such devices might be practical for an stationary power company, they would be impractical or even dangerous in a vehicle—picture cars containing incredibly strong magnets driving down the road, repelling or attracting one another as they pass.

480. How does an integrated circuit store so much information?
An integrated circuit is formed by using photographic techniques to sculpt the surface of a silicon crystal, to add chemicals to the silicon, and to deposit layers of other materials on top of the silicon. As part of this sculpting and coating process, a typical computer chip will have tiny memory cells formed on it. These cells usually consist of a tiny pad of aluminum on which a small amount of electric charge can be stored. To store one piece of information, a "bit", on one of these pads, electronic devices called MOSFETs—built right into the silicon surface—are used to control the flow of charge onto the pad. The amount of charge on the pad determines the bit's value. The charge remains on the pad, thus storing the bit, until it's time to recall the bit. At that time, the MOSFETs allow the charge to flow off the pad and into electronic devices that determine what the stored value is.

481. Does microwave cooking break molecular chains? Does any recombination of ions take place in the food and, if so, is there a possibility of eating some type of toxin formed during cooking?
The answers to all of these questions are no. Microwave cooking merely heats the water molecules, which in turn heat the food. The only molecular rearrangements that occur are those that are caused by warming the food toward the boiling temperature of water. In fact, there is less chemistry done during microwave cooking than is done in a normal oven. For example, one of the problems with microwave cooking is that food doesn't brown because the high temperatures needed to chemically modify the food molecules (and cause browning) aren't reached in microwave cooking. So you shouldn't have any fear of food cooked in a microwave oven. The microwaves don't damage it any more than heating it in boiling water would.

482. How does a microwave oven heat food?
A microwave oven uses a vacuum tube called a magnetron to create intense microwaves inside the cooking chamber. These microwaves are electromagnetic waves with a frequency of 2.45 gigahertz or 2,450,000,000 cycles per second. They are similar to normal radio waves, except that they have a higher frequency. Because of these microwaves, the electric field at any point inside the cooking chamber fluctuates back and forth 2.45 billion times each second. That means that an electrically charged particle at any point in the cooking chamber will be pulled first one way and then the other, back and forth 2.45 billion times each second. While water molecules aren't electrically charged overall, they do have electrically charged ends—one end is positively charge and the other is negatively charged. In the presence of the microwave radiation, these water molecules find themselves twisted back and forth very rapidly. As they twist, they rub against one another and friction heats them up. The water becomes hot and this hot water, in turn, cooks the food. Food that doesn't contain water (like salt or oil) won't get hot. Neither will food in which the water molecules can't turn (like ice or frozen food). That's why it's hard to defrost frozen food in a microwave.

483. Why are you required to have an item in the microwave oven while it is operating?
When a microwave oven is cooking food, electrons move rhythmically back and forth inside the magnetron tube and create the microwaves. These microwaves flow through a metal pipe and into the cooking chamber, where they are absorbed by the water in the food and thus heat the food (the twisting back and forth of the water molecules, described elsewhere on this page, not only heats the food—it also absorbs the microwaves). If there is no food in the cooking chamber, the microwaves build up in the cooking chamber until they are so intense that large numbers of them flow backward through the pipe to the magnetron. These microwaves reenter the magnetron and disrupt the motion of electrons inside it. The magnetron begins to misbehave and can be damaged as a result. To avoid such damage, you want to be sure that there is something in the cooking chamber to absorb the microwaves before they return to the magnetron and cause trouble. In short, don't run the microwave empty for any long periods of time.

484. During a total solar eclipse, does the moon make first contact with the sun on the eastern limb or the western limb? Can you explain this to me?
The moon orbits the earth from west to east. By that, I mean that if the earth were to stop turning, the moon would then rise in the west and set in the east. During a total solar eclipse, the moon is drifting directly in front of the sun. Since the moon moves from west to east, it will first block the western edge of the sun, the western limb. In contrast, during a total lunar eclipse, the moon is drifting into the earth's shadow. Since it is moving from west to east, its eastern edge will enter the shadow first.

485. Is it possible to create a "fog" in a small enclosed area without using dry ice or ultrasound?
The two techniques you mention, dry ice and ultrasound, are both intended to make tiny droplets of water in the air, effectively producing an artificial cloud. While I can't think of any better ways to make such water droplets, I can think of ways to make fogs of other materials. Tiny particles of any clear material will work because what you are seeing is the random scattering of light as it's partially reflected from the front and back surfaces of clear particles. I'd suggest a chemical process that produces tiny clear particles. The easiest one I can think of is to place a dish of household ammonia (ammonium hydroxide—ammonia gas dissolved in water) and a dish of hydrochloric acid (hydrogen chloride gas dissolved in water, sold as muriatic acid by hardware stores) in your enclosed area. The two gases will diffuse throughout your enclosure and react to form tiny clear particles of ammonium chloride. The enclosure will fill with a dense white fog. The particles are so small, that they will remain in the air for a very long time, but they will eventually settle on surfaces and leave a white powdery residue. So, unlike a water fog, this chemical fog is a little messy. You shouldn't breathe the fog, either.

486. How do color-changing eyeglasses work?
These eyeglasses are made from a special photochromic glass that contains about 0.01% to 0.1% silver halide crystals. These crystals are transparent and so small that they leave the glass almost perfectly clear. But when the glasses are exposed to bright sunlight, which contains substantial amounts of ultraviolet light, the silver ions in those crystals are reduced to silver atoms and begin to form tiny silver particles inside the glass. Like the particles that form in black and white photography, these silver particles are so jagged and imperfect that they're light absorbing rather than shiny. The glasses thus darken when exposed to sunlight. But when the eyeglasses are returned to the dark, the halogen gas atoms recombine with the silver atoms and reform the silver halide crystals. The eyeglasses once again become clear. Incidentally, the glasses can also be rendered clear by exposing them to elevated temperatures, so a short time in the oven should help to clear them up if darkness alone doesn't do the trick. That assumes, of course, that you don't melt the frames, overheat the glass, or expose the glass to sudden thermal shocks.

487. What would happen if you saturated the uranium side of a fusion bomb with cobalt? I think it would destroy our planet.
A fusion bomb, also known as a thermonuclear or hydrogen bomb, releases enormous numbers of fast-moving neutrons. Neutrons are uncharged subatomic particles that are found in the nuclei of all atoms except the normal hydrogen atom. A normal cobalt nucleus contains 32 neutrons and is known as cobalt 59 (for its 59 nuclear particles: 32 neutrons and 27 protons). When a neutron collides with a cobalt 59 nucleus, there is a substantial probability that the cobalt 59 nucleus will capture it and become cobalt 60 (for its 60 nuclear particles: 33 neutrons and 27 protons). Cobalt 60 is radioactive—it falls apart spontaneously with a 50% probability each 5.26 years. When a cobalt 60 nucleus decays, it begins by emitting an electron and an antineutrino to becomes nickel 60 (for its 60 nuclear particles: 32 neutrons and 28 protons). But this nickel 60 has extra energy in it and it soon emits two high-energy gamma rays (electromagnetic particles, with more energy than x-rays) to become normal nickel 60, a common form of the nickel atom. A fusion bomb containing cobalt 59 could be expected to make lots of cobalt 60, which would then undergo this radioactive decay over the next few decades, releasing gamma rays as it does.

So a fusion bomb containing cobalt would release a large amount of cobalt 60 into the environment. This would certainly give the bomb long lasting radioactive fallout that would make it much more damaging to the environment than a pure fusion bomb would be. Whether it would destroy the planet, I can't say. The bomb's explosion wouldn't be any more destructive, but its long-term toxic effect to animals and plants certainly would be.

488. What is the difference between a fruit and a vegetable?
A fruit is the ripened ovary of a flowering plant, often with a woody stem, while a vegetable is the edible product of a herbaceous plant (a plant with a soft stem).

489. Why is steam so efficient when cooking food and can you explain how so much heat is released when the steam changes phase, i.e. condenses?
Steam is the gaseous form of water and consists of independent water molecules. When steam comes in contact with relatively cool food, the water molecules have the opportunity to stick to one another and the steam condenses into liquid water. While most small molecules bind relatively weakly to one another, water molecules bind remarkably strongly. They form hydrogen bonds, in which negative charge on the oxygen atom of one water molecule attracts positive charge on a hydrogen atom of another water molecule. Water's hydrogen bonds are so strong that water remains a liquid well above room temperature while most other small molecules (carbon dioxide, methane, nitrogen, oxygen, etc.) are gases even at very cold temperatures. So when water molecules condense from steam to liquid, they form strong bonds with one another and release a great deal of energy. This energy takes the form of heat and it quickly raises the temperature of the food on which the water is condensing. That's why steam cooks food so quickly and efficiently.

490. If microwaves "bounce" or reflect inside the cooking chamber, is it important for all of the surfaces (walls) of the oven to be flat? What would happen if the cooking chamber were cylindrical or circular? Would the microwaves bounce off the walls and then cancel each other out?
A microwave oven with a cylindrical or spherical cooking chamber would have a problem with non-uniform cooking. But before I look at why, I should note that even a microwave oven with a box-like cooking chamber exhibits non-uniform cooking. That's because the microwaves that are bouncing around inside the cooking chamber are all coherent—they are parts of a single, giant wave—and they can interfere strongly with one another. That means that several reflected microwaves can cancel or enhance one another as they cross, leading to regions inside the cooking chamber that cook quickly and other regions that cook slowly. That's why it's important to move the food around the cooking chamber during cooking—so that the food cooks evenly.

If the oven's cooking chamber weren't box-like, there would be a new problem to contend with: a tendency for the microwaves to be concentrated or focused in a particular region. Just as a cylindrical or spherical mirror bends the light rays it reflects, so the curved walls of a non-boxlike cooking chamber would bend the microwaves it reflects. It would tend to focus those microwaves in particular regions (such as the center of the cylinder or sphere) so that there would certain regions inside the chamber where the microwaves would be particularly intense and cooking would proceed very quickly.

491. When you make a telephone call, you send an analog signal from your phone to a central station. Is this direct current or alternating current? How do you and your neighbors share the line?
When you are talking to a friend over the telephone, the telephone company uses a special power supply to send a constant (direct current) through your telephones. Your telephone and your friend's telephone share this current so that if your telephone draws more, your friend's telephone receives less. When you talk into the microphone of your telephone, the current your telephone draws fluctuates up and down with the air pressure fluctuations at the microphone. As a result, the current through your friend's telephone fluctuates down and up, the reverse of the current fluctuations in your telephone. A speaker in your friend's telephone uses these current fluctuations to recreate the sound of your voice. When there are other extensions active in your home, they are all sharing this current so that talking into one telephone causes sound to be reproduced in all of the other telephones, both in your home and in your friend's home. While modern electronics have changed the telephone system extensively, so that this direct current sharing isn't quite the reality it was 30 years ago, all of the complicated electronic circuitry works to simulate this same relationship.

492. What is the difference between a tube amplifier and a solid-state amplifier? Does the human ear prefer one over the other?
The only difference between a well-designed tube amplifier and a well-designed solid-state amplifier is the device doing the amplification. In fact, a vacuum tube and an metal-oxide-semiconductor field-effect-transistor or MOSFET are extremely similar in behavior, so that amplifiers built with the two devices can be extremely similar. If these amplifying devices are used properly in a good amplifier, that amplifier should only boost the power of its input signal and shouldn't add anything that wasn't present in the input signal. As a result, you shouldn't be able to tell whether the audio amplifier you are listening to is based on tubes or on solid-state components.

493. Please explain pectin and why sugar and acid are needed when making jelly.
The molecules of pectin contain enormous chains of atoms, often hundreds or even thousands of atoms long.. Such chains are also found in cellulose and starch, and are used by plants to give them strength and structure. These chain-like molecules are naturally occurring polymers or plastics. The giant molecules in pectin are based on small molecular units of D-galacturonic acid that have joined together like strings of paper dolls. The presence of acid groups on the pectin molecules help to make pectins very water soluble and also sensitive to the acid-base balance of their environment. I am not an expert in the exact structure and chemistry of pectin, or in the proper pH needed for jellymaking, so I can't give you an exact explanation for how to control the jelling process with acids. But the jell forms because these giant molecules spread out in the viscous solution of sugar and fruit juice, and form a tangled network of filaments that span the entire container. At high temperatures, there is enough mobility in the molecular chains to allow the mixture to flow, but at room temperature, the tangle of molecular filaments prevents flow. In the language of polymer or plastic science, the mixture goes from a liquid flow regime at high temperature to an elastic plateau regime at low temperature. When you deform cold jelly, you are pulling the filaments tight but they can't disentangle themselves enough to allow the jelly to actually flow. When you deform the cold jelly too far, the filaments begin to break and the jelly tears into fragments. However, when you warm the jelly, thermal energy allows the filaments to move past one another and the jelly begins to flow like a thick (or viscous) liquid.

494. What effects, if any, does storage temperature have on the height of a tennis ball's bounce?
I suspect that cool storage will prolong the life of a tennis ball in an opened can. That's because the ball's bounciness depends on its retaining air inside its rubber shell. As the ball loses air by diffusion through the rubber, it loses its ability to bounce high. Diffusion is a thermally activated process in which the individual air molecules move between the rubber molecules and migrate through the material. At lower temperatures, the air molecules will move much more slowly through the rubber and the pressure inside the ball will stay high for a longer time.

495. What is the Reaumur Scale for temperature and how does it compare to degrees F, C, and K?
The Reaumur Scale was created in 1730 by French scientist Rene-Antoine Ferchault de Reaumur, who set 0 R as the freezing point of water and 80 R as the boiling point of water. Though in common use for a time, the Reaumur Scale had more or less disappeared by the end of the eighteenth century. Each degree R is equal to 5/4 of a degree C, so T(C)=T(R)*5/4. Similarly, T(F)=T(R)*9/4+32 and T(K)=T(R)*5/4+273.15.

496. What is the efficiency of a 60-watt bulb to convert electricity to light?
Since only about 80% of the heat a 60-watt bulb releases is thermal radiation and only about 12% of that thermal radiation is visible light, the bulb emits about 6 watts of visible light. A halogen bulb is a little more efficient than this and a long-life bulb is a little less efficient than this.

497. How do a diode and a transistor work?
A diode is normal built by touching two different pieces of semiconductor together to form what is called a "p-n junction." Semiconductors are materials that are in between good conductors and good insulators. A pure semiconductor is a very poor conductor of electricity. With careful chemical processing, a semiconductor can be made into n-type semiconductor—a semiconductor that contains a small number of mobile electrons that permit it to carry electric current. With different processing, a semiconductor can also be made into p-type semiconductor—a semiconductor that contains a small number of mobile holes for electrons that permit it to carry electric current. It may seem strange that a hole for an electron can allow electricity to flow, but imagine a highway packed with cars (electrons) bumper to bumper. If there are a couple of empty places (holes) in the bumper-to-bumper traffic, then cars (electrons) can rearrange enough that the traffic can flow. Both mobile electrons and mobile holes allow these two chemically treated semiconductors to carry current.

When an n-type semiconductor touches a p-type semiconductor, a diode is formed. The mobile electrons at the edge of the n-type semiconductor flow over the boundary (a p-n junction) and fill the mobile holes at the edge of the p-type semiconductor. This rearrangement creates a depletion region—a region near the p-n junction in which there are neither mobile electrons nor mobile holes. This depletion region normally won't carry electricity at all. But if you push electrons onto the n-type semiconductor, they will flow toward the p-n junction and replenish the missing mobile electrons. As these mobile electrons approach the p-n junction, they will repel the electrons that are filling the mobile holes on the p-type side of the junction and reopen the mobile holes. Electrons will begin to cross the p-n junction and current will flow through the diode. However, if you push electrons onto the p-type semiconductor, they will fill even more of the mobile holes there and the depletion region near the p-n junction will grow larger and more uncrossable. No current will flow through the diode. Thus a diode (a p-n junction) only carries current in one direction—electrons can only flow from the n-type semiconductor side to the p-type semiconductor side.

There are many types of transistors, so I will only describe an n-channel Metal-Oxide-Semiconductor Field Effect Transistor, or n-channel MOSFET. In this device, three layers of semiconductors are sandwiched together: an n-type piece (the source), a long, thin p-type piece (the channel), and another n-type piece (the drain). Two p-n junctions form between these three components and, since the junctions are arranged in opposite directions, they completely block current flow from the source through the channel to the drain. But a metal surface (the gate) that's separated from the channel by an extremely thin layer of oxide insulator can control the number of electrons on the channel material. If you put even a tiny bit of positive charge on the gate, it will attract electrons onto the channel and turn it from p-type semiconductor to n-type semiconductor. When that happens, both p-n junctions vanish and current can flow from the source to the drain. The MOSFET goes from being an insulating device when there is no charge on the gate to a conductor when there is charge on the gate! This property allows MOSFETs to amplify signals and control the movements of electric charge, which is why MOSFETs are so useful in electronic devices such as stereos, televisions, and computers.

498. How is powder coating done?
Powder coating is done by combining the components of the coating (the binder—a polymer having giant chain-like molecules, the pigments, and the additives) to form a uniform solid, which is then pulverized to a dry powder and sprayed onto the surface to be coated. This coating is then baked to form a continuous film. There are two main classes of powder coatings: thermosetting and thermoplastic coatings. In a thermosetting film, crosslinking occurs between the molecules in the powder during baking. This crosslinking turns the baked film into a single giant molecule that can't melt or flow. In a thermoplastic film, thermal energy makes the binder molecules mobile enough to become entangled so that a continuous film forms and this film hardens upon cooling. While a thermoplastic film can still melt or flow, it can do that only at elevated temperatures. The powders are often given electric charges during spraying so that electrostatic forces will hold them in place until they're baked on.

499. How are tessellations used in roofing, tiles, and quilts?
Tessellation is the covering of a surface without gaps or overlaps using one or a small number of basic shapes. It's a natural activity for roofers, tilers, and quilters, since those activities involve forming complete surfaces with a limited number of shapes. Since there are an infinite number of possible tessellations, people are always trying to create interesting new ones. You can find these in a tile catalog or a quilting guide. Tessellations appear in physics in the context of crystal structure, where surfaces and volumes must be filled completely with a few basic molecular arrangements. Quasicrystalline materials—materials with orientational order but no longer-range order—are a particularly interesting example of tessellation in physics.

500. How does an electric eel produce an electric charge? I know that it can produce up to 600 volts, but what does 600 volts mean without knowing the amount of current?
The eel produces this voltage by rearranging ions in specialized muscle cells called electroplaques. While I'm not an expert in this, I suppose that they use energy derived from food to pump ions through the cell membranes of these electroplaques in order to create charge imbalances between the two surfaces of those cells. By stacking hundreds or thousands of electroplaques in series, they succeed in separating positive and negative charges to great distances on their bodies and thus produce voltage drops in excess of 600 V.

You're correct that current is an important issue here, since even household static electricity can separate enough positive charge from negative charge to reach thousands of volts. However, static electricity can reach very high voltages because there is no current flow to deplete the separated charge. In the case of an electric eel in water, the water conducts current well enough that the eel must continue to separate charge to maintain the 600-volt potential difference between its ends. I'm not sure how much current flows through the fresh water in this situation, but I would guess that it's at least 1 ampere and possibly more. That means that the eel is moving a considerable amount of charge each second and using in excess of 600 watts of power. If the eel were a salt-water fish, it wouldn't be able to reach a 600-volt potential difference at all because salt water conducts current far to well and an enormous current would flow in that case.

501. How does a mass spectrometer work and why must it be evacuated before being used?
A mass spectrometer is a device that measures the masses of the atoms or molecules in a sample. There are many different types of mass spectrometers but they all work on roughly the same principle: they give each atom or molecule a single electric charge and look at how easy or hard it is to accelerate that atom or molecule by pushing on it with electric or magnetic fields. The more mass the atom or molecule has, the more slowly it will accelerate in response to a particular force. Some mass spectrometers use an electric field to push the atoms or molecules forward until they all have the same amount of kinetic energy and the more massive particles end up traveling more slowly than the less massive particles. Their masses can then be determined by timing how long it takes them to travel a certain distance or by sending them through a magnetic field that bends their flight paths. Because the force that a magnetic field exerts on a moving particle increases with that particle's speed, the paths of slow moving massive particles bend less than those of fast moving less massive particles. Since all of this mass analysis occurs while the particles are traveling through space, it's important that they not collide with any gas particles inside the mass spectrometer. That's why the mass spectrometer must be evacuated before use.

502. What are the two substances in a Lava Lamp, and why do they react the way they do?
I'm afraid that I'm unable to determine exactly what substances the lamp contains. However, I believe that one of them is water and the other is a high-density wax. When the lamp is cold, the wax is a crystal solid with a density slightly higher than that of water. Because the buoyant force this wax experiences from the water is less than its weight, the wax sinks to the bottom of the lamp. But when the lamp is on, the bottom of its container heats up and the wax begins to melt. Like most materials, wax's liquid phase is substantially less dense than its solid phase. As it melts, the wax expands so much that its density drops below the density of water and it floats upward to the cool top of the container. Once it reaches the top, the wax begins to solidify. As it solidifies, the wax contracts so much that its density rises above the density of water and it sinks downward to the bottom of the container. Thus when the lamp is in full operation, the rising bubbles of wax are liquid and the descending bubbles of wax are solid. Dyes are added to the two materials to make them more visible—the water is colored by a water-soluble dye (perhaps food coloring) while the wax is colored by an oil-soluble dye (like those used in permanent markers).

503. Both hydrogen and oxygen fuel flame, but together they make water and that can put out a flame. Why?
In a sense, water is the "ash" that forms when hydrogen burns in oxygen. Like all fully burned materials, water can't burn any further. When you put cold water on a fire, it extracts heat from the fire because the water is much colder than the fire and heat naturally flows from hotter objects to colder objects. Since heating the water doesn't cause the water to burn (it can't burn), the heat that's lost by the fire doesn't create new fire (as would be the case if you threw gasoline on the fire instead of water). So the water gradually cools down the fire until the fire no longer has enough thermal energy to sustain its own chemical reactions. The fire then goes out.

504. Why is it that when you have water on your skin and an air current travels over it, your skin gets cold?
Whenever water is exposed to air, the water and air begin to exchange water molecules. By that, I mean that water molecules leave the surface of the liquid water to become water vapor in the air and water molecules that are already vapor in the air leave the air to become liquid water. If the relative humidity of the air is less than 100% (meaning that the air can still hold more water vapor), more water molecules will leave the liquid water than will return to it and the liquid water will gradually evaporate into water vapor. If the relative humidity of the air is greater than 100% (meaning that the air is holding more water vapor than it can tolerate), more water molecules will return to the liquid water than leave it and the water vapor will gradually condense into liquid water.

For a water molecule to leave the surface of liquid water, it needs a substantial amount of energy because it must break several hydrogen bonds which are holding it to its neighbors. It obtains this extra energy from nearby molecules and they become colder. Whenever a water molecule returns to the surface of liquid water, it returns this energy to the nearby molecules and they become hotter. Thus whenever liquid water is evaporating, the water molecules that leave the liquid water are taking away its energy so that it becomes colder. And whenever water vapor is condensing, the water molecules that return to the liquid water are giving it energy so that it becomes hotter.

When your skin is wet and water is evaporating from it, your skin also becomes colder. Blowing additional air across your skin prevents any build-up of humid air near its surface so that far more water molecules leave your skin than return to it. The evaporation then proceeds rather quickly and your skin feels quite cold.

505. Are divining rods and their abilities to locate ground water fact or myth?
I'm afraid that I think they're myth. Despite extensive searches, physicists have found only four forces in nature: gravity, the electromagnetic force, the strong force, and the weak force. Of these, only gravity and the electromagnetic force are noticeable outside of atoms. Since ground water has no electric charge, it can't affect a divining rod through the electromagnetic force. That leaves only gravity as a possibility and the gravity between modest sized objects such as a stick and a pool of water is so incredibly weak that I can't imagine anyone detecting it with their hands. Having eliminated all the possible external forces that would bend a stick downward when it's near water, it's clear that this bending is done by the hands of the person holding it. Perhaps a good dowser can see features in the environment that prompts the dowser, consciously or unconsciously, to believe that water is nearby. In short, I think that there are people who are good at identifying signs that indicate ground water is present and who can find that water. The divining rod itself is unimportant.

506. Why did Fahrenheit choose 32° for the freezing point of water and 212° for the boiling point of water? These seem like such awkward numbers to use.
Daniel Gabriel Fahrenheit chose as the zero of his temperature scale the temperature at which ice melts when it's mixed 50/50 with salt. He then set the temperature at which pure ice melts to be 30° above zero and normal body temperature to be 90° above zero. These values were adjusted several times over the years as temperature measurements became more accurate and are now 32° and 98.6° respectively. Having established the temperature scale based on these various situations, he had no choice about water's boiling temperature. Water's boiling temperature at normal atmospheric pressure simply turns out to be roughly 212° on his temperature scale.

507. When you freeze water, are the minerals separated from the molecules of the water? (When I freeze store-bought water and it then thaws out, there is a glob of nasty looking minerals that settle to the bottom of the bottle.)
When water freezes, it forms ice crystals. Crystals are very orderly arrangements of molecules in which each molecule has a particular position and orientation. Each crystal grows from a tiny initial seed crystal by adding one molecule after another to the surfaces of the crystal. Since each molecule that attaches to the crystal must fit into a particular position and have a particular shape and orientation, molecules that are different from those in the crystal tend to be excluded from the growing crystal. Thus an ice crystal that's growing in dirty water will nonetheless consist almost exclusively of water molecules. Only if the water freezes very quickly will it trap large numbers of impurities by not giving them time to get out of the way. Even gases are excluded from the ice, which is why air bubbles often appear as water freezes into ice.

The minerals that you see in the thawed bottle of water were originally dissolved or suspended in the water. But as the water froze, the ice crystals excluded those impurities and they remained in the liquid portion of the water. Eventually the liquid portion of the water dwindled away and the minerals were forced to come out of solution as solid particles. When the water thawed, those minerals failed to redissolve (they're often only weakly soluble in water and have great difficulty redissolving).

This phenomenon whereby crystallizing a liquid separates out its impurities is very useful in chemistry—many important chemicals, notably medicines, are purified in this manner. Similarly, freezing water is an important way of purifying it in some locations—native people in cold countries have used sea ice (the pure ice that forms when seawater freezes) as a source of fresh water for centuries. And you may have noticed that when you eat frozen juice, you can suck away the sweet flavored portion and leave behind only the pure ice portion—because the sugar and flavors have been excluded from the pure ice crystals during freezing.

508. How does an electric guitar amplify the sound from the strings?
As the steel strings of an electric guitar vibrate, they move back and forth across electromagnetic pickups on the guitar's surface. Each of these pickups consists of a coil of wire with a permanent magnet passing through its center. This permanent magnet has a north magnetic pole at one end and a south magnetic pole at the other end. Surrounding the permanent magnet are lines of magnetic flux that arc gracefully through space from the magnet's north pole to its south pole. These magnetic flux lines are associated with the forces that magnets exert on one another. Some of these flux lines pass very near the permanent magnet on their way from the north pole to the south pole and thus pass inside the coil of wire around the magnet. Other flux lines arc far outward and pass outside the coil of wire around the magnet. And a few of the flux lines pass through the steel string that lies just above one pole of the permanent magnet. Steel is a ferromagnetic metal, meaning that it easily develops strong north and south poles of its own when exposed to another magnet. This ferromagnetism is the result of a remarkable ordering process that takes place among the electrons inside the steel. The steel string is magnetized by its proximity to the permanent magnet in the pickup and it interacts strongly with the magnetic flux lines that pass near it. Some flux lines leaving the north pole of the permanent magnet connect to the south pole of the magnetized string and an equal number of flux lines leaving the north pole of the magnetized string connect to the south pole of the permanent magnet. Thus when the steel string vibrates back and forth, it pulls some of the flux lines with it. The paths that these flux lines take shift back and forth rhythmically as the string vibrates.

Whenever magnetic flux lines move, they create electric fields. An electric field is a phenomenon that exerts forces on charged particles, such as the mobile electrons in the coil of wire around the permanent magnet. As the string vibrates and the magnetic flux lines shift back and forth with it, electric fields appear in the wire coil and begin to push electrons through that coil. These electrons flow back and forth in the wire as the string vibrates. Wires connecting the pickup's coil to an electronic audio amplifier carry these moving electrons (actually an electric current) to the amplifier, where they are detected and used to control a much larger electric current. When this amplified current is sent through a speaker, the speaker produces a very loud sound that's an amplified version of the sound that the string itself is making as it pushes weakly on the air.

509. How does a UPC scanner work?
UPC labels are the bar codes placed on consumer goods to identify them as they pass over a glass window containing a UPC scanner. Although UPC labels were first conceived by Norman Joseph Woodland in the late 1940's, the scheme to read those codes required a very bright and narrow beam of light that could be scanned rapidly across the bars in order to measure their widths. Conventional light sources barely worked and the idea didn't catch on until lasers became available. A modern UPC scanner begins with a laser that emits a tightly collimated beam of light. Early scanners used helium-neon lasers, but new scanners use cheaper and more reliable solid-state or diode lasers. In a typical scanner, the red beam from a laser is directed toward a spinning object—either a carefully faceted and mirrored disk or a flat disk containing a carefully designed hologram. Laser light that reflects from the spinning object emerges from the glass window above the scanner and sweeps rapidly through the space like a tiny searchlight. When this light beam encounters a UPC label, each dark bars absorbs the beam while each light bar reflects it. Thus as the beam scans across the UPC label, the amount of light the product reflects fluctuates up and down in a characteristic manner. When a photodetector in the UPC scanner detects such a fluctuating reflected light signal, it determines that the laser beam is hitting a UPC label. A computer studies the sequence of the light and dark bars to determine exactly what UPC label is being hit and identifies the product to the store's computers.

510. Is it true that cold water in a pan will boil faster than hot water in a pan?
No. It takes more heat to bring a pan of cold water to boiling than it does to bring an equivalent pan of hot water to boiling. You can see that this must be true by noting that the cold water must first reach the temperature of the hot water, after which both pans will be equivalent. But there are a few interesting peculiarities with freezing and boiling water. One worth noting is that water that has recently been boiled will freeze more easily than water at an equal temperature that hasn't been boiled. That's because boiling drives the dissolved gases out of the water so that it can crystallize more easily. The ice that forms from boiled water tends to be unusually clear because it contains very few air bubbles. Water that contains lots of dissolved air traps those air bubbles as it freezes and the air bubbles slow the freezing process.

511. How does the wattage of a candle compare to the wattage of a light bulb?
A 60 watt light bulb emits about 6 watts of visible light while wasting the remaining 54 watts of electric power as other forms of thermal energy. A candle probably also consumes about 60 watts of chemical energy (the paraffin wax) but emits much less than 3 watts of visible light. The light bulb is clearly not very efficient at converting electric power into visible light but the candle is even less efficient. That's because the candle flame operates at a lower temperature (about 1700° C) than the filament of the light bulb (about 2500° C) and the spectrum of light emitted by a hot object depends strongly on its temperature. The cooler flame emits relatively more infrared light and less visible light (particularly blue light) than the hotter filament.

512. How does a magnet work and is there a way that I can determine which end of the magnet is north and which end is south?
The magnetic fields that are responsible for the interesting behaviors of magnets can be created either by (1) moving electric charge or (2) changing electric fields. We can ignore the second process because it has very little to do with permanent magnets. Instead, let's focus our attention on the first process: moving electric charge producing magnetic fields. Whenever electric charges flow through a wire, a phenomenon that we call an electric current, they create magnetism. Many appliances use electricity and electric currents to create magnetism, notably televisions, motors, and audio speakers. But a permanent magnet doesn't use an obvious electric current to create its magnetic field. Instead, it uses the spinning character of the electrons inside the material from which that magnet is made. Electrons are electrically charged and they have an intrinsic spinning character. A simplistic view of an electron is as a spinning, electrically charged ball. Since its charge is in motion, an electron acts as a magnet and has both a north pole and a south pole. In most materials, the magnetic electrons are turned in opposite directions, canceling out one another's magnetism so that the overall material is non-magnetic. But in a few special materials, including most steels, the cancellation is imperfect and some magnetism remains. In a permanent magnet, this remaining magnetism is particularly apparent. The material is, in effect, a big collection of magnetic electrons that all work together to create a large magnet.

To determine which end of a permanent magnet is its north pole and which is its south, take a compass and hold it a reasonable distance from one end of the magnet. If the north end (often the red end) of the compass needle points toward this end of the magnet, you know that this end of the magnet is a south pole! That's because opposite poles attract and the "north" end of the compass needle, a north pole, is attracted to south poles. Interestingly enough, the magnetic pole near the earth's geographic north pole is actually a south magnetic pole. That's why the north pole of the compass needle points toward the earth's north geographic pole. When you use a compass to detect which pole of the magnet is north, be careful not to bring the compass needle too close to the permanent magnet. A strong permanent magnet can remagnetize the compass needle and reverse its poles. To make sure that this hasn't occurred, check to see whether the compass still points toward the north pole after you bring it near strong permanent magnets.

513. How do power lines work and what is the purpose of all the electrical things you see behind the fences with signs saying "Warning: High Voltage"?
Electric power is distributed over long distance using high voltages and relatively low currents. Since the amount of power that flows through a wire is equal to the product of its voltage (the amount of energy carried by each unit of electric charge) and its current (the number of units of electric charge that flow through the wire each second), the electric company can distribute its power either as a large current at low voltages or a small current at high voltages. But it turns out that the amount of power that's wasted by electricity as it flows through a wire is proportional to the square of the current in that wire. Thus the more current that flows through a wire, the more power that wire turns into thermal energy (or heat). To minimize this energy loss, the power company uses transformers to convert the electricity to small currents at very high voltages for transmission cross country. Near each community, there is then a power substation at which this very high voltage power is converted to lower voltage forms. Even in neighborhoods, they use medium currents at moderately high voltages to avoid power wastage. Only in the vicinity of your home is the electricity finally converted by transformers to a large current at low voltage for safe delivery to your appliances. You've probably seen those final transformers as the gray oil-drum sized units on utility poles or the green boxes on front lawns. But despite all this effort to minimize power loss, something like 6% of the electric power generated in this country is lost in the delivery process.

514. How does magnetic recording work?
During the recording process, an electromagnet in the recording head magnetizes the surface of a specially coated tape. This tape is coated with a thin layer of plastic that's impregnated with tiny cigar-shaped magnetic particles. As the tape moves past the recording head, the head magnetizes these particles back and forth to a certain depth, according to the audio signal reaching the recorder from the microphone. The higher the pitch of the sound, the more frequently the direction of magnetization reverses. The louder the volume of the sound, the deeper the magnetization extends into the layer. During playback, this magnetized layer moves past the playback head and induces electric currents in it. These currents are then amplified and used to reproduce the sound. A much more detailed discussion of this process appears in my book.

515. How were tape recorders invented?
Magnetic recording dates to 1898, when Danish engineer Valdemar Poulsen developed a method for recording sound on a steel wire. He stretched this wire across his laboratory and put the recording apparatus on a trolley that traveled along that wire. He would run along with the moving trolley, talking into its microphone to record sound on the wire. To play back this sound, he would roll a second trolley containing the playback equipment along the wire and it would reproduce the sound. Having proven the principle of magnetic recording, Poulsen and others began to develop wire recorders. In these devices, a wire rolling from one drum to another was used to record and play back sound. In 1927, American inventor J. A. O'Neill replaced the wire with a magnetically coated ribbon and since then magnetic tape recorders have dominated the recording industry.

516. How does the pressure inside a mercury vapor lamp affect its spectral distribution, particularly as a source of ultraviolet light?
At low pressure, a mercury vapor lamp emits mostly short wavelength ultraviolet light at a wavelength of 254 nanometers. This light comes from the dominant atomic transition in the mercury atom, between its first excited state and its ground state. However, as the pressure and density of mercury atoms inside the lamp increase, two things happen. First, the high density of mercury atoms in the lamp makes it difficult for the 254-nanometer light to escape from the lamp. Each time a 254-nanometer photon (particle of light) is emitted by one mercury atom, a nearby mercury atom absorbs it. As a result, the 254-nanometer light becomes trapped inside the lamp and diminishes in brightness. With so much energy trapped inside the lamp, the mercury atoms are able to reach more highly excited states than at low density. Second, frequent collisions between the now highly excited mercury atoms allow those mercury atoms to emit wavelengths of light that are normally forbidden in the absence of collisions. The mercury atoms begin to emit light at a wide variety of wavelengths, including substantial amounts of visible light. That's why a high-pressure mercury lamp is a brilliant source of visible light—most of the ultraviolet light is trapped by the mercury vapor and a substantial fraction of the light emerging from the lamp is visible light.

517. How are some light emitting diodes able to emit more than one color? Can light emitting diodes emit different amounts of light or can they only be on or off?
Light emitting diodes (LEDs) that emit more than one color are actually two different LEDs connected to a single circuit in opposite directions. When current flows in one direction around that circuit, one of the LEDs emits light. When the current reverses directions, the other LED emits light. And when the current reverses directions rapidly, both LEDs emit light alternately. If one LED emits red light and the other green light, then the overall device will appear yellow or orange when they are both operating alternately in rapid sequence. The amount of light that an LED emits depends on the current flowing through it—the more electrons that are falling into holes in the p-type semiconductor, the more light that's being emitted. However, many devices that use LEDs just turn them on or off because that's easier than controlling the current flowing through them. Some day, flat panel displays may use three colors of LEDs—red, green, and blue—in order to present full color images like those on a current television screen. For that scheme to work, the LEDs must be able to emit different brightnesses, so the current flowing through each one must be adjustable.

518. How do light emitting diodes work and what is responsible for their different colors?
Light emitting diodes are diodes that have been specially designed to emit light rather than heat during their operations. Whenever current is flowing through a diode, electrons are moving from the n-type semiconductor on one side of the diode's p-n junction to the p-type semiconductor on the other side of the junction. Once an electron (which is negatively charged) arrives in the p-type semiconductor, it's attracted toward an electron hole (which is positively charged) and the two move together. The electron soon fills the hole and it releases a small amount of energy when it does. In a normal diode, electrons lose energy at a rate of 0.6 joules of energy per coulomb of charge as they recombine with the electron holes. That means that the current flowing through the normal diode loses 0.6 volts as it flows through the diode. The missing energy becomes thermal energy or heat.

But in a light emitting diode (an LED), each electron that arrives in the p-type semiconductor after crossing the p-n junction recombines with an electron hole in a remarkable way. It gives up its extra energy as light! Each time an electron and an electron hole recombine, they emit one particle of light, a photon, and the frequency, wavelength, and color of that light depends on the amount of energy given up by the electron as it falls into the electron hole. The semiconductor material from which an LED is made has a characteristic called its band gap. This band gap measures the energy needed to pull an electron away from an electron hole in the material. If this band gap is small, the LED will emit infrared light. If this band gap is larger, the LED will emit red, orange, yellow, green, or even blue light (the farther to the right in that list, the more energy is required). Because each electron loses more energy in recombining with an electron hole in an LED than it would in a normal diode, the current flowing through an LED loses more voltage (typically 2 volts for red LEDs and as much as 4 volts for blue LEDs) than does the current flowing through a regular diode (typically 0.6 volts).

Physicists, chemists, materials scientists, and engineers have been working for years to perfect the materials used in LEDs, making them more and more efficient at turning the electrons' energies into light. Until recently, there were no suitable materials from which to build blue LEDs, but recent developments of large band gap semiconductors have made blue LEDs possible. In fact, even blue laser diodes are now being made. A laser diode is a specially designed LED in which all of the photons are copies of one another rather than being emitted independently by the individual electrons as they drop into their respective electron holes.

One final note: it's now possible to obtain a "white" LED! This device is actually a blue LED, combined with a fluorescent phosphor that converts the blue light into white light.

519. How do helicopters fly with such small wings without them breaking off?
As you suggest, the blades of a helicopter are really rotating wings. But unlike the wings of a normal airplane, the helicopter blades are always moving through the air, even when the helicopter's body is not. That's why a helicopter can obtain an upward "lift" force from the air while it's hovering motionless—the wings keep moving and obtaining that lift force. A second difference between a helicopter's rotating blades and the wings of a normal aircraft is that a helicopter's blades are under enormous tension. Were it not for this tension, the end of each blade would naturally travel in a straight line at constant speed, a behavior that we associate with inertia—objects that are free of outside forces travel at constant velocity (they follow straightline paths at constant speeds). To make the end of its blade travel in a circle (which is certainly not a straight line), the helicopter must pull the end of the blade toward the pivot about which the blade is turning. Thus as the blades turn, each blade experiences an enormous tension pulls the parts of the blade toward the pivot. This tension is what stiffens the blade, just as tension stiffens the strings of a guitar or a violin. Just as it's hard to break a guitar string by bending it, it's hard to break a helicopter blade by bending it. However, both guitar strings and helicopter blades will snap if they're exposed to more tension than they can tolerate. The manufacturers of the blades work hard to make each blade strong enough to withstand the enormous tension it experiences in use. As long as the blades can tolerate this tension, they won't break and will have no trouble supporting the body of the helicopter.

520. What is the principle behind adding salt to water to keep the boiling temperature lower? Do other substances have the same effect?
Actually, it's the other way around! Adding salt or sugar (or anything else that dissolves in water and that doesn't boil easily itself) to water actually raises the water's boiling temperature! That's because the salt or sugar molecules interfere with the evaporation of water molecules and boiling is just a special type of evaporation.

Boiling occurs when the evaporation of water molecules becomes so rapid that bubbles of evaporating water molecules form inside the body of the water itself and are able to grow larger and larger, despite the crushing pressure of the surrounding atmosphere. Below water's boiling temperature, any bubble of water vapor that forms inside the body of the water will be smashed almost instantly. But at water's boiling temperature, the pressure of water vapor inside each bubble is high enough to keep the bubble from being crushed. However, adding sugar or salt to the water makes it harder for water molecules to enter one of these water vapor bubbles because the water molecules in the water cling to the salt or sugar molecules and thus don't evaporate as often. With fewer water molecules entering a water vapor bubble, that bubble can't sustain itself and is crushed. Only when you heat the salty or sugary water above the boiling temperature of pure water is there enough evaporation into each water vapor bubble to support it against atmospheric pressure.

521. Why do people put salt on icy sidewalks in the winter?
Whenever a molecule dissolves in water, the water molecules bind to that molecule and surround it, forming a shell of water molecules around the impurity. Salt water is filled with these tiny balls of water, each one surrounding a single salt ion (either a sodium positive ion or a chlorine negative ion). These little water balls can't crystallize into ice because ice can't fit a sodium ion or a chlorine ion into its orderly structure. As a result, the presence of salt in the water makes it harder for the water to crystallize into ice. The water has to exclude the salt from the crystals that form as it freezes and this difficult process requires that the salt water be cooled below the freezing temperature of pure water before it will freeze. The more salt the water contains, the lower the temperature at which that salt water will freeze. This effect even works when you just sprinkle salt on ice. As long as the temperature of the ice isn't too cold, the salt will begin to dissolve in the water molecules of the ice and ice's crystalline structure will begin to break down. The result will be a puddle of cold salty water. That's why people use salt to melt the ice on sidewalks. But if the ice is too cold, the salt will remain separate and the ice will stay pure ice. That's why salting only works when the temperature isn't too far below freezing.

522. How does Styrofoam work?
Styrofoam is a rigid foam consisting of gas trapped in the closed bubbles of polystyrene. Polystyrene itself is a clear plastic that's used in many disposable food containers. It's a stiff, amorphous solid at temperatures below 100° C, where amorphous means that it has none of the long-range order associated with crystalline solids. The long, chain-like polystyrene molecules are arranged like a tangled bowl of spaghetti noodles. Amorphous plastics tend to be clear because they're very homogeneous (uniform) internally and let light passes through them without being deflected or reflected. Plastics that are partially crystalline tend to be white. I think that items bearing the #5 recycling label are made of polystyrene.

But when air or another gas is injected into melted polystyrene and the mixture is beaten to a froth, it forms a stiff white solid when it cools. The whiteness comes about because of inhomogenieties—the gas spoils the uniformity of the plastic so that light is deflected and reflected as it passes through the material. The Styrofoam retains the rigidity of the polystyrene plastic below 100° C, so that it's suitable for beverage containers for liquids that are no hotter than boiling water. At one time, one of the gases used to make polystyrene foams was Freon, but I believe that Freon is no longer used for this purpose.

523. Who invented the microwave oven and how did he think of it?
In 1945, American engineer Percy Le Baron Spencer was working with radar equipment at Raytheon and noticed that some candy he had in his pocket had melted. Radar equipment detects objects by bouncing microwaves from them and Spencer realized that it was these microwaves that had heated the candy (as well as his body...oops!). Raytheon soon realized the potential of Spencer's discovery and began to produce the first microwave ovens: Radaranges. These early devices were large and expensive and it wasn't until 1967, when Amana, a subsidiary of Raytheon, produced the first household microwave oven, that microwave ovens became widely available.

524. Does light have mass? If so, then how can it travel at the speed of light? Doesn't the mass of an object (particle) approach infinity as its velocity approaches the speed of light?
Light has precisely zero mass and that makes all the difference. You're right that taking a massive particle up to the speed of light is impossible because doing so would, in a certain sense, give the particle an infinite mass. But the more important issue here is that doing so would require an infinite amount of energy and momentum.

Most physicists use the word mass to mean a particle's mass at rest—its rest mass—and as you bring the particle to higher and higher speeds, its rest mass doesn't change. However, the relationship between the particle's energy and its momentum does change with speed and the particle's momentum begins to increase more rapidly than it should according to the older, pre-relativistic mechanical theories. In an effort to explain this anomalous increase in momentum while retaining the old Newtonian laws of motion, people sometimes assign a fictitious "mass" to the particle; one that equals the rest mass when the particle is stationary but that increases as the particle's speed increases. As a particle approaches the speed of light, its momentum increases without limit and so does its "mass." Not surprisingly, the limitless rises in energy, momentum, and "mass" prevent the massive particle from ever reaching the speed of light.

As for light, it really does have zero mass and therefore can't be described by the Newtonian laws of motion. All light has is its momentum and its energy. In fact, light can't travel slower than the speed of light because that would require it to have a mass! So the world of particles is divided into two groups: massless particles that must travel at the speed of light and massive particles that can never travel at the speed of light.

525. Can the light from a fluorescent lamp be collimated into a beam of parallel rays?
While a converging lens or a concave mirror can always direct light from a bright source in a particular direction, the degree of collimation (the extent to which the rays become parallel) depends on how large the light source is. The smaller the light source, the better the collimation. Spotlights and movie projects use extremely bright, very small light sources to create their highly collimated beams. Since fluorescent lamps tend to be rather large and have modest surface brightnesses, I'm afraid that you would be disappointed with the best beam that you could create from that light. The ultimate collimated light source is a laser beam. In effect, the identical photons of light in a laser beam all originate from the same point in space, so that the collimated beam is as close to perfectly collimated as the nature of light waves will allow.

526. What is the difference between the magnetic and electric ballasts used in fluorescent lights?
Fluorescent lights work by sending an electric current through a vapor of mercury atoms in what is known as an electric discharge. Unfortunately, electric discharges are very unstable—they are hard to start and, once started, tend to draw more and more current until they overheat and damage their containers and power sources. Thus a fluorescent light needs some device to control the flow of current through its discharge. Since normal fluorescent lamps are powered by alternating current—that is, the current passing through the discharge stops briefly and then reverses direction 120 times each second in the United States and 100 times each second in many other countries (60 or 50 full cycles of reversal, over and back, each second respectively)—the current control device only needs to keep the current under control for about 1/120 of a second. After that the current will reverse and everything will start over.

Older style fluorescent lights use a magnetic ballast to control the current. This ballast consists essentially of a coil of wire around a core of iron. As current flows through the wire, it magnetizes the iron. Because energy is required to magnetize the iron, the presence of the iron inside the coil of wire slows down the current when it first appears in the wire by drawing energy out of that current. This effect, typical of devices known to scientists and engineers as "inductors", prevents the current passing through the ballast and then through the discharge from increasing too rapidly once it starts. The magnetic ballast is able to slow the current rise through the fluorescent lamp long enough for the alternating current to begin reversing directions. In fact, as the current in the power line begins to reverse, the ballast begins to get rid of the energy stored in its magnetized core. This energy is used to keep the discharge going longer than it would on its own. The ballast thus smoothes out the discharge so that it stays under control and emits an almost steady amount of light.

Modern electronic ballasts still control the current through the discharge, but they use electronic components to achieve this control. Just as an electronic dimmer switch can control the current through an incandescent light bulb in order to adjust the bulb's brightness, such electronic devices can control the current passing through the discharge in a fluorescent lamp to keep that current from growing dangerously large.

527. Why are there dimples on golf balls? - DM
If there were no turbulence around a golf ball as it moved through the air, there would be regions of slow-moving high-pressure air in front of it and behind it, and regions of fast-moving low-pressure air around its sides. Because of their symmetry, these pressures wouldn't exert any overall force on the golf ball and it would fly through the air without experiencing any air resistance. But there is turbulence behind a moving golf ball and this turbulence spoils the high-pressure region behind the ball. Since there is less high-pressure behind the golf ball to push it forward, the ball experiences a backward force—the slowing force of pressure drag. The size of this pressure drag force is roughly proportional to the size of the turbulent wake.

The size of the turbulent wake depends on the airflow behind the ball. On a smooth ball, air flowing into the rising pressure behind the ball experiences friction with the ball's surface and loses energy. This surface air soon reverses its direction of flow, triggering a large turbulent wake. A golf ball's dimples complicate the airflow very near the ball's surface so that new, rapidly moving air is able to flow in close to the ball's rear surface, where it can delay the onset of the flow reversal. The turbulent wake that eventually forms is relatively small, so that the golf ball experiences less pressure drag than a smooth ball. That's why a golf ball can travel so far before slowing down.

528. I've heard two explanations for how flight is achieved: (A) through lift generated by differential pressure (Bernoulli's effect) and (B) through elastic collisions between air molecules and the underside of a wing. Which is correct? How does the fact that planes can fly upside down enter into this picture?
Explanation A is entirely correct and explanation B is partly correct. If you extended explanation B to include all collisions between air molecules and the entire wing, then it would also be correct. Explanation A is the continuous fluid picture of flight and the revised explanation B is the granular fluid picture of flight. To the extent that gases are incompressible fluids (as required for Bernoulli's equation to be completely valid), these two explanations are essentially equivalent.

The lift experienced by a plane's wing depends on its shape and on its tilt or "angle of attack" into the wind. In general, wings are airfoils—curved shapes that are designed to obtain significant lift forces while experiencing minimal drag forces. Most airplane wings are more highly curved on their tops than on their bottoms and obtain upward lift forces as a result. These lift forces occur because the stable airflow that forms around such a wing involves faster-moving and thus lower-pressure air above the wing than beneath it. However, some airplane wings are symmetric—they have equal curvatures on top and on bottom. These symmetric wings compensate for their symmetry by attacking the air at an angle. When they are tipped so that their leading edges are higher than their trailing edges, these wings also experience upward lift forces. The air again flows more rapidly over than under the wings and the pressure is lower above the wings than beneath them. Even an inverted non-symmetric wing can adjust its angle of attack to obtain an upward lift force, which is how a plane can fly upside down.

In all of these cases, the forces are really exerted on the plane's wings by the impacts of countless air molecules. These air molecules hit harder and more often beneath the wings than above them and thus exert a net upward force on the plane. The fact that some wings have more surface area on their highly curved tops doesn't lead to larger downward forces because many of the collision forces exerted by molecules on the top surface of the wing cancel one another, in the same way that forces exerted on opposite sides of a sheet of paper cancel one another.

529. How does a three-way light bulb work? - AER
A three-way light bulb has two filaments inside it. One filament is smaller than the other, consuming less electricity and emitting less light. At the low light setting, only the smaller filament has current running through it and the bulb emits a dim light. At the medium light setting, only the larger filament has current running through it and the bulb emits a medium light. At the high light setting, both filaments have currents running through them and the bulb emits a bright light. To control the two filaments, the bulb has three electrical connections. The two filaments share one of the connection and each has one additional connection of its own. A complicated switch in the lamp determines whether to deliver current to one filament or the other or both. In each case, current flows toward the filament through one connection and returns from the filament through the other connection.

530. How do glaciers flow downhill? — LO and NB, Bothell, WA
Ice is a rather soft material and its crystals can deform permanently when exposed to sufficient stress. If you squeeze ice hard enough, its crystals will gradually change shape in much the same way that a copper penny will change shape if you squeeze it in a press. Since the pressures at the base of a glacier are enormous, the ice crystals there gradually deform to relieve the stress they're experiencing. This slow deformation allows the whole structure of the glacier to move gradually downhill. If ice crystals were harder, like those in most rocks, glaciers wouldn't flow. But they are very soft and so the glacier slowly flows downhill.

531. Why does white noise cancel out the wide range of frequencies in the real world? What range of frequencies does this technology affect? Can you block out the low thud of a neighbor walking in the unit above yours? — EH, Chelmsford, MA
In the context of sound, a source of white noise emits random, non-repetitive sound waves that have equal acoustic powers at all frequencies. That means that the source emits the same amount of energy each second at each frequency, over the entire audible spectrum. What white noise does is to numb your hearing by creating a featureless, uniform background noise at every frequency you can hear. Since your sensitivity to sound volume is logarithmic, meaning that the acoustic power in a sound has to double before you notice that it's substantially louder, this uniform background makes it extremely difficult for you to hear small sounds. Regardless of a small sound's frequencies, the white noise is already exposing your ears to those frequencies and the small sound only makes a small change in the volumes of these frequencies. For an analogy, think about how much more you would notice a small blinking red light in the dark than in bright white sunlight. Similarly, white noise creates the acoustic equivalent of white illumination, making it hard for you to notice small noises that would be very easy to hear against complete silence. If the sounds your neighbor makes are small enough, this numbing effect should make them much less noticeable.

There are also much more sophisticated devices that really cancel noise out. However, these look like earphones and must be worn directly on your ears. These devices use microphones to measure the pressure fluctuations in the sounds and then cause the earphones to create exactly the opposite pressure fluctuations. With these noise cancellation devices properly adjusted, the air pressure fluctuations that are sound never reach your ears at all—they are simply cancelled away to nothing before they arrive.

532. How does a tsunami form and how far does it go when it hits land? — JM, Berkley, MI
A tsunami is simply a giant surface wave on water. Surface waves have several important characteristics, one of which is wavelength—that is, the distance between one crest and the next. The longer its wavelength, the faster a surface wave moves and also the deeper it extends below the surface of the water. In general a surface wave extends downward about one wavelength, so that if the crests are 100 meters apart, the wave is about 100 meters deep.

The wavelength of a tsunami is enormous—hundreds or even thousands of meters. As a result, a tsunami travels hundreds of kilometers per hour and extends downward deep into the ocean. Because it disturbs so much water, it carries a great deal of energy and it delivers this energy to the shore when it hits. Tsunamis are normally created by earthquakes or volcanic eruptions that sudden shift the supporting surfaces of a large amount of water. The water experiences a sudden impulse when the land or seabed shifts and a wave is emitted. You can launch a similar wave simply by shaking the end of a basin of water. But when a large region of land or seabed moves, the wave that's launched has a very long wavelength and tremendous energy. This tsunami heads off with enormous speed until it encounters the gradual shallowing of a seashore. There it becomes deformed because the lack of water in front of it causes its crest to become incomplete. Eventually the tsunami breaks in churning surf. The height of this breaking wave crest and the distance it travels onto shore before it stops depends on the total energy of the tsunami, but heights of 10 or 20 meters are not uncommon. Such waves can travel hundreds of meters up a beach or oceanfront if the slope is sufficiently gradual.

533. How efficient are solar energy cells and windmills in producing energy for everyday use? — JJ, San Antonio, TX
There are several ways to measure their efficiencies. One way is to compare the energy these devices extract from sunlight or from the wind to the electric energy they produce. By that measure, solar cells are roughly 15% efficient and windmills are roughly 50% efficient. However, you're probably most interested in their cost efficiency—in how much power these devices can produce for a given operating cost. By that measure, both devices are somewhat more expensive to build and operate than conventional fossil-fuel power plants. As a result, the United States continues to rely on fossil-fuel plants because they cost less for each kilowatt-hour of electric energy produced. Nonetheless, solar cells are gradually becoming cheaper and they may become cost effective in the next decade or two. Windmills are already cost effective in some countries that rely entirely on imported fossil fuels. Denmark, for example, uses windmills extensively for electric power. While windmill power plants do exist in the United States, they are largely the results of regulation rather than market forces. But that, too, may change in the next decade or two.

534. Why does my voice sound different to me when I listen to a recording of myself?
When you hear yourself speak directly, much of the sound that reaches your ears travels to them through the bones and tissues of your head. This type of sound conduction tends to emphasize the low frequencies in your voice so that your voice sounds lower to you than it does to other people. When you listen to a recording of your voice, you are hearing your voice as other people hear it, without the modifying effects of bone and tissue conduction. Everyone else listening to the tape thinks that your voice sounds normal but you think it sounds higher than normal.

535. How do radio waves transport energy? — AD, Manaus City, Amazonia, Brazil
Radio waves consist of nothing more than electric and magnetic fields that are perpetually recreating one another as they travel through space at the speed of light. An electric field is a phenomenon that exerts forces on electric charges and a magnetic field is a phenomenon that exerts forces on magnetic poles. Both electric and magnetic fields contain energy because they are capable of doing work on and thus transferring energy to electric charges or magnetic poles that they encounter. In a radio wave, this energy or capacity to do work moves along with the fields at the speed of light. The radio transmitter uses electric power to create the radio wave and the radio wave delivers that power to the receiver. While most modern receivers use local electric power to amplify the information arriving in the radio wave, simple "crystal radios" are able to reproduce sound using on the power that is arriving in the radio wave itself.

536. How can we polarize a molecule? — AD, Manaus City, Amazonia, Brazil
Some molecules, including water, are naturally polarized. This means that they have a positively charged end and a negatively charged end. But even normally non-polar molecules such as carbon dioxide can be polarized by exposing them to strong electric fields. Electric fields exert forces on electric charges and cause the electric charges in a molecule to rearrange—the positive charges in the molecule shift in one direction and the negative charges in that molecule shift in the other. As a result of this applied electric field, the molecule acquires a polar character—a negatively charged end and positively charge end. However, this polar character disappears as soon as the electric field is removed.

537. Why do the earth's oceans appear blue to an observer on the moon?
The earth's oceans and sky both appear blue to everyone who observes them. They do this because water absorbs blue light less strongly than it absorbs other colors. When ocean water is exposed to sunlight (white light), it absorbs most of the red light quickly and a good fraction of the green light. But the blue light penetrates to considerable depth in the water and there is a reasonable chance that this light will be scattered back upward to an observer on the shore, in the air, or even on the moon.

538. What's the difference between fluorescent, phosphorescent, and triboluminescent? - DS
Fluorescence is the prompt emission of light from an atom, molecule, or solid that has extra energy. For example, when some of the dyes used in modern swimwear and clothing are exposed to ultraviolet light, they absorb the light energy and promptly reemit part of that energy as visible light—typically brilliant greens and oranges. In contrast, phosphorescence is the delayed emission of light by an atom, molecule, or solid that has extra energy. Glow-in-the-dark objects are phosphorescent—they are able to store the extra energy they obtain during exposure to light for remarkably long times before they finally release that stored energy as visible light. Systems that exhibit phosphorescence rather than fluorescent are those that have special high-energy states that have enormous difficulty radiating away energy as light. Finally, triboluminescence is the emission of light from a surface experiencing sliding friction. Since sliding friction introduces energy into the surfaces that are sliding across one another, it's possible for that energy to be emitted as light.

539. I've seen tops that rest with their large parts down but that flip up onto their handles when you spin them. What is the reason that they have a different equilibrium when they are spinning versus when they are not? — CH, Renton, WA
While I'm not an expert on these "tipple tops," I believe that I understand how they work. These tops have large round heads and look like wooden mushrooms. When you hold the handle (the mushroom's stem) and spin it with its head down, it quickly flips over so that it spins on its handle. The flipping is caused by a torque that friction exerts on the top's round head as the tops surface slides across the table. If the top were perfectly vertical as it spun on its head, friction between the top and the table would exert a torque (a twist) on the top that would simply slow the top's rotation. But when the top isn't perfectly vertical, the torque that friction exerts on it does more than slow its rotation. This torque also causes the top to precess (change its axis of rotation) in such a way that the top's handle gradually becomes lower and the top's head gradually becomes higher. Eventually, the top's axis of rotation inverts completely so that it begins to rotate on its handle. Once that happens, the precession stops because the handle is too narrow for anything but the slowing effects. Only when the top stops spinning does it shift from this dynamically stable arrangement (handle down) to its statically stable arrangement (head down).

540. What is a microwave and what does it do? — AH, Rochester, MN
A microwave is an electromagnetic wave with a frequency and a wavelength that are intermediate between those of a radio wave and those of light. An electromagnetic wave consists of both an electric field and a magnetic field. These two fields travel together in space and perpetually recreate one another as the wave moves forward at the speed of light. An electric field is a phenomenon that exerts forces on electric charges, while a magnetic field is a phenomenon that exerts forces on magnetic poles. Electric and magnetic fields are intimately connected, so that whenever an electric field changes, it creates a magnetic field and whenever a magnetic field changes, it creates an electric field. By combining a changing electric field and a changing magnetic field, the electromagnetic wave uses their abilities to create one another to form a self-perpetuating entity—the wave's changing electric field creates its changing magnetic field and its changing magnetic field creates its changing electric field.

If you were to freeze an electromagnetic wave at one instant and look at its structure in space, you would find that its electric and magnetic fields had a periodic spatial structure. Just as a water wave has crests and troughs, an electromagnetic wave has spatial fluctuations in its two fields. The distance between adjacent "crests" in either one of these fields is that wave's wavelength. Different types of electromagnetic waves have different wavelengths. Radio waves have long wavelengths that range from about 1 meter to hundreds or even thousands of meters and visible light has short wavelengths that range from about 400 billionths of a meter (400 nanometers) to about 750 billionths of a meter (750 nanometers). Microwaves are those electromagnetic waves with wavelengths between 1 millimeter and 1 meter. The microwaves used in microwave cooking have wavelengths of 12.2 centimeters.

Microwaves are often used to carry information in satellite communication and telephone microwave links. Whenever you see a dish antenna (a satellite dish or a communication link dish on a building or tower), you are looking at a microwave system. Astronomers use radio telescopes to look at microwave emissions from celestial objects. Radar bounces microwaves from objects to determine where they are or how fast they're moving. And microwave ovens use microwaves to add thermal energy to water molecules in order to heat food.

541. How much natural pressure is around us when we are on the ground? Does this pressure decrease in higher places? Why don't people in aircraft explode because the pressure is lower?
Near sea level, the air around us has a pressure of about 100,000 newtons per square meter or 15 pounds per square inch. That means that each square meter of surface on your body is exposed to an inward force of 100,000 newtons or that each square inch of your body is exposed to an inward force of 15 pounds. Your body is thus exposed to enormous inward forces. However, you don't notice these forces because your body is composed of solids and liquids that resist compression ferociously. To see that this is so, try to squeeze a sealed bottle of soda or to squash a coin by stepping on it. It's very hard to shrink the volume of a solid or liquid by squeezing it.

The origin of the large pressure around us is the weight of the atmosphere overhead. The air near you is supporting the weight of several miles or kilometers of air overhead and the weight of this air is squeezing the air down here. When you ascend a mountain, the amount of air overhead decreases and so does the pressure of the air around you. Your body becomes less tightly squeezed by the air around it. However, you don't explode because releasing the pressure on you doesn't change your volume very much. Solids and liquids don't expand very much when the pressure on them is released.

542. What does the SPF on sun screens mean? - RC
Sunscreens contain pigments that absorb invisible ultraviolet radiation. While they appear clear and transmit visible light so that you can't see them when they're on your skin, sunscreens are almost opaque to ultraviolet light. A sunscreen's SPF is related to the fraction of ultraviolet light that it absorbs. An SPF of 15 means that a normal layer of this sunscreen on your skin transmits only 1 part in 15 of the ultraviolet light that reaches it from the sun. An SPF of 40 means that a layer of this sunscreen transmits only 1 part in 40 of the ultraviolet light. The true transmission of the sunscreen depends somewhat on how you apply it and how much you apply, so these SPF ratings are only approximate. A sunscreen contains a mixture of dye molecules that transmit visible light but absorb ultraviolet light (and convert its the light's energy into thermal energy). Most if not all of these dye molecules are artificial organic compounds that have been carefully selected to be non-toxic and non-irritating. The first popular sunscreen contained a compound called PABA that caused skin reactions in many people, but more recent dye choices are less likely to cause skin trouble.

543. If airplane cabins are pressurized to provide adequate oxygen for the passengers to breathe, what provides this compressed air? - EL
The air that you breathe inside an airplane is actually pumped into the cabin through the jet engines. The first component of a jet engine is a compressor that takes the low-density air outside and boosts its pressure and density. While most of this air then continues through the engine to the combustion chamber, part of it is diverted to the cabin. But before it can be released into the cabin, the air must be chilled by an air conditioner. That's because compressing air adds energy to it and raises its temperature. The compressed air leaving the jet engine's compressor is hot, even though no combustion has taken place yet. So the air is first cooled and then sent into the cabin.

544. What is a shockwave and a sonic boom? - EL
A plane that is flying faster than the speed of sound is outrunning its own sound. As a result, its sound spreads out behind it as a conical structure, with the plane located at the apex of that cone. This cone moves along with the plane. Since the planes sound is all contained inside the cone, you can't hear the plane until the cone passes by you. When the edge of the cone does pass you, you hear a great deal of sound all at once. In fact, there is a pressure jump right at the surface of the cone (sound and pressure are closely related) and this cone itself is a shockwave. As the shockwave (or cone surface) passes you, you hear a loud booming sound, a "sonic boom". Note that the sonic boom occurs when the shockwave passes your ears, not when the plane "breaks the sound barrier". When you hear the sonic boom depends on where you are relative to the moving plane, so different people hear it at different times.

545. What is the "sound barrier"? - EL
The "sound barrier" is more a psychological barrier than a real impediment. In the early days of high-speed flight, there was concern that a plane flying at or beyond the speed of sound in air would encounter unanticipated phenomena that would rip it apart. However, when Chuck Yeager finally did exceed the speed of sound for the first time in 1947, he found the transition from subsonic to supersonic uneventful. The only way that he could tell he was traveling faster than the speed of sound was with the help of his instruments.

546. Does the volume in the cooking chamber of a microwave oven affect the rate at which it cooks the food? In other words, which cooks faster, a small microwave oven or a large one? - RP
The size of a microwave oven's cooking chamber should have little or no effect on how quickly it cooks food. The oven's magnetron tube delivers a certain amount of microwave power to the cooking chamber and virtually all of that power will eventually be absorbed in the food. It may take a few moments longer for a large cooking chamber to fill with microwaves when you first start the oven, but soon the food inside it will be exposed to the same intensity of microwaves as food cooking inside a smaller microwave oven with a similar magnetron power.

On the other hand, the magnetron's power does affect cooking speed so that an oven with a more powerful magnetron will cook food faster than one with a less powerful magnetron. The speed of cooking in a microwave oven also depends on how much food it contains because the food shares the microwave power. In general, doubling the amount of food in the microwave doubles the cooking time.

547. How do light sticks work? - AE
When you bend a plastic light stick, you break a small glass ampoule and allow two chemicals that are contained inside the stick to mix. One of these chemicals is a powerful oxidizing agent and the other is a chemical that when oxidized ("burned") is left in an electronically excited state. In other words, the chemical reaction between the molecules of the two chemicals creates a new molecule that has excess energy in it. The molecule releases this energy as a particle of light, a photon. Although I am not certain exactly which chemicals are used in a modern light stick, I believe that one is hydrogen peroxide (the oxidizer) and the other is luminol (the chemical that is oxidized). Upon oxidization, luminol emits a photon of blue or ultraviolet light. The green light that you see emerging from a typical light stick is actually a second photon that is emitted by a fluorescent dye contained in the light stick. This dye absorbs the blue or ultraviolet photon emitted by the luminol and then reemits a new photon with somewhat less energy and a green color.

548. Can a compound have triple bonds? If so, please give an example. — BA, IL
Yes, some compounds contain triple bonds. Acetylene is the simplest such molecule, with two carbon atoms connected by a triple bond. Each carbon atom has one hydrogen atom attached to it, so the entire molecule is a four-atom chain: hydrogen-carbon-carbon-hydrogen. The triple bond between carbon atoms is extremely strong—the atoms are sharing 6 electrons between them.

549. What is red mercury, where does it come from, and where is it used?
Mercuric sulfide, a red mineral known as cinnabar, is the world's principal source of mercury and has also been used frequently as a vermilion paint pigment. It has been mined in Almaden, Spain for several thousand years and occurs in a number of other countries, including the United States.

550. How does a light bulb work? — DH, Casselberry, FL (and also KH)
In a common incandescent light bulb, an electric current flows through a double-spiral coil of very thin tungsten wire. As the electric charges in the current flow through this tungsten filament, they collide periodically with the tungsten atoms and transfer energy to those tungsten atoms. The current gives up its energy to the tungsten filament and the filament's temperature rises to about 2500° C. While all objects emit thermal radiation, very hot objects emit some of the thermal radiation as visible light. A 2500° C object emits about 12% of its heat as visible light and this is the light that you see coming from the bulb. Most of the remaining heat emerges from the bulb as invisible infrared light or "heat" light. The glass enclosure shields the filament from oxygen because tungsten burns in air.

551. How do you boil ice water? (I think it has something to do with a vacuum.) - MW
You're right, it does have to do with a vacuum. While water molecules can evaporate from the surface of liquid water at almost any temperature, boiling can only occur when the evaporation rate is high enough to support the appearance of evaporation bubbles inside the body of the liquid water. Normally, atmospheric pressure pushes inward on cold water so hard that any evaporation bubble that appears inside the water is immediately crushed out of existence. But in water that's at 100° C, evaporation is so rapid that the evaporation bubbles in the liquid water can survive and grow, despite the crushing inward forces of atmospheric pressure. The hot water boils.

Water boils not because it's hot but because any evaporation bubble that forms inside it is able to survive and grow despite the surrounding atmospheric pressure. At normal atmospheric pressure, the water does have to be hot for this to happen. But if you remove the atmospheric pressure, the water can boil at much lower temperatures. In fact, at sufficiently low pressures, even ice water will boil! It's funny to see ice cubes floating in a container of boiling water, but it happens when you remove the air from around the ice water.

552. Can we add a section to a microwave oven that gets the food or drinks cold? - MH
Not without adding a full-blown refrigerator. While it's relatively easy to add thermal energy to food or drink, it's much harder to remove that thermal energy. Since energy is conserved, the thermal energy that you remove from the food must be transferred elsewhere. Since heat (moving thermal energy) normally flows from a hotter object to a colder object, you must make something colder than the food before the heat will leave the food. While it's possible to cool an object to a temperature lower than its surroundings, this cooling process requires a heat pump, a device that actively pumps heat from a cold object to a hot object (against its natural direction of flow). A refrigerator is such a heat pump.

553. Are there any risks, other than a case of implosion, with regards to exposure to normal fluorescent lighting? - RR
While the phosphors in fluorescent lamps are not considered to be toxic, they do contain a tiny amount of mercury. This mercury is an essential part of the operation of the lamp (it is what creates the initial light during the electric discharge). While most fluorescent lamps are simply discarded into landfill, some facilities (including the University of Virginia) dispose of them more carefully. The University of Virginia breaks the lamps to collect the phosphors and then distills the mercury out of the phosphors. The phosphors are then entirely non-hazardous and the mercury is recycled.

554. How do I figure out how much energy is used to heat the water in our gas hot water heater? I know that 1 BTU is the energy to heat 1 lb. of water 1°. Do I figure out how many gallons in 1 lb. of water; and then multiply that by the difference in room temperature and 140°? — JH, Maple Grove, MN
Yes. A gallon of water weighs about 8.3 pounds, so a typical 40-gallon hot water heater tank holds 332 pounds of water. To raise that water from its delivery temperature (about 60° F) to its final temperature (about 140° F) takes about 26,560 BTUs.

555. Is there any substance that can stop magnetic fields — K, Mendenhall, MS
Magnetic fields are related to what are call magnetic flux lines. These magnetic flux lines extend unbroken from north magnetic poles to south magnetic poles. Where the flux lines are close together, the magnetic field is strong. Thus to avoid magnetic fields, you need to keep magnetic flux lines away. Because magnetic flux lines can't be broken, they can't simply be made to disappear. To "stop" a magnetic field in a particular region of space, you have to either terminate the flux lines at a magnetic pole or you have to divert the flux lines away the region that you're interested in. The first strategy has a problem: no isolated magnetic poles (so-called "magnetic monopoles") have ever been found. That means that every north pole you find has a south pole attached to it. Thus you can't simply end the flux lines with magnetic poles because for each flux line you end with a south pole, you'll start a new one with the attached north pole. But the second strategy is reasonable. There are many materials that divert magnetic flux lines. One of the most important of these is a metal called "mu metal," an alloy that's made from nickel, iron, chromium, and copper. Mu metal attracts flux lines. It draws flux lines through itself so that if you were to wrap yourself in a layer of mu metal, any magnetic flux lines that would have gone through you (and thus exposed you to magnetic fields) will go through the mu metal instead. Mu metal and similar alloys are used routinely to shield objects that can't tolerate magnetic fields.

556. How do you calculate total speaker impedance? For example, 4 speakers wired in series or parallel. Is there a formula? — PV, Atlanta, GA
You can calculate the impedance of a collection of speakers the same way you would calculate the resistance of a collection of resistors. Each time two speakers are connected in series, so that the electric current must pass through one and then the other to get to its destination, their impedances add. Thus two 4-ohm speakers in series are equivalent to one 8-ohm speaker (4 ohm + 4 ohm = 8 ohm). Each time two speakers are connected in parallel, so that the electric current can pass through one or the other to get to its destination, the reciprocals of their impedances add to give the reciprocal of their overall impedance. Thus two 4-ohm speakers in parallel are equivalent to one 2-ohm speaker (1/4 ohm + 1/4 ohm = 1/2 ohm). Once you have figured out the impedance of a pair of speakers, you can treat it as though it were one speaker and proceed to figure out the impedance of a larger group of speakers. For example, four 4-ohm speakers in series have an overall impedance of 16 ohms and four 4-ohm speakers in parallel have an overall impedance of 1 ohm.

557. What are some everyday examples of friction? (For example, we couldn't walk without friction.)
Before giving some examples, I'll note that there are two different types of friction. First, there's the static friction between two surfaces that are pressed together but are not sliding across one another. Second, there's the sliding or dynamic friction between two surfaces that are moving across one another. Static friction allows objects to push one another sideways but doesn't create thermal energy. Sliding friction also creates thermal energy (or heat).

Your example of walking is a case of static friction: your feet push backward on the sidewalk and the sidewalk reacts by pushing your feet (and you) forward. As further examples of static friction: holding a pencil, screwing in a light bulb, pulling a rope toward you hand over hand, pedaling a bicycle so that the ground pushes the wheel forward, keeping the dishes and silverware from blowing off a level picnic table on a windy day...

As examples of sliding friction: skidding the wheels of a automobile during a rapid start or stop, sliding down the pole in a fire station, skiing or skating, squeezing a bicycle's caliper brakes against the wheel rims, shaping metal with a grinding wheel, sharpening a knife, sanding a wooden desktop...

558. How do the different jet engines work on aircraft—the turbojet, the turbofan, and the turboprop? — KB, Charlottesville, VA
All three engines start with a turbojet engine. In a turbojet engine, a stream of air is first compressed by a rotary compressor. The air is then mixed with fuel and the mixture is burned. Finally, the hot burned gases are allowed to expand through a rotating turbine and they flow out of the back of the engine at very high speed.

To understand how all of this works, let's follow the flow of energy through the turbojet engine. Assuming the plane is moving forward, the air is moving fast when it encounters the engine's inlet duct. This inlet duct slows the air down substantially and the change in its speed causes the air's pressure to rise—an effect observed by Bernoulli. The air's energy doesn't change, but its kinetic energy (energy of motion) is partially converted to pressure potential energy. The now pressurized air is further pressurized by its passage through the rotary compressor at the front of the turbojet. The compression process adds energy to the air by doing mechanical work on that air. Now fuel is added to the high-pressure air and the mixture is burned. This combustion adds an enormous amount of energy to the air. The exhaust gases immediately expand and their speeds increase substantially as they pour out of the combustion chamber. These gases flow through a rotating turbine on their way out of the back of the engine. Even though the gases do work on the turbine, they still have lots of energy and flow out of the jet engine at a much greater speed than the air had when it arrived. Much of the fuel's chemical potential energy has become kinetic energy in these exhaust gases. The turbine provides the mechanical work that operates the rotary compressor, or the fan of a turbofan or the propeller of a turboprop. Overall, the exhaust gases leave the turbojet engine traveling faster than the air did when it arrived. Since the gases carry backward momentum with them as they leave the engine, they have evidently pushed the engine forward to give the engine and the plane forward momentum.

That's all there is to a turbojet engine. A turbofan engine uses the mechanical work from an enlarged turbine to operate a large fan that's in front of the turbojet engine itself. This fan takes air that has slowed down on entry into the jet's inlet duct and adds energy to this air. The air then speeds up as it flows out the jet's outlet duct and the air leaves the engine traveling faster than when it arrived. Once again, the engine experiences a forward thrust force as it pushes this air backward.

A turboprop engine uses mechanical work from an enlarged turbine to operate a propeller. The propeller pushes air flowing past the engine backward and the air pushes the engine and airplane forward. Because there is no duct around the propeller blades, the air passes the blades at full speed (a turbofan engine uses its duct to slow the air down before pushing on the air with its fan blades).

559. If you were in freefall from a jet airplane, would the airplane overtake you in the fall (assuming that the plane started freefalling as you jumped out the door)?
It would overtake you immediately. Airplanes are designed to experience extremely small drag forces and are remarkably aerodynamic as a result. In contrast, you would experience severe air drag (air resistance) once you left the plane. The plane would coast past you at high speed while you would slow enormously in the first second or two of exposure to the air.

560. What is a convection oven and what are its advantages?
The main mechanism by which heat is transferred to food in a normal oven is convection. In this mechanism, air heated by the gas or electric burner at the bottom of the oven rises because of buoyant forces (i.e., hot air rises) and carries heat to the food. But natural convection is slow and imperfect—if you overfill the oven, you block convection and the food cooks unevenly. In a convection oven, a fan stirs the air rapidly. Heat flows quickly and evenly from the burner to the food. Cooking occurs more quickly and you can also put more food in the oven without danger of uneven cooking.

561. What is the purpose of the grid on the glass door of the microwave oven?
The metal grid reflects microwaves and keeps them inside the oven. Electromagnetic waves are unable to pass through holes in conducting materials if those holes are significantly smaller than their wavelengths. The wavelengths of visible light are very short, so light has no trouble passing through the holes in this grid. But the microwaves used in the oven have wavelengths of about 12.4 cm and are unable to propagate through the grid. Thus you can see the food cook while the microwaves are trapped inside the oven.

562. At what angles do light rays reflect out of a prism? — BC, Farmersville, TX
It depends on the shape of the prism and the angle at which the light arrived at the prism. Whenever light's speed changes as it passes through a surface at an angle, the light bends. Since light travels faster in air than in glass (or plastic), it bends when it goes from air to glass or from glass to air. When light enters glass, it slows down and it bends toward the normal to the surface (toward the line that's at right angles to the surface). When light leaves glass, it speeds up and it bends away from the normal to the surface. To know exactly how far the light bends, you need to know how much the glass slows light (the glass's refractive index) and the angle at which the light encountered the glass surface (the angle of incidence). You can then use one of the basic laws of optics, Snell's law, to determine the angle at which the light continues through the glass. You can then do the same for the light's emergence from the glass and determine the angle at which it leaves.

563. In a CD player, how is the digital optical signal transformed into an electrical signal? — IM, Oxford, UK
The ridges and flat regions on a compact disc's aluminum layer determine how laser light is reflected from that layer. As the disc turns and the player's laser scans across ridges and flat regions, the intensity of the reflected light fluctuates up and down. This reflected light is directed onto an array of silicon photodiodes that provide both the signals needed to keep the laser focused tightly on the aluminum layer and the signal that the player uses to recreate sound. The sound is encoded in the lengths of the ridges. A computer monitors the amount of light returning from the disc to determine how long each ridge is and how much spacing there is between it and the next ridge. The computer uses this information to obtain a series of 16 bit binary numbers for each of the two sound channels that are represented by an audio CD. A digital-to-analog converter uses these 16 bit numbers to produce currents that are eventually amplified and used to produce sound.

564. How does hair spray work? — KC, IL
While I don't know exactly what chemicals are used in hairspray, the main constituents are almost certainly polymer molecules—otherwise known as plastics. In the container, these polymer molecules are dissolved in a volatile solvent such as an alcohol or water, and pressurized with a chemical such as propane or a hydrofluorocarbon. When you spray the mixture onto your hair, the solvent evaporates and leaves the polymer molecules clinging to the hairs. These molecules are very long chains of atoms that form a stiff web around each hair and stiffen it. In general, the characteristics of polymers change with temperature and chemical environment. The polymer used in hairspray should be in the "glassy" regime, meaning that its atoms and molecules are essentially immobile at room temperature. Once the solvent is gone, the web of polymer molecules on the hairs is stiff and keeps the hairs from changing shape. Before you panic at the idea of spraying plastic onto your hair, consider that starch is also a polymer, as is hair itself. So putting hairspray on your hair is no different from putting starch on clothes.

565. How does a turbine flow meter work?
There are many different types of flow meters, some specialized to handling gases and others to handling liquids. In each case, a true flow meter transfers gas from its inlet to its outlet one unit of volume at a time and it measures how many of those volumes it transfers. There are also some flow rate meters that measure how quickly a gas or liquid is flowing. These devices normally use of turbines to measure the speed of the passing fluid and measurements from these flow rate meters can be integrated over time to determine how much gas or liquid has passed through them. However, because flow rate meters don't measure each volume of gas directly, they aren't as accurate as true flow meters.

Let me assume that you want to know about a turbine flow meter for gas. The most common of these is a device that's half filled with liquid. The "turbine" is actually a set of blades that spin in a vertical plane and spend half their times immersed in the liquid. When one of the turning blades emerges from the liquid, the empty space that appears beneath it is allowed to fill with the gas being measured. This gas flows in from the meter's inlet. Soon another blade begins to emerge from the liquid and a volume of gas is then trapped between the first blade and the second blade. Once the blades have turned almost half a turn, the first one begins to submerge again in the liquid. The gas that was trapped between it and the next blade is then squeezed out from between those blades by the liquid and flows out the meter's outlet. A geared arrangement measures how many turns the blades make and therefore how many volumes of gas have been transferred from the meter's inlet to its outlet.

566. Years ago I heard or read that some incandescent bulbs in Thomas Edison's house are still burning after being turned on back early in the 20th century. Is this true? What are they made of?
From comments that I've received over the web, this story is apparently true. However those bulbs must be operating at reduced power levels and are glowing dimly as a result. There is no magic filament material that can operate indefinitely at yellow-white heat. The life of a filament is determined by how quickly its atoms evaporate (actually sublime) from its surface. Modern tungsten filaments operate at about 2500° C. At that temperature, the filament loses atoms slowly enough that it lives for about 1000 hours. If you were to operate the filament several hundred degrees colder, it would live much, much longer but it wouldn't emit nearly as much light and what light it did emit would be relatively reddish. The design of incandescent bulbs is a trade-off of energy efficiency and operating life. Long-life bulbs are substantially less energy efficient than normal bulbs—you don't have to replace them as often but they cost more to operate. Getting back to Edison's bulbs: they can only live long lives by operating at less than normal temperatures. In that case, they may live a hundred years but have very poor energy efficiencies.

567. Does hot water really freeze quicker than water at room temperature? — MH, Dallas, TX
In most cases the answer is no. All things being equal, the room temperature water will have a head start and will freeze first. The hot water must first cool down to room temperature and then it will simply follow the behavior of the room temperature water. However, in the special case where the water is held in an insulated container that's open at the top, it's possible for hot water to freeze faster. That's because evaporation of water molecules from the exposed surface of the hot water makes an important contribution to the cooling process in that case and a significant fraction of the water molecules will have left the container by the time it reaches room temperature. Since it takes less time to freeze a smaller quantity of water, the container of hot water can freeze before the container of room temperature water. However, there will be less ice in the container that was once filled with hot water.

There is another interesting effect that occurs when freezing hot water. If you boil water, you will drive most of the dissolved gases out of it. You see these gases emerge as bubbles on the sides of a pot as the water heats up when you put the pot on the stove. If you freeze boiled water, it will probably freeze slightly faster than unboiled water. That's because the dissolved gases also come out of solution during the freezing process and these gases form bubbles in the ice. These bubbles slow the flow of heat through the ice and delay the freezing of its center. Thus, while room temperature water will freeze quicker than hot water, previously boiled water that's now at room temperature will freeze even quicker than normal room temperature water. The boiled water will also form clearer ice cubes—they won't have any bubbles in them.

Finally, John Newell points out an interesting practical reason why hot water may sometimes freeze faster than cooler water in a household refrigerator—the temperatures of those refrigerators fluctuate because their thermostats have hysteresis. Once it has stopped operating, a refrigerator's compressor won't turn on again until the refrigerator temperature drifts upward significantly. If you put cool water in the refrigerator's freezing compartment, it may be quite a while before the compressor turns on and the refrigerator begins to pump heat out of the freezing compartment. But if you put hot water in the compartment, you may raise the temperature of the refrigerator enough to start the compressor, thus accelerating the freezing of the water.

568. How can you make a makeshift water distiller with common materials? - VL
To distill water, you need to condense steam on a cold surface and collect the condensate. You could boil water in a teapot and allow the steam flowing out of its mouth to pass across the underside of a clean metal bowl full of ice water. The steam would condense as pure distilled water on the outside of the bowl. If you then placed a clean cup under the metal bowl, the distilled water would drip into that cup.

569. Were microwaves invented for the microwave oven?
While microwaves were known long before anyone know how to produce them efficiently, they became important during World War II as the basis for radar. The ability to detect and locate enemy aircraft at long distances and at night was crucial to the defense of Allied cities during the war. The 1945 discovery that microwaves also cooked food was an accidental offshoot of radar development.

570. Who invented the telephone dial or rotary portion of the telephone? — B, R, B, D, and S at Northlake Elementary, Richardson, TX
Let me start a little earlier, with the automatic telephone exchange: This exchange was invented in 1892 by Alman B. Strowger, an undertaker from Kansas, who first installed it in La Poste, Indiana. The system used electromagnetic relays to recognized a series of pulses and to make the appropriate connections between telephones. While the "Strowger system" remained in use until the advent of modern electronic switching systems, it was improved many times. The pulses that controlled this system were originally made with push buttons and one of the most important improvements was to replace the push buttons with a rotary dial that created the pulses automatically. However, the rotary dial wasn't so much invented as developed and I haven't found any record of the individuals who contributed to that development. No doubt it's a patented device and the patent record probably includes the names of the people responsible. If I can find that patent, I'll add it here.

571. Could you slow down the molecules to cool food quickly instead of heating it up?
Heat naturally flows from hotter objects to colder objects. As a result, you can heat food by putting it in hotter surroundings and cool food by putting it in colder surroundings. However, you can also heat food by converting an ordered form of energy into thermal energy, right inside the food. For example, microwaves can penetrate the food and their energy can become thermal energy inside the food, speeding up the cooking process.

However, there is no analogous way to reach inside the food and extract its thermal energy. You must wait for the thermal energy inside the food to drift to its surface and to be transferred to the colder surroundings. This requirement is the result of the laws of thermodynamics, which govern the interconversions of work and heat. While it's easy to turn mechanical work into heat (just rub your hands together), it's very difficult to turn heat into work. Because of this difficulty, thermal energy must usually be transferred elsewhere. You can't build a "microwave refrigerator" that turns thermal energy into microwaves inside the food.

572. How can glass be shattered with sound? — JI, Rapid City, SD
When sound shatters glass, it breaks the glass in the usual way: by distorting the glass to its breaking point. Whenever glass is bent too far, a crack propagates into the glass from its surface (usually at a defect) and the glass tears. For sound to cause this tearing process, the sound must distort the glass substantially. An extremely loud sound can distort the glass to its breaking point in a single motion. For example, an explosion shatters windows when a surge in air pressure (which you hear as a very loud "pop" sound) exerts so much force on those windows that they bend and break.

However, a moderately loud tone can also break certain glass objects by pushing on those objects rhythmically until they distort beyond their breaking points. To understand how that's possible, recall that you can get a child swinging strongly on a playground swing either by giving the child one hard push or by giving the child many carefully timed gentle pushes. The gentle pushes transfer energy to the child via a mechanism called resonant energy transfer—the child is exhibiting a natural resonance and you are using that resonance to transfer energy to the child a little bit at a time.

While most glass objects exhibit only very weak natural resonances and are therefore extremely difficult to break via resonant energy transfer, a good crystal wineglass is resonant enough to be broken by a loud tone. You can hear the appropriate tone by flicking the wineglass with your finger. If the wineglass emits a clear bell-like tone, you will be able to break that wineglass by exposing the wineglass to a loud version of that same tone. When the wineglass is exposed to this tone, it begins to vibrate in its natural resonance. Each rise and fall in air pressure associated with the tone adds energy to the vibrating wineglass until its surface is distorting wildly. If the tone is loud enough and its pitch is exactly right, the wineglass will distort a remarkable amount and it may shatter. I know from experience with this effect that the distortion a crystal wineglass can undergo without shattering is amazing—it usually won't break until it's upper lip is almost as oval-shaped as an egg. Finding the right tone and holding that tone accurately enough and loudly enough requires sophisticated equipment. Few humans have any chance of breaking a wineglass because the pitch accuracy and volume needed are beyond the abilities of all but the most remarkable opera singers. However, Enrico Caruso was apparently able to do this trick with a wineglass held directly in front of his mouth. Note also that normal window glass and normal drinking glasses are made from soft forms of glass that exhibit no strong resonances—if you tap them, you hear only a dull "thunk" sound, not a bell-like tone. As a result, you can't break them with tones.

573. How are light and sound the same? How are they different? — JS, Binghamton, NY
There are so many answers to these questions that I'll have to pick and choose. For their similarities, I'll note that they're both disturbances that travel through space and that both have wavelengths and frequencies. Sound is a pressure disturbance in the air (or in another material) and consists of compressions and rarefactions that travel outward from their origin. The distance between adjacent regions of compression (or rarefaction) is the sound's wavelength and the number of compressed regions that pass by a particular point each second is the sound's frequency (or pitch). Light is an electromagnetic disturbance in space itself, although materials that are present in that space can alter its characteristics somewhat. It consists of electric and magnetic fields that travel outward as waves from their origin. The distance between adjacent regions of maximum electric field (or magnetic field) in one direction is the light's wavelength and the number of regions in which the electric field points maximally in a particular direction that pass by a particular point each second is the light's frequency (or color). I hope that you can see some of the similarities in these descriptions.

As for differences, sound is a longitudinal wave—meaning that the air involved in the pressure fluctuations moves back and forth in the direction of the wave's travel. Thus if sound is moving from left to right, the air is also fluctuating back and forth from left to right. In contrast, light is a transverse wave—meaning, that the electric and magnetic fields involved in the wave fluctuate back and forth at right angles to the direction of the wave's travel. Thus if light is moving from left to right, the electric and magnetic fields associated with it are fluctuating either up and down or toward you and away from you (or both). Another difference is that sound travels about 300 meters per second and its speed depends on the speed of the air through which it travels. Light, on the other hand, travels about 300,000 kilometers per second and its speed in vacuum (empty space) is absolutely constant. The speed of light is one of the fundamental constants of the universe.

574. How do some paints and stickers glow in the dark? — DD, Sandy, UT
Glow in the dark paints and materials contain molecules that are able to store energy for long periods of time and then release that energy as light. To understand how this delayed emission works, let's examine the interactions of molecules and light. The electrons in any molecule are normally arranged in what is called the "electronic ground state," an arrangement that gives those electrons the least possible energy. However, the electrons in a molecule can also be arranged in one of many "electronically excited state," in which they have more than the minimum energy. Whenever a molecule is exposed to light, its electrons may rearrange and the molecule may find itself in one of the electronically excited states. If that occurs, the molecule will have absorbed a particle of the light, a "photon," and used the photon's energy to rearrange its electrons.

In a typical molecule, the extra energy is released almost immediately, either as light or as the vibrational energy that we associate with heat. But in a few special molecules, this extra energy can become trapped in the molecule. When an electron shifts from one arrangement to another and the total energy of that molecule decreases, the missing energy may leave as a photon of light. But electrons behave as though they were spinning objects and in shifting between arrangements, the electron normally can't change the direction of its spin. In most rearrangements that lead to the emission of light, the electron spins remain unchanged.

However, a glow in the dark molecule is one in which there is an electronically excited state that can only shift to the ground state if one of the electrons changes its direction of spin as the photon of light is being produced. In some molecules, this process is almost totally forbidden by the laws of physics and proceeds so slowly that the molecule may wait for minutes, hours, or even days before it emits the photon and returns to its ground state. When you expose a material containing these molecules to light, its molecules become trapped in these special electronically excited states and they then glow in the dark for a long while afterward.

575. How does a paper airplane fly? — CL, South Bend, IN
A paper airplane flies for roughly the same reason that a normal airplane flies: the air pressure below its wings is somewhat higher than the air pressure above its wings. As a result of this pressure difference, the paper airplane experiences an overall upward force due to air pressure and this upward force is strong enough to balance the airplane's downward weight.

In a paper airplane, the most important effect is a rise in pressure below the wings. To understand why this pressure rise occurs, think about the movement of air from the perspective of a bug that's riding on the airplane. To the bug, the air is flowing toward the front of the airplane. As this stream of air encounters the undersurface of the wing, the air slows down. You can think of this air as hitting a slanted wall. Whenever a moving stream of air slows down, its pressure rises. You experience this pressure rise when you hold your hand out of the window of a moving car and feel the slowing air push your hand toward the back of the car.

The dynamics of the air above the wing is more complicated and depends on the design of the wing. But in any case, the air above the wing doesn't slow down and its pressure never rises above atmospheric pressure. In a well-designed wing, it actually drops below atmospheric pressure! Since the air pressure rises under the paper airplane's wing and doesn't rise above the airplane's wing, the wing experiences an upward force due to pressure. It's this upward force that supports the airplane.

576. Why is it harder to balance on a stationary motorbike compared to a moving one? Is it a gyroscopic effect on the wheels? — DF, Morley Perth, Australia
As you suspect, gyroscopic effects do play a role. Because it has only two wheels, a motorbike is inherently unstable. When it's stationary, it is only in equilibrium—that is it experiences no net force or torque—when it's perfectly upright. The slightest tip causes it to fall over. You must be very careful and agile to keep it balanced. A physicist would say that the motorbike is statically unstable or that it has an unstable static equilibrium.

For the motorbike to remain upright, you must keep the overall center of gravity (yours and the motorbike's) directly above the wheels (actually the line formed by their contact points on the ground). That's very hard to do when the motorbike is stationary. But when the motorbike is heading forward, it naturally steers itself under the center of gravity. If the motorbike begins to tip to one side, its front wheel automatically steers in the direction of the tip and the forward moving motorbike soon drives its wheels back under the center of gravity. This automatic steering is due to both gyroscopic precession in its spinning front wheel and to the shape and angle of the front wheel fork. If you hold the motorbike (or a bicycle) off the ground, spin its front wheel the right direction, and then tip the motorbike, you'll see its wheel turn toward the direction of the tip because of gyroscopic precession. If you return the motorbike to the ground and then tip it to one side, you'll see that its wheel will automatically turn toward that side because of the fork shape.

With both effects helping the motorbike steer under the center of gravity, the moving motorbike is very stable. A physicist would say that it is dynamically stable. Everything I've said also applies to bicycles and was pointed out by British physicist David Jones in 1970. Bicycles are so dynamically stable that almost anyone can ride them without hands and not tip over!

577. What are the different types of light bulbs and how do they work? - BS
An incandescent light bulb works by heating a solid filament so hot that the filament's thermal radiation spectrum includes large amounts of visible light. A fluorescent tube uses an electric discharge in mercury vapor to produce ultraviolet light, which is then transformed into visible light by fluorescent phosphors on the inner surface of the tube. A gas discharge lamp uses an electric discharge in a gas inside that lamp (often high pressure mercury, or sodium vapor, or even neon) to produce visible light directly.

578. How does a steam engine work? — MP, New Fairfield, CT
Like the internal combustion engines used in automobiles, a steam engine is a type of heat engine—a device that diverts some of the heat flowing from a hotter object to a colder object and that turns that heat into useful work. The fraction of heat that can be converted to work is governed by the laws of thermodynamics and increases with the temperature difference between the hotter and colder objects. In the case of the steam engine, the hotter the steam and the colder the outside air, the more efficient the engine is at converting heat into work.

A typical steam engine has a piston that moves back and forth inside a cylinder. Hot, high-pressure steam is produced in a boiler and this steam enters the cylinder through a valve. Once inside the cylinder, the steam pushes outward on every surface, including the piston. The steam pushes the piston out of the cylinder, doing mechanical work on the piston and allowing that piston to do mechanical work on machinery attached to it. The expanding steam transfers some of its thermal energy to this machinery, so the steam becomes cooler as the machinery operates.

But before the piston actually leaves the steam engine's cylinder, the valve stops the flow of steam and opens the cylinder to the outside air. The piston can then reenter the cylinder easily. In many cases, steam is allowed to enter the other end of the cylinder so that the steam pushes the piston back to its original position. Once the piston is back at its starting point, the valve again admits high-pressure steam to the cylinder and the whole cycle repeats. Overall, heat is flowing from the hot boiler to the cool outside air and some of that heat is being converted into mechanical work by the moving piston.

579. Can you suspend an object in midair with magnetism? — JA, Holmen, WI
Yes. However, you can't suspend a stationary object in midair with permanent magnets. Instead, you must either use a moving object or you must use electromagnets that can be adjusted in strength in order to balance the object. Such magnetic suspension is an important issue because people are trying to suspend trains above tracks using magnetic forces. Magnetic levitation is useful because it eliminates the friction and wear that occur between wheels and track. Some of these schemes are based on electronic feedback that turns electromagnets on or off in order to keep the train floating properly. Other schemes use electromagnetic induction to turn the metal track into a magnet so that the moving magnetic train automatically hovers above the track. I should also note that there is a wonderful toy called a Levitron that's a spinning permanent magnet that hovers above a permanent magnet in its base. The spinning behavior of the magnetic top keeps it stably suspended about an inch above the base. It's a fantastic invention.

580. How does a battery work? How many different kinds of batteries are there? - BW
Batteries use chemical reactions to move electric charges from one terminal to another. A chemical reaction is a process that rearranges molecules—you begin with a certain collection of molecules and end up with a different collection of molecules. As the atoms in those molecules rearrange, they stick to one another more tightly than before and they release some of their chemical potential energy. This released energy then takes another form. While some chemical reactions such as burning will turn this released energy into thermal energy, a battery uses this released energy to move electric charges from one place to another. The battery moves extra positive charges onto its positive terminal and extra negative charges onto its negative terminal. While you can't see those charges, you can tell that they're there. If you use wires to connect the terminals to the two sides of a light bulb, the charges will rush through the wires and the light bulb will glow.

There are many types of batteries, but two of the most important modern batteries are alkaline batteries (used in flashlights and toys) and lead-acid batteries (used in automobiles). An alkaline battery uses a reaction between zinc metal and manganese dioxide to move electric charges between its two terminals. The battery's negative terminal is made of powdered zinc and its positive terminal is surrounded by manganese dioxide. Between the two terminals is an alkaline paste of potassium hydroxide. As the chemical reaction proceeds, negative charges are transferred to the battery's negative terminal and positive charges are transferred to the battery's positive terminal. As these charges are used by the flashlight or toy, the battery replaces them with new charges. Since each transfer of charges consumes some of the battery's original chemicals, the more the battery's charges are used, the more its chemicals are consumed. Eventually the powdered zinc is gone and the battery stops working. Once the powdered zinc has been used up, it can't be replaced.

A lead-acid battery uses a reaction between lead metal, lead oxide, and sulfuric acid to move electric charges. It, too, consumes its original chemicals while transferring charges. However, a lead-acid battery can be recharged easily by pushing charges through it backward. When a car is running, its generator pushes charges backward through the lead-acid battery and converts the consumed chemicals back into their original forms. This recharged battery is almost as good as new, so it can be used over and over again and lasts for several years.

581. Can you get electricity or some sort of energy or power from fruit? — J, Embrun, Ontario
The answer is yes, but the method may not be what you had in mind. While it's possible to make a battery by inserting two dissimilar metal strips into the fruit, the battery that results is really powered by the metals themselves. The fruit juice just acts as an "electrolyte"—an electrically conductive liquid that facilitates the movement of electric charges. Claiming that the fruit is responsible for the energy is like claiming that the stone in "stone soup" (an old tale about a beggar who tricks the villagers in a community into contributing vegetables to spice up the soup that he's making with his magic stone) is really the basis for the soup.

The best way to obtain energy from the fruit is to eat it! The sugars and starches in the fruit have plenty of chemical potential energy that's released when those chemicals are oxidized in your body. This released energy is what allows you to live, work, and play.

582. How does an amplifier work and what are the basic components of one? — WT, Albuquerque, NM
A typical amplifier examines the current flowing in its input circuit and produces a current in its output circuit that's proportional to but much larger than this input current. The factor by which the amplifier multiplies the input current to produce the output current is sometimes called the amplifier's "current gain." The tiny currents produced by a microphone attached to an audio amplifier's input circuit are boosted into huge currents that flow through speakers attached to the amplifier's output circuit. Since your voice is controlling these large currents, the speakers reproduce the sound of your voice.

While there are many techniques used to amplify currents, most modern audio amplifiers use transistors to do the amplification. A transistor is a device that permits a small current or electric charge to control the flow of a much larger current. The transistors inside the amplifier examine the current in the amplifier's input circuit and these transistors control the current passing through the amplifier's output circuit. Because the current in the output circuit needs electric power to continue flowing, a power supply inside the amplifier provides that current with power. As you talk into the microphone, the transistors adjust the current flowing through the output circuit so that that current is proportional to the current flowing through the input circuit.

583. If you microwaved bean plant seeds over a period of weeks while they were growing, would they grow faster or longer, and if they would, would that be due to the heat or some effect of the microwave radiation? - DS
Microwaving the bean plant seeds would be no different from heating them, except that the distribution of temperatures in the seeds and soil might be a little different from what you would get if you simply used a space heater. The particles or photons of ultraviolet light, X-rays, or gamma rays have enough energy to cause chemical changes in organic molecules and can induce mutations in living organisms. However, the photons of microwaves have so little energy that all they can do is heat living things. The most likely result of microwaving the bean plant seeds will be that the seeds will overheat and won't grow at all. You'll have bean stew.

584. My mother owns a microwave oven that is about 20 years old. It looks like new and has always been well taken care of. However, I was wondering whether it is still safe to use. Should I have it tested for leakage? — KE, Milwaukee, WI
As long as it still cooks, it's probably fine. Leakage of microwaves can only occur if the cooking chamber has holes in its metal walls. These walls include the metal grid over the front window and the seals around the door. If the metal grid is intact and the door still appears to close properly, the oven shouldn't leak any more microwaves now than it did 20 years ago. However, to set your mind at ease, you can have it tested or test it yourself. sells a simple microwave leak tester for $30. You can probably find similar devices at local appliance stores or, for a more accurate and reliable test, take your microwave oven to a service shop for inspection with an FDA certified meter. [Note added 1/10/97: I have finally found one microwave oven that leaks enough that a simple tester identifies it as dangerous—it's the microwave oven in my laboratory and I've moved it around frequently and taken it apart several times for my classes. Evidently, I damaged its door hinges during my experiments because the door now sags a bit and doesn't seal properly. The tester worked nicely in finding the leaks.]

585. Where does the white go when the snow melts? - JM
To start with, light slows down when it moves from air to ice and speeds up when it moves from ice to air. That's because the electric charges in matter can delay a light wave and slow it down. Since electric charges are more concentrated in ice than they are in air, light travels more slowly in ice than it does in air. Next, some light reflects whenever light changes speed. That's why a glass windowpane reflects some light from both its front and back surfaces. Similarly, light reflects from each surface of an ice crystal. Finally, snow is a jumbled heap of ice crystals. These clear crystals partially reflect light in all different directions like billions of tiny mirrors. The result is a white appearance. You see this exact same effect when you look at white sand, granulated salt, granulated sugar, clouds, fog, white paint, and so on. Each of these materials consists of tiny, clear objects that partially reflect light in all directions. Since they reflect all colors of light equally and spread that light in all direction equally, they appear white.

When the snow melts and becomes water, it stops having all those tiny surfaces to partially reflect light. Instead, it has only its top surface and this surface continues to partially reflect light. That's why you see reflections in the top of a puddle.

586. Do regular fluorescent lights emit ultraviolet light? If so, how does the ultraviolet level compare to what we would receive if we were outside? — GF, Barstow, CA
While the electric discharge in the tube's mercury vapor emits large amounts of short wavelength ultraviolet light, virtually all of this ultraviolet light is absorbed by the tube's internal phosphor coating and glass envelope. As a result, a fluorescent lamp emits relatively little ultraviolet light. I think that the ultraviolet light level under fluorescent lighting is far less than that of outdoor sunlight.

587. What is the composition of the phosphors used in fluorescent light bulbs? - M
The exact composition depends on the color type of the bulb, with the most common color types being cool white, warm white, deluxe cool white, and deluxe warm white. In each case, the phosphors are a mixture of crystals that may include: calcium halophosphate, calcium silicate, strontium magnesium phosphate, calcium strontium phosphate, and magnesium fluorogermanate. These crystals contain impurities that allow them to fluoresce visible light. These impurities include: antimony, manganese, tin, and lead.

588. How does a telephone switching system work? Why was it so hard to trace telephone calls? In movies we see people pulling wires in order to trace the origin of a call. - AZ
Before the advent of electronic telephone switching systems, the automatic switching was done by electromechanical relays. These remarkable devices were essentially 10-position rotary switches that were turned by a series of electric pulses—the same pulses that were produced by the rotary dial of a telephone. When you dialed a "5", your telephone produced a series of 5 brief pulses of electric current and one of these relays advanced 5 positions before stopping. Each number that you dialed affected a different relay so that your called was routed through one relay for each digit in the number that you called. To trace a called, someone had to follow the wires from relay to relay in order to determine what position each relay was in. From those positions, they could determine what number had been dialed. The first few digits of the telephone number determine which exchange (which local switching system) was being called, so those first relays were located in the caller's telephone exchange building. The last few digits determine which number in the answerer's exchange was being called, so those relays were located in the answerer's telephone exchange. As you can imagine, finding your way through all those relays and wires in at least two different buildings was quite a job.

589. If E=mc2 and we know light exists, why is it that light doesn't have infinite mass and consequently why aren't we all squashed? - M
The equation that you present is a simplification of the full relationship between energy, mass, momentum, and the speed of light, and is really only appropriate for stationary massive particles. In it, E is the particle's energy, m is the particle's rest mass, and c is the speed of light. Since light has no rest mass, the previous equation is simply not applicable to it. I should note that this equation is sometimes used to describe moving massive particles, in which case the m is allowed to increase to reflect the increasing energy of the moving particle. But the use of this equation for moving particles and the redefinition of mass as something other than rest mass often leads to confusion.

A better way to deal with moving particles, particularly massless particles, is to incorporate momentum into the problem. The full equation, correct for any particle, is E2=m2c4+p2c2. In this equation, E is energy, m is the rest mass of the particle (if any), p is the momentum of the particle (if any), and c is the speed of light. While light has no rest mass, it does have momentum and it's this momentum that gives light an energy. Light travels along at the speed of light with a finite momentum and a finite energy. On the other hand, the momentum of a massive particle increases without limit as the particle approaches the speed of light and so does the particle's energy. Thus massive particles can't ever reach the speed of light.

590. How does a magnetically levitated transit vehicle work? — LB, West Palm Beach, FL
Although there are a variety of schemes for magnetically levitating trains, perhaps the most promising is a technique called electrodynamic levitation. In this scheme, the train contains very strong magnets (probably superconducting magnets like those used in MRI medical imaging systems) and it travels along an aluminum track. The train starts out rolling forward on wheels but as its speed increases, the track begins to become magnetic. That's because whenever a magnet moves past a conducting surface, electric currents begin to flow in that surface and electric currents are magnetic. Thus the moving magnetic train makes the aluminum track magnetic. For complicated reasons having to do with electromagnetic induction, the track's magnetic poles are oriented so that they repel the magnetic poles of the train—the two push apart. While the track can't move, the train can and it floats upward as much as 25 cm (10 inches) above the track. Once the magnetic forces can support the train, the wheels are retracted and the train floats forward on its magnetic cushion. To keep the train moving forward against air resistance (and a small magnetic drag force), there is also a linear electric motor built into the train and track. This motor uses additional electromagnets in the train and track to push and pull on one another and to propel the train forward (or backward during braking).

591. What are watts and amps? - NS
The watt is the standard unit of power—that is, it's the way in which we measure how much energy is being transferred to or from sometime each second. 1 watt is equivalent to 1 joule of energy per second. A 100 watt light bulb consumes 100 joules of electric energy each second. Anytime energy moves from one place to another, you can determine how much power is flowing. For example, the food energy in a jelly donut is about 1 million joules, so if you eat 1 jelly donut in 100 seconds, you receive 10,000 watts of power. Since your body only consumes about 100 watts of power while you are resting, it will take you 10,000 seconds to use up all that food energy.

The amp (or ampere) is the standard unit of electric current—that is, its the way in which we measure how many electric charges flow past a certain point each second. 1 amp is equivalent to 1 coulomb of electric charge per second. Since 1 coulomb of electric charge is the charge on 6,240,000,000,000,000,000 protons, even a current of only 1 amp means that a great many electric charges are passing each second. The current passing through a 100-watt light bulb is roughly 1 amp on average, while the current used in starting a car is about 100 amps.

592. How does an internal voltage regulator type auto alternator work and are they any better than an external regulator type? - H
An alternator is a device that uses rotary motion to generate electricity. As the car engine turns, it spins a magnet (the rotor) in the alternator and this spinning magnet induces electric currents in a set of stationary wire coils (the stator). The alternator's ability to generate electric currents by spinning a magnet past stationary wires is an example of electromagnetic induction. Induction is a general phenomenon in which a moving or changing magnetic field creates an electric field, which in turn pushes electric charges through a conducting material. Overall, some of the engine's mechanical energy is converted into electric energy.

The amount of energy given to each electric charge that flows through the wires in the stator depends on the speed with which the magnet turns and the strength of that magnet. Whether it's internal or external, the voltage regulator monitors this energy per charge—also known as the voltage—to make sure that it's correct. If not, it adjusts the strength of the alternator's magnet. It can do this because the alternator's magnet is actually an electromagnet and its strength depends on how much current is flowing through its wire coils. The voltage regulator carefully adjusts the current flowing through the electromagnet in order to obtain the proper output voltage from the alternator. Actually, the alternator itself produces alternating current, so a set of solid-state diodes converts this alternating current into direct current. A car's electric system, particularly its battery, operates on direct current. Since the alternator's operation is the same whether the voltage regulator is inside it or external to it, neither version should be better than the other.

593. How does waterpower work? - MA
By "waterpower" I assume that you mean hydroelectric power. In that case, water from an elevated source enters a pipe and travels downhill to a generating plant. As the water descends, its gravitational potential energy (the stored energy associated with height and the earth's gravity) becomes pressure potential energy (the stored energy associated with pressure) and kinetic energy (the energy of motion). By the time the water reaches the generating plant, it has enormous pressure and a modest speed.

This moving, high-pressure water is then sent through a fan-like turbine. As the water moves toward the low pressure beyond the turbine, it does work on the turbine's rotating blades and its energy is transferred to those blades. The water gives up its energy and the turbine takes away this energy in its rotary motion. The turbine is attached to an electric generator, which uses moving magnets and wire coils to turn the turbine's rotary energy into electric energy. The electric energy is carried away on wire to be used elsewhere. Overall, the water's gravitational potential energy has become electric energy.

594. How does a prism work? — RH, Louisville, KY
When light enters a material such as glass, the light slows down. That's because the electric charges in the material delay a light wave by interacting with the wave's electric and magnetic fields. The higher the frequency of the light wave, the more it interacts with the charges in most materials and the more that light wave slows down. Thus high-frequency violet light slows more than low-frequency red light as the two enter a piece of glass.

Because of this slowing effect, light bends when it encounters a glass surface at an angle. The wave has a width and as it encounters the glass surface, one side of the wave reaches the glass before the other side of the wave. Since the side that arrives first also slows first, the whole wave bends so that it travels more directly into the glass. Since violet light slows more than red light, the violet light also bends more than the red light. The two colors thus follow different paths through the glass.

The same bending occurs in reverse when the light leaves the glass. Light speeds up as it leaves glass and again the violet light bends more than the red light. In a prism (or any carefully cut glass, crystal, or plastic), the colors of light bend differently at each surface and follow slightly different paths both in and out of the prism. The light rays then appear separately when they strike a surface outside the prism or when you look at those light rays with your eyes.

595. How much life is consumed each time you turn on a fluorescent lamp? — BL, San Jose, CA
The starting process erodes the electrodes of a fluorescent tube through a phenomenon called sputtering. A typical fluorescent tube will last about 50,000 hours if left on continuously but only 20,000 hours if it's turn on for just 3 hours at a time. From that tidbit, I think its fair to say that a fluorescent tube can only start about 10,000 times. If the tube costs $5, you are spending about 0.005 cents per start. If the electricity to operate that tube costs about 0.2 cents per hour, then turning the tube off for about 1.5 minutes saves the same amount of money in electricity as it costs in tube life when you turn the tube back on. In short, if you turn the lamp off for less than about 1 minute, you're wasting money. But if you turn it off for more than 10 minutes, you're saving money. In between, it's not so clear. There is a myth that turning on a fluorescent lamp consumes a huge amount of electricity so that you shouldn't turn the lamp off and on. There is simply no basis to that myth.

596. How does a sewing machine work? — RD, APO
A sewing machine uses a spinning shaft to push a needle up and down through fabric. The rod that controls the needle's height is attached to the spinning shaft away from the shaft's axis of rotation so that as the shaft spins, the rod and needle move up and down. This motion resembles that of a child on a tricycle: as the front wheel turns, the child's legs move up and down.

Thread from a spool held above the fabric passes through an eye in the needle's tip, so that as the needle pierces the fabric, it carries the thread with it. A device beneath the fabric catches hold of this thread and pulls it rapidly around a smaller spool of thread (the bobbin). The thread from above the fabric thus fully encircles the thread from this bobbin and the two threads become permanently locked together. When the needle withdraws from the fabric, some of the thread that it carries remains behind, locked around the thread from the bobbin below. With each stroke of the needle, a new joint is created between the thread from above the fabric and the thread from below the fabric. If there are several pieces of fabric lying on top of one another, these pieces become locked together by the intertwined threads.

597. What does the heat anticipator do on a furnace thermostat? Does it have anything to do with the dwell (temperature rise) of the unit? — BV, Burton, MI
A simple thermostat turns the furnace on when the temperature it senses falls below a certain value and turns the furnace off when the temperature it senses rises above that value. Because it takes time for the furnace to respond to signals from the thermostat, for the heat from the furnace to travel to the thermostat, and for the thermostat to respond to changes in the temperature around it, the furnace tends to stay on for too long after the thermostat turns it on and then to stay off for too long after the thermostat turns it off. The result is an oscillation in temperature: the home or building alternately overheats and then overcools. To reduce this oscillation, a thermostat with a heat anticipator limits the amount of time that the furnace stays on. Since the furnace turns off earlier, the temperature doesn't overshoot as much on the high side and the furnace turns back on again more quickly once the home or building drifts below the set temperature of the thermostat. Overall, the temperature still oscillates above and below the set temperature, but those oscillations are smaller and faster.

598. How does the power/frequency of the earth's magnetic field compare to the magnetic fields of electrical appliances? — MC, Independence, KA
Although I haven't been able to find detailed lists of the magnetic fields near common appliances (such lists do exist), those fields are unlikely to be stronger than the earth's own magnetic field. That's because the magnetic fields in most appliances are created by electric currents and you must be quite near a relatively large current before the magnetic field of that current exceeds 0.5 gauss, the strength of the earth's magnetic field. But while an appliance's magnetic field is likely to be no greater than that of the earth, the appliance's magnetic field does change with time. It reverses each time that the alternating current from the power line reverses. In the United States, that's 120 reversals per second (60 full cycles of reversal, over and back, each second).

599. How do motion detectors work? — MK, Port St. Joe, FL
According to Gabriel Lombardi of Torrance, CA, most home motion detectors use infrared light to sense motion. Moving objects change the amount of infrared light striking a detector at the focus of an array of fresnel lenses. He points out that you can see this array on the front of many motion sensors. Such devices are known as passive infrared or PIR detectors. The motion detectors used in automatic door openers, such as those at the supermarket, usually use radio frequency electromagnetic waves to detect motion.

600. What are the most important energy-efficient household appliances? How do their efficiencies compare with those of standard appliances? — LM, Klong Luang, Pathumthani, Thailand
I can think of three important energy-efficient household electric devices: (1) heat pumps, (2) electric discharge lamps (including fluorescent lamps), and (3) microwave ovens.

A heat pump is a device that transfers heat against its natural direction of flow. If you use one to heat your home, the heat pump uses electricity to transfer heat from the colder outside air to the hotter inside air, so that the inside air becomes even hotter and the outside air becomes even colder. The electricity that the heat pump uses also becomes thermal energy inside your home. Since both the electric energy and the thermal energy pumped from the air outside end up inside your home, a heat pump provides more heat than a simple space heater can provide with the same electricity. The energy efficiency of a heat pump decreases as the temperature difference between inside and outside becomes greater, but it typically provides 4 or more times as much heat to your home as a normal electric space heater would provide with the same amount of electricity. Incidentally, when the heat pump is reversed, so that it pumps heat out of your home, it is then an air conditioner.

Electric discharge lamps are between 2 and 5 times as energy efficient as normal incandescent light bulbs. The hot filament of an incandescent lamp delivers only about 10% of its electric power as visible light. In contrast, a fluorescent lamp delivers about 25% of its electric power as visible light and some gas discharge lamps (particularly low-pressure sodium vapor) deliver as much as 50% of their electric powers as visible light.

A microwave oven transfers about 50% of its electric power directly into the water molecules of the food that you are cooking. Cooking occurs quickly and because the cooking chamber doesn't get hot, there is no power wasted in heating the oven itself or the room surrounding the oven. Depending on how large an object you are cooking, a microwave oven probably uses between 5 and 20 percent of the electricity it would take you to cook the same food in a standard oven.

601. Is 2.45 gigahertz the best frequency for a microwave oven? Is that frequency at or near a water molecule resonant frequency? Do water molecules have a resonant frequency?
The frequency of the microwaves used in most microwave ovens, 2.45 gigahertz or 2,450,000,000 cycles per second, isn't related to any resonance of the water molecules themselves. While the isolated water molecules in steam or moist air have clear resonances associated with various vibrational and rotational modes of oscillation, these resonances are smeared out in liquid water. The water molecules in liquid water touch one another and their resonances are disturbed in much the same way that the resonances of a bell are disturbed when you touch it.

Rather than interacting with the water molecules via a resonance, the microwaves in an oven heat the water by twisting its molecules rapidly back and forth so that they rub against one another. The molecules are heated by the molecular equivalent of sliding or dynamic friction. The choice of 2.45 gigahertz gives the water molecules about the right amount of time to twist in each direction. The precise frequency isn't important, but microwave ovens are required to operate at exactly 2.45 gigahertz so that they don't interfere with communication systems using nearby frequencies. I believe that there are 2 other frequencies allocated to microwave ovens, but only a few ovens make use of those frequencies.

602. I was wondering if a pitot tube is a very efficient way to measure airflow and, if so, what would be the conversion formula to cfm? - BN
A pitot tube determines airspeed by measuring the pressure rise that occurs when the airstream is slowed to a stop. Any time moving air encounters a closed chamber head-on, the air stops and it exchanges its kinetic energy—its energy of motion—for pressure potential energy—energy stored in the form of an elevated pressure. By measuring this elevated pressure, you can determine what the air's kinetic energy was while it was moving and thus how fast it was moving.

Pitot tubes are used to measure airspeed in airplanes. They're the cigar-shaped objects that project forward from the undersurfaces of airplanes near their noses. I suppose that you could use a pitot tube to measure the speed of air flowing through an air duct, but to determine the volume of air flowing through that duct, you'd need to know the dimensions of the duct. The relationship between pressure in the pitot tube and the airspeed is complicated and so is the relationship between airspeed in a real duct and the volume of air it's carrying. Overall, this doesn't look like an easy job.

603. What is the relationship between dark material and heat? Why does dark material absorb more heat than light material? - AR
Thermal radiation consists of electromagnetic waves. These waves are emitted and absorbed by the movements of electrically charged particles, usually electrons. Since all materials contain electrically charged particles, any of them can interact with thermal radiation. However, these interactions differ from material to material. The electrons in some materials are extremely effective at absorbing and emitting thermal radiation and these materials appear black. When the sun's thermal radiation strikes a black material, that material absorbs the sunlight and nothing reflects. That's why the material appears black. When you heat a black material to high temperatures, it also emits thermal radiation extremely well—for example, a hot piece of black charcoal glows brightly with its own red thermal radiation.

Materials in which the electrons are not able to absorb or emit thermal radiation have one of several familiar characteristics. Some are clear, meaning that thermal radiation passes right through them. Others are white, meaning that thermal radiation that strikes them is scattered uniformly in all directions. Still others are mirror-like, meaning that thermal radiation that strikes them is reflected in specific directions. All of these materials are virtually unable to emit their own thermal radiation: clear glass, white sand, and mirror-like aluminum emit very little thermal radiation even when they're "red hot."

Since black objects are best at emitting and absorbing thermal radiation, they are best at transferring heat via radiation. A black object will receive more heat from the hotter sun than a white object of similar dimensions and temperature. A black object will also radiate more heat to its colder environment than a white object of similar dimensions and temperature, although here "black" and "white" refer to the object's behavior regarding its own thermal radiation. Near room temperature, thermal radiation is in the infrared, and many objects that appear white to visible light are actually rather black to infrared light.

604. How do electromagnets work? — HL, Kurtistown, HI
Whenever an electric current—a current of moving electric charges—flows through a wire, that wire becomes magnetic. This phenomenon is an example of the wonderful interconnectedness of electric and magnetic effects—electricity often produces magnetism and vice versa. Because of its magnetic character, a current carrying wire will exert magnetic forces on another current carrying wire and they are both effectively electromagnets.

A more effective electromagnet uses a coil of wire and a core of very pure iron. Wrapping the wire into a coil gives it specific north and south magnetic poles and adding the iron strengthens those magnetic poles dramatically. Iron is a ferromagnetic material, meaning that it's intrinsically magnetic. All materials contain electrons and an electron has a spinning character that makes it magnetic. But the electron magnetism in most materials cancels completely and only a few materials such as iron retain the magnetism of their electrons. While iron's magnetism is hidden as long as its tiny internal magnets are randomly orientated, its magnetic character becomes obvious when it's inserted in an electromagnet or placed near one. When current flows through the wire coil of the electromagnet, the iron's magnetic poles align with those of the electromagnet and the electromagnet becomes extremely strong.

605. How can you make an electromagnet from a battery and copper wire?
You'll need a large steel nail or bolt, too. Wrap about 100 turns of copper wire around the nail, keeping the turns fairly uniformly spaced. Make sure that both ends of the wire coil, start and finish, project out from the windings. When you're done winding the coil, strip off about 1 cm of the insulation from each end of the wire. Now connect one end of the wire to the positive terminal of a AA alkaline battery and the other end of the wire to the negative terminal of that battery. The nail will become a strong magnet and will be able to pick up other nails or paper clips with ease. Electricity will also heat the wire, so be prepared for the electromagnet to become uncomfortably hot. Detach the wires from the battery when you're no longer able to hold everything safely.

606. What is the difference between a low impedance output and a high impedance output of an audio distribution amplifier? What kind of output does consumer equipment need? — JH, Eugene, OR
Some audio amplifiers provide several different outputs, each characterized by the impedance of its expected load (e.g., the impedance of the speaker that you should attach to that output). This impedance measures the relationship between voltage and current that the load needs to function optimally. The higher the impedance, the more voltage the amplifier must provide to propel a particular electric current through the speaker. If the speaker that you attach to the amplifier has the wrong impedance, the amplifier won't be able to deliver its maximum audio power to the speaker and you may damage the amplifier, speaker, or both.

Since a typical household speaker has an impedance of 8 ohms, you should connect it to an amplifier's 8 ohm output. However, if you connect more than one speaker to the same output, you should be careful to determine the combined impedance. For example, two 8-ohm speakers in series have a combined impedance of 16 ohms while two 8-ohm speakers in parallel have a combined impedance of 4 ohms. Many amplifiers are designed to accommodate these arrangements.

When a distribution amplifier must send current long distances through thin wires, it will often use higher voltages and lower currents to minimize power losses in the wires. Such an amplifier expects its load to have an unusually large impedance. In this situation, the speaker that is used must either have a large impedance, so that it can use this high voltage/low current power directly, or there must be an impedance matching transformer between the amplifier and the speaker.

607. Why does a microwave oven heat organic material and not inorganic material? — JM, Columbus, OH
A microwave oven heats anything that contains liquid water. Since many organic materials contain water, they will become hot in a microwave oven. But some organic materials such as pure salad oil don't contain water and won't become hot in a microwave oven. There are also some inorganic materials such as damp unglazed pottery that contain water and that will become hot.

608. Does cooking in a microwave oven destroy the nutritional value of foods? Are microwaves radioactive? Does radiation "leak" from the oven? - DL
Microwaves are essentially high frequency radio waves. They heat food by twisting its water molecules back and forth so that those water molecules rub against one another. Like all electromagnetic waves, microwaves are absorbed and emitted as particles or "photons," but the photons of microwaves have so little energy that they are unable to cause chemical changes in the molecules they encounter. They simply heat food; they don't "irradiate" it. The only way a microwave oven damages the nutritional value of foods is if it overheats. Microwaves are not radioactive—radioactivity is the spontaneous fragmentation of the nuclei of atoms and is usually associated with the emission of high-energy particles; particles that can induce chemical changes in the molecules they encounter. Finally, if a microwave oven was properly constructed and hasn't been damaged, virtually no microwaves leak from it. A small amount of microwaves won't hurt you anyway—they are present all around us already because of satellite transmissions, cellular telephones, and even the thermal radiation from our surroundings.

609. I've heard from many people that you should not stand directly in front of and no closer than 30 feet while a microwave oven is on? Why? If this is a myth how did it get started?
This idea is just a myth. There should be virtually no microwaves leaking from the oven so it shouldn't matter where you stand. If you're concerned about microwaves, you can buy a microwave oven tester from a local appliance store or from (or for a more accurate and reliable measurement, take your microwave to a service shop for inspection with an FDA certified meter). I have no idea how such a myth got started, but it's clear that microwave ovens scare people because they don't understand them. Given how easy it is to burn yourself on a conventional oven, I'd guess that there are fewer health risks with microwave cooking than with conventional cooking.

610. Is magic a real possibility? — LM, Dartmouth, Nova Scotia
I would define magic as any phenomenon that can't be explained by the normal laws of nature. In that case, I'm afraid that it isn't a possibility. Like most physicists, I'm convinced that the laws of physics can ultimately explain everything that we observe. Violations in those laws would have such terrible complications that even a single "magic" event just can't occur. No doubt, there are people who believe in magic and that view physicists as just another group with a different and incorrect opinion about the world. That's just wishful thinking. Physics has been extraordinarily successful at explaining how the world works. Unlike magic, physics has an internal consistency that is astonishing and it has the ability to predict behavior with enormous accuracy.

611. What is gravity and how do you define it?
There are two levels at which to work. First, there is Newtonian gravity—an attraction that exists between any two objects and that pulls each object toward the center of mass of the other object with a force that's equal to the gravitational constant times the product of the two masses, divided by the square of the distance separating the two objects. For example, you are attracted toward the earth's center of mass with a force equal to the gravitational constant times the product of the earth's mass and your mass, divided by the square of the distance between the earth's center of mass and your own center of mass. This force is usually called "your weight." The earth is attracted toward your center of mass with exactly the same amount of force.

Second, there is the gravity of Einstein's general relativity—a distortion of space/time that's caused by the local presence of mass/energy. Space is curved around objects in such a way that two freely moving objects tend to accelerate toward one another. As long as those objects aren't too large or too dense, this new description of gravity is equivalent to the Newtonian version—they both predict exactly the same effects. But when one or both of the objects is extremely massive or very dense, general relativity provides a more accurate prediction of what will happen. In reality, mass/energy really does warp space/time and general relativity does provide the correct view of gravity in our universe. The next level of theory, quantum gravity (which will reconcile the theory of general relativity with the theory of quantum physics), is still in the works.

612. How does a Bourdon tube pressure gauge work? - AM
A Bourdon tube pressure gauge works on much the same principle as a party favor that inflates and unrolls when you blow in its tube. The hollow Bourdon tube of the pressure gauge isn't circular in cross-section—it's somewhat oval. When the pressure inside the tube increases, the tube's oval walls are distorted and the tube's cross-section becomes slightly more circular. However, the tube is wrapped in a coil and as its walls become more circular, the tube uncoils slightly. The amount of uncoiling that occurs is almost exactly proportional to the pressure inside the Bourdon tube. As the tube uncoils, its motion activates a rack-and-pinion gear system that turns the needle on the pressure dial of the gauge. While all that you see when you look at the gauge is this needle pointing at the current pressure, you should understand that there is a small, bent tube that's coiling and uncoiling with each change in the pressure inside that tube.

613. What is a photoconductor? — MN, Chicago, IL
A photoconductor is a material that behaves as an electric insulator in the dark but becomes an electric conductor when exposed to light. An insulator is unable to transport electric charges because its own electrons can't respond to modest electric forces. Because of quantum physics, electrons can only follow specific paths called "levels" as they move through a material and all of the easily accessible levels in an insulator are completely filled. For reasons of symmetry, there are always as many electrons traveling to the right in an insulator as are traveling to the left, so that on average, no electrons move anywhere, even when they are exposed to electric forces. But when light energy shifts some of the electrons from the filled levels to a collection of formerly unoccupied levels that previously weren't accessible, these shifted electrons can respond to electric forces and transport electric charge through the material. In the light, a photoconductor stops acting as an insulator and starts acting as a conductor. Such photoconductors are the basis for xerographic copiers and laser printers.

614. Can plastic melt in a microwave oven? How does this process work? Can plastic burn in a microwave oven? - HD
Most plastics are unaffected by microwaves and do nothing at all in a microwave oven. For them to absorb energy from the microwaves, the plastics must either conduct electricity or their molecules must undergo the twisting motions that water molecules experience in the microwave oven. There are a few conducting plastics and these may melt or burn in a microwave as the microwave electric fields propel electric currents through them. There are also some plastics that trap water molecules and these may also melt or burn as the water molecules gather energy from the microwaves. I suppose that there are also a few plastics that have polar molecules in them that respond to the microwaves the way water does. However, most plastics do none of these and only melt or burn if they accidentally come in contact with very hot food or pieces of metal that happen to be in the microwave oven.

615. Is the fact that the small magnetic fields generated by appliances change due to the alternating electric current the reason that EMFs may cause health problems? — MC, Independence, KS
I believe that the alternating nature of the electromagnetic fields around appliances is at least part of the reason they're suspected of causing health problems. Since these fields are created by an electric current that alternates in direction, they alternate in direction, too. However, I have not seen any credible evidence for there being a relationship between these appliance-related fields and health problems, nor have I heard any sensible physical theory for such a possibility. On the contrary, I have read a number of compelling arguments for why the tiny electromagnetic fields around appliances should have no biological effects at all. I think that the worries about EMFs are unfounded.

616. How does an internal combustion engine work? — RT, Kitchener, Ontario
An internal combustion engine burns a mixture of fuel and air in an enclosed space. This space is formed by a cylinder that's sealed at one end and a piston that slides in and out of that cylinder. Two or more valves allow the fuel and air to enter the cylinder and for the gases that form when the fuel and air burn to leave the cylinder. As the piston slides in and out of the cylinder, the enclosed space within the cylinder changes its volume. The engine uses this changing volume to extract energy from the burning mixture.

The process begins when the engine pulls the piston out of the cylinder, expanding the enclosed space and allowing fuel and air to flow into that space through a valve. This motion is called the intake stroke. Next, the engine squeezes the fuel and air mixture tightly together by pushing the piston into the cylinder in what is called the compression stroke. At the end of the compression stroke, with the fuel and air mixture squeezed as tightly as possible, the spark plug at the sealed end of the cylinder fires and ignites the mixture. The hot burning fuel has an enormous pressure and it pushes the piston strongly out of the cylinder. This power stroke is what provides power to the car that's attached to the engine. Finally, the engine squeezes the burned gas out of the cylinder through another valve in the exhaust stroke. These four strokes repeat over and over again to power the car. To provide more steady power, and to make sure that there is enough energy to carry the piston through the intake, compression, and exhaust strokes, most internal combustion engines have at least four cylinders (and pistons). That way, there is always at least one cylinder going through the power stroke and it can carry the other cylinders through the non-power strokes.

617. Does the pull of the moon have any effect on a person's behavior? — PSC, Summerville, WV
No, but for an interesting reason. While the moon's gravity acts on people, it also acts on everything around them and everything falls toward the moon at the same rate. Because of this uniform falling, we don't feel the moon's gravity at all. This effect is identical to the one that astronauts feel as they orbit the earth—the earth's gravity pulls on them and on their spaceship, but they are falling freely under the influence of that gravity and they don't feel it—they feel weightless. Since we are falling freely under the influence of the moon's gravity, we don't feel it either—we feel moon-weightless.

Since we are being pulled toward the moon by the moon's gravity, you might wonder why we don't crash into the moon. That's because we're traveling sideways so fast that we perpetually miss the moon and circle it once every 27.3 days. Similarly, the moon perpetually misses the earth and circles it, too.

The only significant effect of the moon's gravity is to create the tide. The earth's oceans are so large that they're sensitive to variations in the moon's gravity. The moon's gravity decreases with distance from the moon, so that the oceans on the near side of the earth are pulled harder than the oceans on the far side of the earth. The result is two bulges in the oceans—one on the near side of the earth and one on the far side of the earth. These bulges create the familiar high and low tides that we observe at the seashore.

618. Hydrogen atoms can form a single bond to each other, oxygen atoms can form a double bond to each other, and nitrogen atoms can form a triple bond to each other. Is there any element that can form a quadruple bond? — KC, Mendenhall, MS
The bonds that you are referring to are call "covalent bonds," in which two atoms share a pair of electrons in order to lower their total energy. When two electrons are shared in this manner, the electrons are able to spread out over two atoms rather than one. This broadening of their territories lowers their kinetic energies because of quantum mechanical effects. The electrons also spend large portions of their times between the atoms, where they lower the electrostatic potential energies of the two atoms. Lowering the total energy of the two atoms binds them together.

The number of covalent bonds that form between two atoms depends on the number of electrons in those atoms. Hydrogen atoms have only one electron each and can form only one covalent bond. Oxygen atoms have two electrons each that they can share and form two covalent bonds. Nitrogen atoms have three electrons to share and form three covalent bonds. And carbon atoms have four electrons to share, so you might expect them to form four covalent bonds. But there's a hitch...

In the first covalent bond that forms between two atoms, the pair of electrons positions itself directly in between the atoms. This arrangement is most effective for lowering the energy of the system and binding the two atoms together. Chemists call this arrangement a "sigma bond." In the second covalent bond, the two electrons position themselves on both sides of the sigma bond. If you picture the atoms as two people facing one another and holding hands, the electrons are located along the arms of the two people. This arrangement is reasonably effective for lowering the energy of the system and is called a "pi bond." The third covalent bond is also a pi bond, but it forms 90° from the first pi bond, as though the two people are now touching their heads and their feet together along with their hands. With a sigma bond and the two pi bonds between the atoms, there is no room for additional electrons. The fourth covalent bond that two carbon atoms would like to form with one another simply can't form. While two carbon atoms will bind together with a triple bond, each atom will have one remaining electron that is still seeking a partner. The carbon dimer molecule is a highly reactive double radical that will bind to just about anything it encounters.

619. What is the "optimal" shape for a pinewood derby car — I'm guessing some sort of short, flat, thin rectangle. - BP
The car's biggest obstacle is air resistance, which in this case is a force known as "pressure drag." The pressure drag force is proportional to the size of the turbulent wake the car creates in the air as it passes through the air. Streamlining is important to minimizing this wake. The thinner and shorter you can make the car, the smaller its wake will be. The ideal shape would be an airfoil, like those used in airplane wings and bodies. These carefully tapered shapes barely disturb the air at all and experience very little pressure drag. If you design your car to resemble a wingless commercial jet airliner, you will be doing pretty well.

620. What is the "optimal" weight distribution for a pinewood derby car — in front/behind, above/below the center of gravity? - BP
I'll assume that the car starts on a slope and coasts downhill to a level finish. If that's the case, then you want to put the car's center of gravity as far back in the car as you can get it. That way, the center of gravity will start as high as possible in the tilted car and will finish as low as possible in the level car. During a race, the car obtains its kinetic energy (its energy of motion) from its gravitational potential energy. The farther the car's center of gravity descends during the race, the more gravitational potential energy will be converted to kinetic energy and the faster the car will go.

621. A friend was telling me of a guy who created a TV satellite dish out of chicken wire in his attic — how would you do it, adjust it, and what kind of home-brew receiver would be required to use it? - BP
Since the microwaves used in satellite transmissions have wavelengths of several centimeters or more, they can't pass through holes in a conducting material if those holes are less than about a centimeter in diameter. As a result, chicken wire reflects microwaves as though it were a sheet of solid metal. You can form a dish antenna by bending chicken wire into a parabola. When the microwaves from the satellite strike this parabolic reflecting surface, they are brought together to a focus at a particular point above the center of the parabola. If you then place a microwave receiving device at this focal point, you'll be able to watch satellite TV.

If you want to do this, you should make a cardboard template for the parabolic shape and bend the chicken wire carefully to match this template. The more highly curved the parabola, the closer the focus will be to the dish's surface. You should aim this dish directly at the satellite and put the receiving unit at the focus of the parabola, above its center. However, you'll have difficulty building the receiving device yourself, although there are probably kits you can buy. The receiver should have a tiny antenna, a microwave amplifier, and a frequency down-converter, all together on a single circuit board. Working with microwave-frequency electronics is difficult because the wave character of the electric signals is painfully obvious in those circuits. Designing microwave circuits is a job for experts. In short, you can build the dish, but you should buy the receiver that sits at the center of the dish.

622. Could you explain the microscopic model of temperature in a gas? — DD, SC
Thermodynamics imposes a severe constraint on the meaning of temperature by observing that when two objects are at the same temperature, no heat flows between them when they touch. That constraint leads to the follow possibility: in a gas composed of independent particles, temperature must be proportional to the average internal kinetic energy per particle. By internal kinetic energy, I mean that we are excluding any kinetic energy associated with the movement of the gas as a whole. And by average per particle, I mean to add up all the internal kinetic energies and divide the sum by the number of particles. With this definition of temperature, two bodies of gas that have the same temperature won't exchange heat when they touch. It turns out to be a good definition of temperature and the one that we use in general.

623. What is the relationship between turbulence, laminar flow, and Reynolds number? — DD, SC
The Reynolds number is a measure of the way in which a moving fluid encounters an obstacle. It's equal to the fluid's density, the size of the obstacle, and the fluid's speed, and inversely proportional to the fluid's viscosity (viscosity is the measure of a fluid's "thickness"—for example, honey has a much larger viscosity than water does). A small Reynolds number refers to a flow in which the fluid has a low density so that it responds easily to forces, encounters a small obstacle, moves slowly, or has a large viscosity to keep it organized. In such a situation, the fluid is able to get around the obstacle smoothly in what is known as "laminar flow." You can describe such laminar flow as dominated by the fluid's viscosity—it's tendency to move smoothly together as a cohesive material.

A large Reynolds number refers to a flow in which the fluid has a large density so that it doesn't respond easily to forces, encounters a large obstacle, moves rapidly, or has too small a viscosity to keep it organized. In such a situation, the fluid can't get around the obstacle without breaking up into turbulent swirls and eddies. You can describe such turbulent flow as dominated by the fluid's inertia—the tendency of each portion of fluid to follow a path determined by its own momentum.

The transition from laminar to turbulent flow occurs at a particular range of Reynolds number (usually around 2500). Below this range, the flow is normally laminar; above it, the flow is normally turbulent.

624. I have heard that microwaving can destroy certain nutrient molecules in food, such as vitamins. Is this true? — D, Boulder, CO
A microwave oven heats the food it cooks; nothing more. If it damages nutrients, then it's by overheating those nutrients. Such overheating could happen in a microwave oven if you don't move the food about during cooking. That's because the microwaves aren't uniformly distributed in the cooking chamber and some parts of the food heat faster than others. Some parts of the food could become hotter than you intend and this overheating could damage sensitive molecules. However, I think that microwave cooking is probably less injurious to the food than conventional cooking. It's pretty hard to burn food in a microwave!

625. What material is used in glass to make it polarize light? — FG, Torrance, CA
Actually, the polarizing material you are referring to is a plastic that has been impregnated with iodine atoms. The plastic, polyvinyl alcohol, is heated and stretched to align its long molecules in a particular direction. This plastic is then exposed to iodine, which binds to the long molecules and forms the equivalent of molecular wires along the direction of the aligned plastic molecules. These molecular wires absorb light that is polarized along them because the light's electric field points along its polarization direction and pushes electric charges wastefully along the iodine wires. This light is absorbed and its energy is converted to thermal energy, leaving only light with the other polarization.

626. Exactly what is light? Is it a wave or particles? — MW, Catoosa, OK
Light is an electromagnetic wave—an excitation of the electric and magnetic fields that can exist even in "empty" space. Light's electric field creates its magnetic field and its magnetic field creates its electric field and this self-perpetuating arrangement zips off through space at a phenomenal speed—the speed of light. Light is created by moving electric charges, which first excite the electromagnetic fields. Light is also absorbed by electric charges, which obtain energy from the light's electromagnetic fields.

Like everything else in the universe, light exhibits both wave and particle behaviors. When it is traveling through space, light behaves as a wave. That means that its location is generally not well defined and that it can simultaneously pass through more than one opening (the way a water wave can when it encounters a piece of screening). But when light is emitted or absorbed, it behaves as a particle. It's created all at once when it's emitted from a particular location and it disappears all at once when it's absorbed somewhere else. This wave/particle arrangement is true of everything, including objects such as electrons or atoms: while they are traveling unobserved, they behave as waves but when you go looking for them, they behave as particles.

627. What effects do fluorescent lamps have on household plants? — SN, Milwaukee, WI
Since plants appear green, they are absorbing mostly the red and blue portions of the visible light spectrum. Blue light is particularly important to them. Incandescent light contains relatively little blue light, so it probably doesn't help plants very much. Because fluorescent lighting provides more blue light than incandescent lighting, fluorescent lighting is certainly better for plants.

628. How do sound proof and bulletproof glasses work? - DH
Sound proof glass uses several separate layers of glass to make it difficult for sound to move from one room to another. Each time sound passes through a surface and experiences a change in speed, some of the sound reflects. Sound travels much more slowly in air than in glass, so with each transition into or out of a glass pane, most of the sound is reflected backward. If two rooms are separated by 3 or 4 sheets of glass, each carefully sealed into place so that there are no holes for sound to leak through, the amount of sound that can make it through the overall window will be very small. Most of the sound will be reflected.

Bulletproof glass is actually a multi-layered sandwich of glass and plastic—it's like the front windshield of a car, but with many more layers. When a bullet hits the surface of the sandwich, it begins to tear into the layers. But the bullet loses momentum before it manages to burrow all the way through to the final layers. The bullet's energy and momentum are transferred harmlessly to the layers of glass and plastic.

629. I read recently that scientists at CERN produced some form of antimatter, but that it could not be stored. Why can't it be stored and, if it could, would it be a viable method of propulsion? — BC, Ottawa, Ontario
The antimatter that was formed at CERN was an antihydrogen atom, which consisted of an antiproton and an antielectron (often called a positron). Antiprotons and positrons have been available for a long time, but it has been a challenge to bring them together gently enough for them to stick to one another and form a bound system. An antihydrogen atom is hard to store because, like a normal hydrogen atom, it moves or falls so quickly that it soon collides with its container. For a normal hydrogen atom, that collision is likely to cause a chemical reaction. But for an antihydrogen atom, that collision is likely to cause annihilation. When an antiproton touches a proton, the two can destroy one another and convert their mass into energy. The same is true for a positron and an electron. To store an antihydrogen atom, you must keep it from touching any normal matter. That's not an easy task. Because of its ability to emit its entire mass and that of the normal matter it encounters into energy, antimatter is the most potent "fuel" imaginable. But don't expect it to show up in a rocket ship any time soon.

630. How can one prove to students that the earth rotates. Any instructions on how to build a pendulum to show rotation or some other way? - KC
There are many indirect indications that the earth rotates, including the motions of celestial objects overhead, the earth's winds—particularly the counter-clockwise rotation of surface winds in northern hemisphere hurricanes, and the outward bulge of the earth around its equator. But for a more direct indication, a Foucault pendulum is a good choice.

Unfortunately, a Foucault pendulum isn't easy to interpret or build. It would be easiest to interpret if it were at the north pole, where it would swing back and forth in a fixed plane as the earth turned beneath it. To a person watching the pendulum from the ground, the pendulum's swinging arc would appear to complete one full turn each day. However, elsewhere in the northern hemisphere, the plane of the pendulum does change and the pendulum's swinging arc will appear to complete less than one full turn each day. Nonetheless, the fact that the arc shifts at all is an indication that the ground is accelerating and that the earth is turning.

The problem with building a Foucault pendulum is that it must retain its swinging energy for hours or even days and that it must not be perturbed by activities around it. It must have a very dense, massive pendulum bob supported on a strong, thin cable and that cable must be attached to a rigid support overhead. The longer the cable is, the longer it will take the bob to complete each swing and the more slowly the pendulum will move. Slow movements are important to minimize air resistance. If I were building a Foucault pendulum, I'd find a tall empty shaft somewhere, away from any moving air, and I'd attached a lead-filled metal ball (weighing at least 100 pounds but probably more) to the top of the shaft with a thin steel cable. I'd make sure that nothing rubbed and that the top of the cable never moved. (Over the long haul, there is the issue of damage to the top of the cable because of will eventually break here. Wrapping the cable around a drum so that there is no specific bending point helps.) Then I'd pull the pendulum away from its equilibrium position and let it start swinging slowly back and forth. Over the course of several hours, its swing would decrease, but not before we would notice that its arc had turned significantly away from the original arc because of the earth's rotation.

631. How do you figure out the weight lifting ability of a hot air balloon? — BK, Meraux, LA
The air surrounding an object pushes upward on it with a force equal to the weight of the air the object displaces. The observation is called Archimedes' principle. If the object weighs less than the air it displaces, the object will experience a net upward force and will float upward. Since hot air is less dense and weighs less than cold air, a balloon filled with hot air can weigh less than the air it displaces. To determine the net upward force on the balloon, you subtract the total weight of the balloon (including the air inside it) from the weight of the air it displaces.

At room temperature, air weighs about 12.2 newtons per cubic meter (0.078 pounds per cubic foot). But air's density and weight are proportional to its temperature on an absolute temperature scale (in which absolute zero is the zero of temperature). At 200° F, air weighs about 20% less than at room temperature, or about 9.7 newtons per cubic meter (0.062 pounds per cubic foot). Thus each cubic meter of 200° F air inside the balloon makes the balloon 2.5 newtons lighter than the air it displaces (or each cubic foot of that hot air makes it 0.016 pounds lighter). If the balloon's envelope, basket, and occupants weigh 4000 newtons (900 pounds), then the balloon will have to contain about 1600 cubic meters (56,000 cubic feet) of hot air in order to float upward.

632. How does food cook? — KJ, Irving, TX
There are two parts to this question: how does thermal energy (or heat) reach the food and what does that thermal energy do when it arrives. I'll start with the first part, but first let me define thermal energy as a form of energy associated with the random jittering about of the atoms and molecules in a material. The hotter a material is, the more average thermal kinetic energy (energy of motion) each atom has—in effect, the more vigorously the atoms and molecules jiggle. Thermal energy naturally tends to flow from hotter objects to colder objects, so that when you put cold food on a hot stove or in a hot oven, thermal energy will flow toward the food. This moving thermal energy is called heat.

There are three main mechanisms for heat transfer: conduction, convection, and radiation. Heat that flows via conduction is being passed from atom to atom inside a solid or liquid. In metals, conduction is greatly assisted by mobile electrons (the same electrons that allow metals to carry electricity) that carry heat between atoms far away from one another. Conduction is important on the stovetop, where the food touches the pot and the pot touches the hot stovetop. Heat that flows via convection is carried by a moving gas or liquid. Convection is important in an oven that's heated from below so that hot air rises to touch the food. Heat that flows via radiation is carried by electromagnetic waves (forms of light). Radiation is important in an oven that's heated from above (as in a broiler) so that thermal radiation travels downward to the food's surface.

Once the heat arrives at the food, it raises the food's temperature. As the food becomes hotter, chemical reactions begin to occur and molecules begin to change shape. Thermal energy makes it possible for chemical bonds within and between the molecules to come apart so that new bonds and new molecules can form. Water and other small molecules evaporate more and more rapidly until the water begins to boil. Sugar molecules rearrange to form caramels and carbon. Protein molecules rearrange and stiffen. These molecular changes, together with the increased temperature of the food, are what we associate with cooking.

633. How does electricity work and is it possible to design a light bulb that will let you know when it is about to stop working? — LS, Chicago, IL
Electricity involves electric charges. While static electricity involves stationary electric charges, the electricity you are probably referring to is dynamic: electricity in which the electric charges move. Most (dynamic) electricity is the movement of electrons—tiny negatively charged particles that form the outer part of atoms. The electricity in the wires leading to and from a lamp is the flow of electrons through those wires. A lamp has two wires attached to it because the electrons flow into the lamp through one wire and out through the other wire. However, because the electricity we normally use is alternating current, the direction in which the electrons flow through those two wires reverses 120 times a second (60 full cycles of reversal, over and back, each second).

As the electrons flow through the lamp's filament, they leave behind much of their energy. This energy is deposited in the tiny filament and the filament becomes extremely hot. It begins to emit much of its thermal energy as thermal radiation, part of which is visible light. So you can think of the electricity as a steady stream of tiny delivery trucks (the electrons), carrying energy to the lamp's filament, and then returning to the power company to pick up some more energy. The filament sends this energy into the room as heat and light.

When a light bulb burns out, it's because the filament has became so thin that a section of it has overheated and melted. This thinning process is caused by the slow evaporation (or actually sublimation) of tungsten atoms from the filament. A thinned filament usually fails as you turn the bulb on because that's the time of maximum power delivery to the filament and thus maximum stress. Unfortunately, it's very hard to tell in advance whether the filament will be able to tolerate the next attempt to turn it on. Probably the best predictor is the number of hours the bulb has been on. If you always replace a bulb after it has operated for 750 hours at full power, you'll probably avoid most outages.

634. How does a magnetron work? — MM, Czech Republic
A magnetron has a ring of resonant electromagnetic cavities around a hot central filament. Each resonant cavity acts like an electromagnetic "tuning fork"—electric charges and electromagnetic waves swing back and forth inside a resonant cavity at a particular frequency; the cavity's resonant frequency. As electrons are "boiled" off the hot filament, a high voltage attracts them toward the walls of the resonant cavities. The resonant cavities tend to have at least small amounts of electric charge "sloshing" back and forth in them at their resonant frequencies and the electrons from the filament are attracted more strongly to the cavities' positively charged walls than to their negatively charged walls.

However, there is also a magnetic field present in the magnetron and this field deflects the streams of electrons so that they hit the wrong walls of the resonant cavities. Instead of canceling the charge sloshing in the walls of the resonant cavities, the newly arrived electrons add to it. As electrons flow to the resonant cavities, more and more charge sloshes in the resonant cavities and these cavities accumulate huge amounts of energy. Some of this energy is tapped by a small wire loop and a microwave antenna. This antenna radiates some of the energy from the cavities into a metal channel that leads away from the magnetron. In a microwave oven, this channel leads to the cooking chamber so that energy from the resonant cavities is delivered to the food in the oven. Energy is extracted from the magnetron slowly enough that the filament and high voltage power supply can replace it and the operation continues indefinitely.

635. What does the inside of a microwave oven look like? Please show illustrations. — Dade County, FL
A microwave oven contains (1) a magnetron that produces the microwaves, (2) a high voltage direct current power supply (a high voltage transformer, a set of rectifiers, and a capacitor) that provides power to the magnetron, and (3) a computerized control system that turns the power supply and magnetron on and off. A metal pipe connects the magnetron to the cooking chamber of the oven. While there are photographs and drawings of the insides of a microwave oven in my book, I can't reproduce them here because of copyright issues.

636. Why is the sky blue? - Z
As it passes through the atmosphere, sunlight can be deflected by a process known as Rayleigh scattering. When sunlight passes through any material, its light waves cause electric charges in the material to jiggle back and forth. That's because light waves contain electric fields and electric fields exert forces on electric charges. When the charges in a material jiggle back and forth, they may emit light. In this case, the material can absorb the sunlight for an instant and reemit it in a new direction. This process, whereby jiggling electric charges in a material absorb a light wave and reemit it in a new direction, is Rayleigh scattering.

Rayleigh scattering is extremely inefficient in particles that are much smaller than the wavelength of the light, so that visible light can travel through miles of molecules in the atmosphere before it experiences significant Rayleigh scattering. But blue light has a shorter wavelength than red light and thus experiences Rayleigh scattering more often than red light. As a result, the atmosphere tends to send the blue portion of sunlight off in every direction. Thus when you look at the atmosphere, it appears blue.

A reader (TAC) points out that the above explanation would seem to imply that the sky should appear violet, since violet light scatters more strongly than blue light. But the spectrum of sunlight peaks in the green—sunlight contains more green light than blue light and more blue light than violet light. The sky combines these two effects together (more green light but better scattering of violet light) and acquires an overall blue appearance.

637. How are luminol and fireflies related? — JH, Minneapolis, MN
There are a few molecules that can be chemically oxidized to produce new molecules that then spontaneously emit light. The chemical reactions that occur in these special molecules leave the resulting new molecules electronically excited—their electrons are in states that have more than the minimum allowed energies. As these energetic electrons subsequently shift to states with less energy, they release some of that energy as light.

In a firefly, the molecule that is being oxidized is called luciferin. It's combined with oxygen and the important biological energy storage molecule ATP (adenosine triphosphate), assisted by a catalyst protein called luciferase. A series of reactions then occurs, culminating in the formation of excited decarboxyketoluciferin. This molecule emits a photon of green light and becomes normal decarboxyketoluciferin.

Luminol, a molecule used in many cold light products, is a somewhat simpler molecule that is much easier to synthesize commercially than is luciferin. When it's oxidized with hydrogen peroxide and potassium ferrocyanide, it forms an excited molecule that emits a photon of blue light. This blue light is often shifted to green or orange with the help of a fluorescent dye. The dye absorbs the blue light and uses its energy to emit green or orange light. This material is commonly used in light sticks and glowing necklaces or toys.

638. Is it possible to make a black bulb that absorbs light rather than emitting it? — KD, Pflugerville, TX
Not unless you will consider a black hole to be a black bulb. For a "bulb" to absorb light that isn't heading toward the bulb, that bulb will have to attract the light toward it. Since light has no electric charge, the only force that the bulb can exert on light is gravitational force. While a black hole's gravity is strong enough to attract and ensnare light, it wouldn't make a very practical bulb. However, it is possible in certain circumstances to add light to previously existing light and, in doing so, create a dark shadow that wasn't present before. This process is called interference, where two light waves cancel one another in a particular region of space and prevent any light from reaching a certain spot. But this cancellation is difficult to achieve, except with lasers, and doesn't occur everywhere in space—the light doesn't vanish, it just gets redistributed. Overall, the idea of a black bulb is just not realistic.

639. How does a CD player work? — NL, Dearborn, MI
A CD player uses a laser beam to determine the lengths of a series of ridges inside a compact disc. Infrared light from a solid-state laser is sent through several lenses, a polarizing beam splitter, and a special polarizing device called a quarter-wave plate. It's then focused through the clear plastic surface of the compact disc and onto the shiny aluminum layer inside the disc. Some of this light is reflected back through the player's optical system so that it passes through the quarter-wave plate a second time before encountering the polarizing beam splitter. The two trips through the quarter-wave plate switches the light's polarization from horizontal to vertical (or vice versa) so that instead of returning all the way to the laser, the light turns 90° at the polarizing beam splitter and is directed onto an array of photodiodes. These photodiodes measure the amount and spatial distribution of the reflected light. From this reflected light, the CD player can determine whether the laser beam is hitting a ridge or a valley on the disc's aluminum layer. It can also determine how well focused or aligned the laser beam is with the aluminum layer and its ridges. The player carefully adjusts the laser beam to follow the ridges as the disc turns and it measures how long each ridge is. The music is digitally encoded in the ridge lengths so that by measuring those lengths, the player obtains the information it needs to reproduce the music.

640. Why does a helium balloon in a car seem to defy Newton's laws? When you accelerate forward suddenly, the balloon moves forward and when you brake, the balloon moves back. Is that because the air inside the car compresses when you accelerate? — CT, Charlottesville, VA
Since the air in the car is denser than the helium balloon, the air's motion dominates the helium balloon's motion. When your car accelerates forward, the air's inertia tends to move it toward the back of the car-the accelerating car is trying to leave the air behind. The balloon moves forward in the car to give the air more room near the back of the car. When you stop suddenly, the air in the car continues to coast forward and accumulates at the front of the car. Again, the balloon moves backward in the car to give the air more room at the front of the car. You'll see exactly this same effect if you watch an air bubble in a bottle of water as you drive the bottle around in a car.

641. When light hits an object, how do we recognize the color? — CM, Levering, PA
White light is a mixture of various light waves with different wavelengths and thus different colors. When white light hits an object, some of the light waves are absorbed while others are not. The light that isn't absorbed may pass through the object or it may be reflected in a new direction. The light that you observe coming from the object is this transmitted or reflected light. If the light that you see doesn't include the same mixture of wavelengths that first hit the object, you won't see this light as white. Instead, you'll see it as colored. If the light you see contains mostly long wavelengths of light, you'll see it as red. If the light contains mostly short wavelengths of light, you'll see it as blue or violet. The wide range of colors that objects have comes from subtle differences in the wavelengths of light they absorb. However, when an object is illuminated with colored light, the light that it transmits or reflects may be altered. After all, it can't transmit or reflect a light wave that never hit it in the first place. Even variations in "white" light can affect an object's color—makeup looks different in incandescent "white" light than it does in fluorescent "white" light because those illuminations contain different mixtures of light waves.

642. How do lasers work?
Lasers use systems with excess energy to amplify light. These systems, typically atoms or atom-like structures in solids, are in excited states—they have more than their minimum amounts of energy. An excited system can get rid of its excess energy in many different ways, but certain systems tend to emit the excess energy as photons—particles of light. While an excited system will emit a photon spontaneously if you wait long enough, it can also duplicate a passing photon if that passing photon has the proper characteristics. Most importantly, the excited system must be naturally capable of emitting the passing photon spontaneously—the passing photon's wavelength and travel path must be such that the excited system is able to duplicate it.

This duplication effect makes it possible to amplify light. When a single photon passes by a number of identical excited systems, those systems may duplicate the photon many times so that many identical photons emerge. This phenomenon is the basis for laser amplifiers. When one of the photons emitted spontaneously by the excited systems is deliberately sent back and forth through those systems with the help of mirrors, the laser amplifier becomes a laser oscillator—it both initiates and amplifies the light. The light that ultimately emerges from the laser oscillator or amplifier differs from normal light because the laser light consists of many identical photons. They all have identical wavelengths (colors) and follow identical paths through space. They also exhibit dramatic wave effects, particularly interference.

643. What is the chemical formula for glass? — GL, Birmingham, AL
Glass isn't a simple molecule that can be represented by a normal chemical formula. It's a network solid in which the atoms are joined in one gigantic non-crystalline structure. In effect, a piece of glass is a single enormous molecule. Window glass is called soda-lime-silica glass and consists mostly of silicon, oxygen, sodium, and calcium atoms. Silicon and oxygen are considered to be network-forming atoms and bind to one another in long atomic linkages that form the backbone of the glass. The sodium and calcium atoms are added to terminate the linkages. This network termination softens the glass, lowers its softening and melting temperatures, and generally makes the glass easier to work with. Harder glasses such as lead "crystal" replace the sodium and calcium with other materials (e.g. lead oxide) that don't weaken the glass as much and produce harder or stronger glasses. Pyrex cookware contains boron instead of sodium and calcium, and is a borosilicate glass.

644. How can MRI pictures show slices through an object? And how do you get an image from using a magnet?
MRI images show where hydrogen nuclei (protons) are located in a person's body. Protons are magnetic particles that have only two possible states in a magnetic field: aligned with the field or aligned against the field (also called "anti-aligned"). This limited range of alignments is the result of quantum physics. Normally, the protons in a person's body are equally divided between aligned one way and aligned in the opposite way. But when a person is placed in a strong magnetic field, the protons in their body tend to align with the magnetic field and the distribution of aligned and anti-aligned protons shifts. There are then somewhat more aligned protons than anti-aligned protons.

Once there are more aligned protons than anti-aligned protons, it becomes possible to flip them about. Flipping these protons from aligned to anti-aligned takes energy and this energy can be provided by a radio wave. But not just any radio wave will do: its frequency must be just right in order to provide the proper amount of energy or the proton won't flip. When the right radio wave is provided, some of the aligned protons will flip to become anti-aligned. This flipping of protons can be detected by a sensitive radio receiver.

By placing the person in a non-uniform magnetic field and by adjusting the frequencies and timings of the radio waves, an MRI device can determine where protons are located in the person's body to with a few millimeters. A computer records where the protons are and then displays information about them as cross sectional images. For example, the computer can display a dense concentration of protons as white and a region with few protons as dark. MRI is particularly good at imaging tissue because tissue contains lots of hydrogen atoms and their protons.

645. Does the air pressure of a basketball and the hardness of the floor surface have an effect on the height of the bounce? — BB, West Unity, OH
Yes to both questions. When a basketball collides with the floor, the ball's kinetic energy—its energy of motion—is temporarily stored as elastic potential energy in two objects: the ball and the floor. The fractions of the collision energy stored in the basketball and the floor depend on how far each of them dents—the more one dents, the larger the fraction of the collision energy it receives. How well the basketball rebounds from the floor depends on how much of the collision energy returns to the ball during the rebound. Some of the stored energy in each dented surfaces is converted to thermal energy and is lost from the bouncing process. A hardwood floor is very springy and returns its share of the collision energy efficiently. A properly inflated basketball is also very springy. Thus when a firm basketball bounces on a good hardwood floor, it bounces well. But if the basketball is underinflated, its surface bends too far so that it receives most of the collision energy and internal friction in the ball's skin wastes most of that energy. The ball bounces weakly. And if you try to bounce the ball on a soft carpet, the carpet dents easily, receives most of the collision energy, and wastes most of it as thermal energy. Again, a weak bounce.

646. How do helicopters work? — KH, Holland, MI
The wings of a normal airplane obtain upward lift forces from the air as the airplane moves forward through the air. That's because the shape and angle of the wings is such that air flows faster over the top surface of each wing than under the bottom surface of that wing and the air pressure above the wing drops below the air pressure below the wing. Each wing experiences a net upward pressure force and these upward forces are enough to support the weight of the plane.

A helicopter spins its wings around in a circle so that they move through the air even when the helicopter itself is stationary. Normally, these rotating wings are called blades. Again, the air flows faster over each blade than beneath it and there is a net upward pressure force on each blade. These upward forces support the helicopter and they also allow it to tilt itself—by adjusting the angle of each blade as the blades turn, the helicopter can obtain twists from the air so that it tilts one way or the other. Once the helicopter has tilted, it can use some of the lift force from its blades to push it horizontally so that it accelerates forward, backward, or toward the side.

647. How do I graph (line or pie) the time it takes different amounts of water to freeze? — LC, TX
First, you must determine what it is that you're really measuring. If you pour a gallon of water onto a huge copper plate that's been cooled to -200° C, the water will freeze in a fraction of a second while if you put a drop of water on a hot frying pan, it will never freeze at all. You must design a sensible experiment and then repeat it with several different amounts of water. The experiment should be sure to focus on the water by avoiding situations where external effects determine the freezing time. For example, you might obtain 4 identical 1-liter containers and fill them with 1/4, 2/4, 3/4, and 1 liter of the same water respectively and then put them simultaneously in a freezer with a uniform cold temperature. Then you can record how long it takes each of them to freeze. Then use an XY graph to plot these times: the x-axis could be the amount of water in the container and the y-axis could be the time it took for the water to freeze. The four points you'll obtain probably won't form a straight line. That's because the amount of heat that must leave the water for it to freeze depends on the water's volume and the time it takes that heat to leave depends on the water's surface area. Doubling the water's volume doesn't double its surface area, so the freezing time will have an interesting and somewhat complicated dependence on the water's volume. Try it!

648. I have heard of a magnetic top that will spin on top of another magnetic field because of the gyroscopic effect. If that is put into a vacuum chamber, would it spin perpetually? — JH, Visalia, CA
Probably not. The magnetic top that you mention is a wonderful invention, sold under the name "Levitron". It uses gyroscopic precession to stabilize what is normally an unstable arrangement: two oppositely aligned magnets, one supporting the other. In air, you can get the Levitron top to stay aloft for a couple of minutes before its spin rate declines to the point where it stops being stable. In a vacuum, I'd expect it to last much longer but not forever. Thermodynamics overwhelms just about everything sooner or later and the Levitron won't be an exception. Even if you get rid of air resistance, the spinning top's strong magnetic field will interact with its environment and will allow the top to exchange energy with that environment. While there is always the possibility that these exchanges will make the top spin faster, such favorable exchanges are relatively unlikely. Instead, the energy exchanges are much more likely to extract energy from the top and slow it down. For example, any conducting surfaces near the Levitron top will exert a magnetic drag force on the top and will convert its energy into thermal energy in those conducting surfaces. Forever is a long time and the top will certainly slow to a stop eventually. Still, it might be interesting to see how long it can stay spinning. I'll bet 10 minutes is the realistic maximum. If I have a chance to test it out, I'll let you know what happens.

649. My son and I are building an electromagnet for a science project. We know that if we wrap the wire around the nail and connect the battery to the wire...presto, a magnet is born. But what is it about flowing current that allows this to happen? — GG, Westfield, NJ
Moving electric charges are inherently magnetic. That's because electricity and magnetism are intimately related and aren't really separate phenomena. To see why this is true, imagine two electrons sitting motionless in front of you—they push one another away with electric forces. But now imagine that you and those two electrons are moving northward in a train and someone standing beside the track is watching all of you pass. From that person's perspective, the two electrons are moving and they exert both electric and magnetic forces on one another. What appears to you to be a purely electric effect appears to the person near the track to involve both electricity and magnetism. Without the appearance of magnetic effects in moving charges, grave inconsistencies would appear in the dynamics of objects view from different perspectives.

So the current in the wire of your electromagnet is inherently magnetic. The magnetic field it produces aligns the tiny magnetic domains in the steel nail so that the nail's magnetic field greatly strengthens that of the current in the wire.

650. If microwave cookers are so energy-efficient, why can't similar machines be used as hot water heaters or in central heating systems? - GB
Microwave ovens transfer about 50% of the electric energy they receive from the electric company to the food. Conventional ovens transfer only something like 10%. Cooking just isn't a very energy efficient process because you're trying to get heat into an object from outside that object. In contrast, an electric space heater transfers 100% of the electric power it receives to the room around it. Home heating is much more energy efficient because you're getting heat into an object from inside that object. In effect, your microwave oven is also 100% efficient at heating your room—every bit of electric energy it consumes eventually enters your room as heat. But it's an expensive sort of "space heater" and you do better just to use conventional heating systems.

651. What would happen if the two magnetic poles of the earth were to be reversed? Would it affect climate and weather? Has this ever happened before? — HP, Birmingham, AL
The earth's magnetic poles have reversed before, many times. A record of the earth's magnetic field is made whenever a magnetic mineral is cooled through a magnetic transition temperature called the Curie point (named after Pierre Curie, the husband of Marie Curie, who first identified it). Volcanic lava often includes such magnetic minerals and as the lava cools, it records a snapshot of the earth's current magnetization. By examine ancient lava flows, scientists have pieced together a detailed record of the earth's magnetization and have found that the earth's magnetic poles have drifted about and reversed many times, typically every few hundred thousand years or so.

I can't think of any mechanism whereby these reversals would seriously affect climate or weather. However, these reversals would affect some migratory animals that use the earth's magnetic field to navigate. In principle, these animals might migrate the wrong direction and die out. However, there are always a few of each species that are born with their magnetic compasses reversed. While these backward animals might not survive during normal times, they would prosper during a reversal and would help to perpetuate their species. Moreover, experiments have shown that individual animals can adapt to the magnetic reversals as well.

652. Where is all the matter "sucked" into a black hole thought to go? — KH, St. Johns, Newfoundland
From our perspective outside a black hole, the matter never quite passes through the black hole's event horizon—the surface from which not even light can escape. That's because time slows down near the event horizon and it takes an infinite amount of our time for the matter to pass through the event horizon. But from the perspective of the matter falling through the event horizon, the passage is uneventful—the matter experiences no sudden changes as it passes through that surface of no return. Instead, the matter continues to accelerate toward the singularity at the center of the black hole—a point of infinite density and infinitely small size. Its approach to the singularity completely destroys the matter's structure. The gravitational tidal forces caused by the differences in gravity at different locations in space tear the matter apart so that it contributes only mass, charge, momentum, and angular momentum to the singularity. The black hole is usual identified with the event horizon rather than the singularity contained inside it. Passage through that event horizon erases any memory of the structure of the matter, leaving only its mass, charge, momentum, and angular momentum observable in the properties of the black hole.

653. I recently acquired a microwave that "doesn't cook as fast as it used to." Does this sound right? What type of service might need to be performed? - W
It is possible for a microwave to lose cooking speed. If the microwave source isn't able to produce as intense microwaves as before or if it doesn't turn on reliably and steadily, it won't cook as fast. For the source to produce less intense microwaves, the high voltage power supply would probably have to be weak. Its storage capacitor could have failed or one or more of its high voltage diodes could have burned out. According to a reader, the most likely cause of weak cooking in a microwave oven is a failed capacitor—with no ability to store separated charge in its capacitor, the oven produces pulsing rather than steady microwaves and delivers less average power. I suppose that the magnetron itself could be dying, with the most common failure (according to that same reader) being shorting out, the result of electromigration of the filament material. For the source to not turn on reliably, it would probably have to have a bad connection to the power line. One good possibility is that the relay that turns on power to the high voltage power supply is not making good contact.

Listen to the microwave as it operates on a medium setting. It should cycle on and off every five or ten seconds. You should hear it hum softly during the on half of the cycle and then stop humming during the off half of the cycle. Different power levels simply vary the fractions of on time and off time. If you don't hear the hum or the hum is intermittent, then something is probably wrong with the power relay or with something else in the high voltage power supply. If the relay is flaky, a little cleaning of its contacts may cure the problem. Be careful of the high voltage capacitor, which can store a lethal charge even when the unit is unplugged.

654. In a microwave oven, does food cook from the inside out or outside in? — KS, Essex, England
If the piece of food isn't too large, it all cooks at once. The microwaves that heat the food pass deep into it and they deposit energy in every part of the food simultaneously. Only if the piece of food is so large that an appreciable amount of microwaves are absorbed before they reach the center will the center cook more slowly than the outside. I doubt that this shielding of the center is a problem with foods small enough to fit inside a normal microwave oven. However, the microwaves in a microwave oven aren't perfectly uniform, so that some parts of a meal will cook a bit faster than others. That's why it's important to move the food about during cooking to achieve uniform heating throughout.

655. What is the scientific explanation of a rainbow? — RS, Salinas, CA
A rainbow is caused by three important optical effects: reflection, refraction, and dispersion, all working together. The rainbow forms when sunlight passes over your head and illuminates falling raindrops in the sky in front of you. This sunlight enters each spherical raindrop, partially reflects from the back surfaces of the raindrop, and then leaves the raindrop and heads toward you. The raindrop helps some of the sunlight make a near U-turn. But the path that the light follows after it enters the raindrop depends on its color. Light bends or "refracts" as it changes speed upon entering water from air and the amount it bends depends on how much its speed changes. Since violet light slows more than red light, a phenomenon called "dispersion," the violet light bends more than the red light and the two colors begin to follow different paths through the drop. All the other colors are spread out between these two extremes.

The colored rays of light then partially reflect from the back surface of the raindrop because any change in light's speed also causes partial reflection. Now the various colors are on their way back toward you and the sun. The light bends again as it emerges from the raindrop and the various colors leave it traveling in different directions. Only one color of light will be aimed properly to reach your eyes. But there are other raindrops above and below it that will also send light backward and some of that light will also reach your eyes. But this light will be a different color. What you see when you observe the rainbow is the lights that many different raindrops send back toward your eyes. The upper raindrops send their red light toward your eyes while the lower raindrops send their violet light toward your eyes. You see a series of colored bows from these different raindrops.

656. How can one measure the vapor pressure of mercury? If it is amalgamated, what is the relationship of vapor pressure with respect to temperature, material content in the amalgam, and free mercury? — BS, Erwin, TN
The vapor pressure of mercury is quite low at room temperature so you'd need a very sensitive pressure gauge and a vacuum system in order to measure it. You'd have to evacuate all of the air from the gauge and expose the empty gauge to a saturated vapor of mercury (mercury vapor that's in contact with liquid mercury) alone. While the pressure will only be a few thousandths of a millimeter of mercury, there are a number of pressure gauges that are capable of measuring pressures in this range.

Once the mercury is amalgamated with other metals, its vapor pressure drops substantially. The mercury atoms bind so strongly into the amalgam that they can remain in it for years, centuries, or even millennia. Mercury's vapor pressure in this bound form is exceedingly low. To measure it, you'd need a mass spectrometer that's capable of counting the atoms in the vapor above the amalgam.

657. If space is curved and gravity is not really a force (as per Einstein), how is it that an object can slingshot around a planet and gain kinetic energy? Where is the extra energy coming from? Which object converts mass to energy; the object or the planet? — EM, Redmond, WA
When a small object such as a satellite arcs around the back side of forward moving planet, the satellite's speed and energy increase while the planet's speed and energy decrease. The planet has given some of its energy to the satellite. Viewed in terms of curved space, the satellite follows a curved path because of the planet's presence and the planet follows a curved path because of the satellite's presence. The satellite's effect on space is very small, but it is enough to change the planet's path slightly. The planet arcs toward the satellite and gives up a small amount of its speed and energy in the process. This energy is transferred to the satellite as the satellite arcs toward the planet. Overall, the planet loses a little of its kinetic energy and the satellite gains an equal amount of kinetic energy. However, neither the planet nor the satellite experience any changes in rest mass. Both objects still have the same numbers of atoms as before and both still have their original masses.

658. What was the profession of A. B. Strowger, the inventor of the Step by Step Switching System? — GC, Pleasanton, CA
Mr. Strowger was apparently a Kansas undertaker when he developed his automatic telephone switching system.

659. A man falls into the center of the earth — how much does he weight? Which way is space bent in the center of the earth? — JW, Virginia Beach, VA
At the center of the earth, the man would be truly weightless and the space around him would be exactly flat (no curvature due to gravity). This special situation occurs because the gravitational effects of the earth around the man are perfectly balanced. With equal amounts of the earth's mass on each side, there is no special direction in which the man would accelerate.

660. I just bought a set of nice chrome wheels with low profile tires for my car. Since these 4 wheels are 40 pounds heavier than the old ones, I removed 40 pounds of weight from the body of the car to compensate. My acceleration times and braking distances have increased dramatically. Why? — DTS, Shawnee, Kansas
When you accelerate forward from a stop, the car's kinetic energy is increasing. The time it takes you to reach cruising speed is largely determined by how fast the car's engine can increase the car's kinetic energy. Stopping speed is similarly determined by how quickly the brakes can remove the car's kinetic energy. While your car still has the same mass that it had before you changed wheels, and thus would seem to require the same transfers of energy to start and stop, that's not the case. Transferring mass from the car's body to its wheels has substantially increased the amount of kinetic energy the car has when it's moving at cruising speed. That's because each spinning wheel has two forms of kinetic energy. First, its center of mass is heading forward at cruising speed, so it has a translational (motion along a line) kinetic energy proportional to its mass. Second, it is spinning about its center of mass, so it has a rotational kinetic energy proportional to its moment of inertia (the rotational equivalent of mass). If most of each wheel's mass is located near its periphery, its rotational kinetic energy will be roughly equal to its translational kinetic energy. The 40 pounds you transferred to the wheels is counting twice as much as before! You've effectively added 40 pounds to the mass of your car. Your new wheels and tires are demanding far more energy from your car's engine and delivering far more energy to your car's brakes than the old wheels did and you'll have to remove an additional 40 pounds from the car's body to compensate.

661. What causes a dropped ball to bounce? - MK
When you lift a ball off the floor, you transfer energy to it. This energy is stored in the gravitational force between the ball and the earth and is called gravitational potential energy. When you release the ball, its weight makes it accelerate downward and its gravitational potential energy gradually becomes kinetic energy, the energy of motion. When the ball hits the floor, both the ball's bottom surface and the floor's upper surface begin to distort and the ball's kinetic energy becomes elastic potential energy in these two distorted surfaces. The ball accelerates upward during this process and eventually comes to a complete stop. When it does, most of the energy that was initially gravitational potential energy and later kinetic energy has become elastic potential energy in the surfaces. However, some of the original energy has been converted into thermal energy by internal frictional forces in the ball and floor. The distorted ball and floor then push apart and the ball rebounds into the air. Some or most of the elastic potential energy becomes kinetic energy in the ball, and the rising ball then converts this kinetic energy into gravitational potential energy. But the ball doesn't reach its original height because some of its original gravitational potential energy has been converted into thermal energy during the bounce.

662. Why does a gas lantern use a silk mantle? How does it produce such intense light — BW, Santa Clara, CA
The mantle of a lantern is actually a ceramic ash. The silk itself burns away completely and leaves behind only of the oxides of materials that were incorporated in the silk mantle when it was manufactured. The principal oxide formed when the standard Welsbach mantle is burned is thorium oxide, with a few percent of cerium oxide and other oxides. This use of thorium oxide or thoria, is a rare example of a radioactive element (thorium is radioactive) permitted in common household use. Thoria glows brightly when heated because it can tolerate extremely high temperatures without melting and because it is a very effective emitter of thermal radiation at temperatures of roughly 2200° C.

The light emitted by these oxide mantles is shorter in average wavelength than can be explained simply by the temperature of the burning gases, so it isn't just thermal radiation at the ambient temperature. The mantle's unexpected light emission is called candoluminescence and is thought to involve non-thermal light emitted as the result of chemical reactions and radiative transitions involving the burning gases and the mantle oxides.

663. Why does an object like metal give off light when it is heated? — ER, Fresno, CA
All objects emit thermal radiation—electromagnetic waves that are associated with the transfer of heat. That's because all objects contain electrically charged particles and whenever electrically charged particles accelerate, they emit electromagnetic waves. Since all objects have thermal energy in them, their electrically charged particles are always undergoing thermal motion and their thermally induced accelerations cause them to emit electromagnetic waves.

At normal temperatures, the electromagnetic waves of thermal radiation are too low in frequency and too long in wavelength for us to see. But when an object's temperature exceeds about 500° C, the object emits a dim glow. By 1800° C, the object emits the yellowish glow of a candle. By 2700° C, the object emits the yellowish-white light of an incandescent bulb. By 5800° C, the object emits the white light of the sun.

664. Is there water on the moon? — JB, Edmonton, Canada
Recent radar studies of the moon's surface have indicated that water may be present at the bottoms of deep craters near the moon's north and south poles. Because sunlight never reaches into these craters, they have cooled by radiating their heat into the empty space overhead and are now extremely cold. They're so cold that water deposited there, probably by comet impacts, has remained as ice for millions of years. While the ice in your freezer slowly disappears because the water molecules sublime—become water vapor—at normal freezer temperatures, extremely cold ice barely sublimes at all and can exist in a vacuum almost indefinitely.

665. How can an insulator carry a charge if it cannot conduct electricity? How can one charge an insulator? Can an insulator be charged by induction? — VV, Washington, DC
While charge can't move through an insulator, there is nothing to prevent charge from being placed on its surface or injected inside it. If you rub the surface of an insulator with a piece of silk, sliding friction will push electrons onto or off of its surface and leave its surface electrically charged. With no way for that charge to move about, the insulator's surface retains the charge indefinitely. A beam of fast moving electrons or other charged particles can be injected into an insulator and will become trapped inside it. Once again, the charges can't move around after the injection. Since charges can't flow in the insulator, you can't charge it by induction—a process in which proximity to a nearby charged object rearranges the charges in a conductor and allows you to trap those charges in a nonuniform arrangement.

666. What is a short in the electrical system of a car? What causes shorts? — BM, Puyallup, WA
A short circuit is a conducting path that allows electric current to flow from its source (typically the positive terminal of a battery) to its destination (typically the negative terminal of that battery) without passing through the equipment that the current is supposed to operate. The conducting path is thus a short cut for the current that allows it to complete its circuit too quickly, hence the name "short circuit." In virtually all automobiles, the whole body of the car is connected to the negative terminal of the battery so that any accidental conducting path from the battery's positive terminal to the body of the car is a short circuit. Since a short circuit doesn't include a device that's designed to consume electric power, the wires of the short circuit must consume that electric power. They often become hot and may cause a fire.

667. How can you make electricity with magnets? - AL
You can make electricity by moving a magnet past a wire. The magnet has a magnetic field around it—something that exerts forces on magnetic poles. If you move the magnet and its magnetic field, you create an electric field—something that exerts forces on electric charges. That's because whenever a magnetic field changes with time, it creates an electric field. This electric field will push on the mobile electrons in a wire. So when you move a magnet past a wire, you are producing a changing magnetic field in the wire. This changing magnetic field produces an electric field and the electric field makes the electrons in the wire accelerate. The moving electrons are electricity. Generators move magnets past wires (or wires past magnets) to produce electricity.

668. Would extreme temperatures affect the strength of a magnet? — PL, Columbus, OH
Yes! High temperatures disorder materials and destroy magnetic order. Permanent magnets can be demagnetized by heating them, often to surprisingly modest temperatures. Many household magnets can be spoiled by putting them in a hot oven. Even electromagnets will lose most of their strength at very high temperatures because they rely on iron and iron undergoes several phase transitions at high temperatures that destroy its magnetic order. You can show that iron loses its magnetism at high temperatures by heating a steel nail red hot with a propane torch and then trying to pick it up with a magnet. Be careful not to burn yourself. The hot nail won't stick to the magnet because it won't have any magnetic order. Once the nail cools, its magnetic order will reappear.

669. How do you calculate the path light takes after going through a lens and how do you measure the curvature of the lens? — AS, Champaign, IL
The surfaces of most lenses are shaped like the surfaces of spheres. Such "spherical" lenses can be characterized by a single distance: the focal length. For converging lenses, those with convex or outward-bulging surfaces, light from a distant object such as the sun will converge together after passing through the lens and will form an image of the object at a distance of the focal length from the center of the lens. You can find this "real" image by holding a sheet of white paper beyond the lens and looking for a clear pattern of light corresponding to the object. If the object is closer to the lens, the image will form a bit farther from the lens. The relationship between the distance to the object (the object distance or OD), the focal length of the lens (F), and the distance to the image (the image distance or OD) is given by a simple formula: 1/F = 1/OD + 1/ID.

This lens formula works for diverging lenses, too, but those lenses have negative focal lengths and produce their images on the object side of the lens. You can only view these "virtual" images by looking at them through the lens itself.

The easiest way of determining a lens's focal length is by measuring the distance between the lens and the real image it forms of a distant object. However, you can measure the curvatures of the lens's surfaces and calculate its focal length. Special gauges exist that touch the lens at several points, usually a circle and a central point, and determine how curved its surface is.

670. How do you create sculptures out of glass? - RD
While I know how to work with glass in principle, I'm certainly not able to make sculptures. Although anyone can shape glass, doing so with artistry and precision requires great skill. In effect, glass is a frozen liquid. Its microscopic structure is very similar to that of a liquid and it softens with temperature rather than melting abruptly. If you heat a piece of glass carefully with a propane torch, it will begin to flow as a thick liquid (like cold honey). In that state, it can be reshaped rather easily. But making it take the shapes you want is a whole other story and something I know little about. I have bent lots of glass tubes in my day, but I often kink the tubing or smash it flat by accident. A skilled glassblower can do seemingly impossible things with glass. I should also note that glass can be cut or shaped by a water-cooled abrasive wheel. Again, anyone can slice and dice glass but it takes great skill to do something attractive. I usually chip the glass pieces that I try to cut.

671. I recently received a "strong magnetic cup" as a gift. According to the claims of the maker, water kept in this cup for a minute can lower blood pressure and reduce weight, etc. Please explain how this works. — AL, Pharr, TX
I'm afraid that it works only by psychological effect, if at all. Water itself is non-magnetic and experiences no significant change when exposed to a magnetic field. Although the magnetic field of the cup has an ever so slight effect on the atomic and molecular structure of the water, this effect vanishes when the water leaves the cup. Water from the cup is just plain old water. There are many people in this world who take advantage of the public's relative inability to distinguish science from pseudoscience. One of the reasons that I enjoy answering questions here is to help people make that distinction. Magnets aren't magic—they are understandable devices and their effects on everything around them are also understandable.

672. In high school physics, we learned that matter and energy can neither be created nor destroyed. Is that true in quantum mechanics? What is quantum mechanics and how did the field come about? — JE, College Station, TX
While modern physics continues to maintain that matter and energy can't be created or destroyed, the picture is a little more complicated than it was before the discovery of relativity and quantum mechanics. First, relativity ties matter and energy together so that matter can become energy and energy can become matter in certain circumstances. As a result, it's only the sum of matter and energy that can't be created or destroyed. Second, there are situations in which that sum of matter and energy can change temporarily in an isolated system. Quantum mechanics and its famous "uncertainty principle" permit brief but important violations of the conservation of mass/energy. The shorter a particular violation, the worse it may be. These violations are never directly observable because all observations are done on long time scales. But there are indirect indications of these temporary violations and they're critical to much of modern high energy and particle physics.

Quantum mechanics developed at the beginning of this century to explain several strange experimental observations, particularly the photoelectric effect and the black-body radiation spectrum. Einstein received his Nobel Prize for explaining the photoelectric effect in terms of quantum mechanics, not for any of his work on relativity.

673. Airplanes can land at many different altitudes above sea level. How does the altimeter work at these different landing altitudes to show zero when the plane has finally landed? — BC, Canada
Their altimeters don't read zero once they have landed—they read the altitude of the airport! Each airport's altitude is reported on the navigational maps that pilots use. As the pilot approaches the runway, the pilot watches the altimeter and expects it to reach the airport's altitude about the time that the plane touches the runway. Before the next take off, the pilot adjusts the altimeter using the airport's official altitude as a calibration point for the altimeter. Some modern planes also used radar equipment to determine the distance to the ground beneath the plane. These devices do read zero at landing. The satellite-based Global Positioning System (GPS) also provides altitude information to pilots. Since this system reports altitude above sea level, it gives the altitude of the airport at landing, not zero.

674. How do the thermometers with digital (electronic) numbers work? — KM, Lincoln, NE
The electronic fever thermometers that you can buy in a grocery store use a thermistor to measure temperature. A thermistor is a semiconductor device that acts as a temperature-sensitive electric resistor. At very low temperatures, a thermistor is essentially an insulator—it has no mobile electric charges and thus can't carry electricity. But as its temperature increases, thermal energy rearranges the charges in the thermistor and it has more and more mobile electric charges. Its ability to conduct electricity increases with temperature fairly dramatically—it gradually becomes an electric conductor. The thermistor used in a fever thermometer is designed to undergo this rapid change in electric resistance at temperatures near 98° F. A simple computer inside the thermometer measures the thermistor's electric resistance and determines the thermistor's temperature. It then uses a liquid crystal-based display to show you what that temperature is.

675. How do these digital thermometers work? I read somewhere that they contain alcohol instead of mercury. — KM, Lincoln, NE
Most modern liquid-in-glass thermometers do contain alcohol rather than mercury, but these aren't the digital thermometers you are referring to. The alcohol thermometers are the ones with the red line that moves upward in a glass tube as the temperature increases. I believe that the digital thermometers you're interested in are the ones with numbers that change colors as the temperature changes. For example, when its 72° F, the number "72" is brightly colored while the other numbers are essentially black. Those thermometers use liquid crystals to measure temperature. More specifically, they use chiral nematic liquid crystals—long asymmetric molecules that arrange themselves in orderly spirals in the liquid. When light strikes these spiral structures, some of it reflects. But the reflection is strongest when the light's wavelength is an integer or half integer multiple of the spiral's pitch—the distance between adjacent turns of the spiral. Since light's wavelength is related to its color, the light reflected by these liquid crystals is colored. Because the pitch of a chiral nematic liquid crystal changes with temperature, so does its color. Slightly different liquid crystals are inserted behind each number on the thermometer so that each number becomes colored at a different temperature.

676. How is the xerographic copying process related to laser and led printers? - BG
The same basic printing process is used in both xerographic copiers and laser or led printers. In all cases, a charge image is formed on the surface of a photoconductor and this pattern of electric charge attracts a pattern of colored plastic powder. The powder is then transferred to paper and melted or pressed into the paper's surface to form a permanent print.

The main difference between a copier and a printer is in the source of light used to produce the charge image. In a copier, lenses and mirrors are used to form a real image of the original document on the surface of the photoconductor. Wherever light from the white portions of the document strikes the photoconductor, the photoconductor becomes an electric conductor and charge is able to move. The pattern of light then becomes a pattern of charge—a charge image.

In a printer, a laser or an array of light emitting diodes is used to form the pattern of light on the surface of the photoconductor. Wherever the light strikes the photoconductor, charge is again able to move about. Dot by dot or row by row, the charge image takes shape. The pattern of charge that's written on the surface of the photoconductor eventually becomes the printing itself.

677. I have heard that diode lasers won't work in ring laser gyroscopes because these lasers are not single frequency. If this is true, will a prism or a diffraction grating isolate one of the frequencies? - M
While most diode lasers operate at several frequencies simultaneously, it's possible to make lasers that function at only one frequency. In fact, such "single mode" diode lasers are extremely stable light sources and the basis for much current research in optical science. For example, the recent observations of Bose condensation in vapors of alkali metal atoms were made with the help of single mode diode lasers.

The phrase "single mode" refers to a single longitudinal wave that travels back and forth through the device while it is operating. This single wave has one frequency and one wavelength. It is selected from other possible waves through the use of interference effects. For the wave to be stable inside the laser cavity (the laser is bounded at each end by a mirror, thus forming an optical cavity), the cavity's length must be an integer or half integer multiple of the light's wavelength. While that criterion alone will allow several possible waves to form, coupling a second cavity to this laser cavity further restricts the wave so that only a single wave can operate inside the laser. The diode laser will then have only a single mode of operation and will emit a single frequency of light.

678. What are the risks of occupational exposure to "black" fluorescent lamps? - MB
By "black" lamps, you mean ultraviolet lamps. Since ultraviolet light is able to cause chemical damage to biological tissue, long-term exposure to this light isn't so good. How much risk there is depends on how much ultraviolet light they produce and how near you are to them. Sunlight contains a considerable amount of ultraviolet, so long exposure to sunlight burns and ages skin. The photons of ultraviolet light contain enough energy to cause changes in molecules and thus upset the cellular machinery that keeps us healthy. Ultraviolet lamps will do the same thing, given enough intensity and time.

679. What is the most explosive and energy releasing combination of chemicals? — RC, Chapman, Australia
A mixture of 1 part hydrogen and 19 parts fluorine by weight is the most energetic possible mixture of chemicals, releasing approximately 13,600 joules of energy per gram. The next most potent mixture is 8 parts oxygen and 1 part hydrogen by weight, releasing approximately 13,400 joules of energy per gram. Because fluorine is such a vigorous oxidizer that tends to cause fires, it isn't practical for rocket propulsion. The hydrogen/oxygen mixture is the basis for the Single Stage to Orbit rockets that are currently being developed. — Thanks to Gary V. Lorenz at NASA for help on this question.

680. What is the "memory effect" of a NiCad (nickel-cadmium) battery? Is it reversible or minimizable? - MF
NiCad batteries are more rechargeable than most batteries because the chemicals that power NiCad batteries remain solid throughout the discharge cycle. The chemicals in most other batteries, including alkaline batteries, go into solution or otherwise change shape during the discharge cycle so that it difficult to reconstruct the original battery electrodes during recharging.

Unfortunately, the two solid electrodes in a NiCad battery are damaged by repeated charging and discharging. These electrodes work best when they are both fine powders (the positive electrode is nickel hydroxide powder and the negative electrode is cadmium metal powder). With repeated use, the powder particles grow larger and larger and they stop contributing to the battery's power. "Memory" appears during the discharge cycle when all the useful small particles have been used up and only the undesirable large particles remain. Repeated charging and partial discharging tends to convert many of the small particles into large particles. You can improve the battery by fully discharging it before recharging it, presumably because this deep discharge breaks up the larger particles so that the battery contains mostly small particles once again.

681. How does radar absorbent materials work. How effective is stealth technology? — DP, Scottsbluff, NE
I believe that most radar absorbing materials are partially conducting plastic composites. As a microwave from the radar transmitter penetrates these composites, the electric field in that wave drives charges back and forth through the composites. Since the composites don't conductor electricity well, they turn the wave's energy into thermal energy and thereby absorb it. A similar effect occurs for light waves when you shine them on a pile of powdered charcoal. (According to David Ingham, some radar absorbing materials include lossy magnetic materials—materials such as ferrite and carbonyl iron that respond to the magnetic field in a microwave.) Because there is always some reflection whenever an electromagnetic wave enters a material that slows the wave down, stealth aircraft are also careful to deflect the reflected wave away from the radar transmitter so that its receiver won't detect the return wave. In fact, these materials can be corrugated so that any microwaves hitting them reflect into the corrugations and have many opportunities to be absorbed. As I understand it, the microwaves that return to the radar receiver from a stealth plane are remarkably weak. I wouldn't be surprised if a whole stealth plane reflected less microwaves back at the radar unit than would reflect from a foil chewing gum wrapper.

682. Does water drain in the opposite direction in the southern hemisphere? - TL
In principle, yes, but in practice, no. To explain why, I'll begin with the origins of directional circulations on earth. Because the earth is turning, motions along its surface are complicated. The ground at the equator is actually heading eastward at more than 1000 miles per hour. The ground north or south of the equator is also heading eastward, but not as quickly. The ground's eastward speed gradually diminishes until, at the north and south poles, there is no eastward motion at all. As a result of this non-uniform eastward motion of the ground, objects that travel in straight lines because of their inertia end up drifting eastward or westward relative to the ground. For example, if you took an object at the equator and threw it directly northward, it would drift eastward relative to the more slowly moving ground. If someone else threw an object southward from the north pole, that object would drift westward relative to the more rapidly moving ground. In the northern hemisphere, objects approaching a center tend to deflect away from that center to form a counter-clockwise circle around it. This process is reversed in the southern hemisphere so that objects approaching a center there tend to form a clockwise circle around it. Thus hurricanes are counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere.

When water drains from a basin in the northern hemisphere, it flows toward a center and should have a tendency to deflect into a counter-clockwise swirl. However, the effect is very weak in a small washbasin. The direction in which the water swirls as it drains is determined by other effects such as how the water was sloshing before you opened the drain or how symmetric the basin is. For this earth's rotation-driven swirling effect (the Coriolis effect) to dictate the direction of a circulation the objects involved must move long distances over the earth's surface. Even tornadoes don't always rotate in the expected direction; they're just not big enough to be spun consistently by the Coriolis effect.

683. How does a gravity powered water pump work? — JA, Hiawassee, GA
I believe that the pump you're interested in is one that uses the energy released when water flows downhill to lift a small fraction of that water upward. While there are many possible designs for such a pump, the classic version used a phenomenon called "water hammer" to lift water upward. In this technique, a column of water is allowed to accelerate downhill through a pipe until it's flowing at a good speed through the pipe. The pump then closes a valve at the lower end of the pipe, so that the water has to stop abruptly. Since water accelerates in response to imbalances in pressure, the stopping process involves an enormous pressure surge at the lower end of the moving water column. A one-way valve at the lower end of the pipe opens during this pressure surge and allows a small fraction of the water to escape from the pipe. The escaping water rises upward through a second pipe for delivery to a home or business. According to a reader, the escaping water actually enters a head tank that is normally filled with air and thus compresses that air. The compressed air is then used to push water through the pump's outlet and provide the pumping action. This pumping scheme is apparently called a "hydraulic ram."

The only trick to operating such a pump is opening and closing the valve at the lower end of the first pipe. This valve must open long enough that the water in the pipe reaches a good speed and then it must close very suddenly to provide the pressure surge that lifts the small amount of water upward for delivery.

684. How does heat conduct through different materials? - B
In electric insulators, heat is carried by motions of the atoms themselves. You can think of this heat transfer as a bucket-brigade process—one atom jiggles its neighbor, which in turn jiggles its neighbor, and so on. If one end of an insulator is hotter than the other, this jiggling effect will gradually transfer thermal energy from the hotter end (more vigorous jiggling) to the colder end (less vigorous jiggling). Imperfections and weaknesses in most electric insulators make them relatively poor conductors of heat, although there are a few exceptional materials such as diamond that use the bucket-brigade mechanism very effectively and are excellent thermal conductors. In electric conductors, mobile electrons help out by carrying thermal energy from one atom to another over long distances. Even in a material that doesn't make good use of the bucket-brigade mechanism, the mobile electrons provide substantial thermal conductivity. Thus good electric conductors, such as copper, silver, and aluminum, are also good thermal conductors.

685. Why does water boil at lower or higher temperatures under varying atmospheric pressures? Do changing vapor pressures above a liquid play a role in changing boiling points of liquids? — KC, East Greenwich, RI
A liquid boils when its vapor pressure reaches atmospheric pressure. While a liquid will evaporate at temperatures below the boiling temperature, that evaporation only occurs from the surface of the liquid. That's because atmospheric pressure crushes any bubbles that try to form within the body of the liquid. Every once in a while, a few molecules of the liquid break free inside the liquid and form a bubble of gas. The pressure inside such a bubble is the vapor pressure of the liquid at its present temperature. If the liquid's temperature is below its boiling temperature, atmospheric pressure is greater than the pressure inside one of these spontaneous vapor bubbles and it crushes the bubble. But once the temperature of the liquid reaches the boiling temperature, the bubbles will have enough pressure to remain stable against atmospheric pressure. Each bubble that forms begins to float upward toward the top of the liquid and more molecules evaporate into it as it rises, so that it grows larger and larger.

If you lower atmospheric pressure, the liquid will boil at a lower temperature because the vapor pressure reaches atmospheric pressure more easily. If you raise atmospheric pressure, the liquid will boil at a higher temperature because the vapor pressure must rise higher before it reaches atmospheric pressure.

686. What are the relative efficiencies and reasons for power losses in sprocket and chain drives, rubber cogged belt drives, pulley drives, and gear drives? — RA, Montreal, Quebec
The only power loss mechanisms I can think of in each case are sliding friction and vibration. The drive system most likely to experience substantial sliding friction is a pulley (or smooth belt) drive. If the belt slips as the pulleys turn, the belt will do work against the force of sliding friction and that work will be converted into thermal energy. But, as one of my readers points out, if the belt is properly tightened, has an adequate coefficient of friction to prevent slipping, and has a high tensile strength so that it doesn't creep across the pulley surface, then it can operate with very little power loss.

In the other drive systems, there is no possibility of slippage so that any power loss that occurs must be due to internal sliding friction within the components, or from vibrations. Flexing a chain involves some internal sliding friction and wastes some power. I suppose this could be minimized with careful chain construction and I wouldn't be surprised if large change drive systems placed bearings in the chain links to eliminate sliding friction altogether. Flexing a rubber-cogged belt also involves some molecular friction within the belt material so it wastes some power. I'm not sure which system is more efficient, the chain drive or the cogged belt drive. Finally, the gear drive is the least likely to waste significant energy. The only sliding friction that occurs is between the gear teeth. If the teeth are designed well and cut carefully, they should slide very little. In that case, the only significant power loss would be through vibrations. If everything is carefully mounted to prevent vibrations, there should be very little power loss in a gear drive.

687. I have a gas steam heating system and the second floor radiators don't heat well. How does this system work and how can I balance the system so that the upstairs radiators warm at the same rate as the first floor radiators?
In a steam heating system, steam rises upward from a boiler in the basement and condenses in the radiators. As the steam transforms into water, it releases an enormous amount of heat and this heat is transferred to the air in the rooms. The condensed water than descends back to the boiler to be reheated. The beauty of this system is that the rising steam and the descending water can both pass through the same pipes, propelled by gravity alone. The low-density steam is lifted upward by the high-density water.

However, there are a few potential problems with this system. If there is air trapped in the pipes, the steam will have trouble reaching the radiators. Even though steam is lighter than air, it will diffuse slowly through the trapped air. That's why each steam radiator has a small bleeder valve. When the steam pressure exceeds atmospheric pressure, it should push the air in the pipes out the bleeder valves of the radiators. You ought to be able to hear the air leaving and the valves may continue to sputter a bit even when the pipes and radiators are essentially full of steam. I suspect that the bleeder valves on your upstairs radiators aren't functioning well so that steam isn't reaching them.

688. Do whales drink salted water? — GR, Montreal, Quebec
No. Marine mammals rely on water obtained from their food. Because they don't sweat, they only lose water through their urine, which they concentrate to minimize the loss of water. What little water these animals do need comes from eating foods that are already relatively low in salt. Most of the lower sea animals, including fish, have active systems—ones that consume ordered energy—for eliminating salt so that when a sea mammal eats one of the lower animals, it inherits that animal's relatively salt-free water. Moreover, metabolizing fats and carbohydrates produces water as a byproduct.

689. I recently visited an audio store where I saw electrostatic speakers. These speakers have no moving parts like conventional speakers and are more expensive. How do they produce sound? — BC, Ottawa, Canada
Electrostatic speakers uses the forces between electric charges (so called "electrostatic forces") to move a thin metal diaphragm back and forth rapidly. The motions of this diaphragm compress and rarefy the air in front of it, producing sound. On each side of the diaphragm is a rigid metallic grill that can hold electric charges. When the speaker is silent, the diaphragm has a large positive electric charge on it and both the metal grills have large negative charges on them (it could be the other way around, depending the speaker's exact design). The diaphragm is then attracted equally toward both grills and the electrostatic forces cancel perfectly. The diaphragm doesn't undergo any acceleration. To make the speaker produce sound, the electric charges on the two grills are changed so that the electrostatic forces on the diaphragm don't cancel. Instead, the diaphragm is pulled strongly toward whichever grill has more negative charge on it (or less positive charge). The charges on the grills fluctuate as the music plays and the diaphragm accelerates back and forth between the grills. It pushes on the air as it does and produces sound. You'll notice that the diaphragm is a moving part, so the claim that the speaker has "no moving parts" is misleading. The speaker cone of a conventional speaker only moves back and forth, too, so it has an equal claim to having "no moving parts." The relative expense of an electrostatic speaker comes from the requirement of careful construction and the need for a high voltage adapter to match an amplifier to the speaker.

690. Don't microwaves change the molecular structure and composition of food, by ejecting some electrons from atoms and forming cancer-causing free radicals? If I should stand away from a microwave to avoid possible leakage, why would I eat microwaved food?
Microwaves don't affect the molecular structure of the food, except through the thermal effects we associate with normal cooking (e.g., denaturing of proteins with heat and caramelizing of sugars). That's because, like all electromagnetic waves, microwaves are emitted and absorbed as particles called "photons." The energy in a microwave photon is so tiny that it can't cause any chemical rearrangement in a molecule. Instead, it can only add a tiny amount of heat to a water molecule. During the microwave cooking process, microwave photons stream into the food and heat it up. But millions of them would have to work together in order to cause non-thermal chemical changes in the food molecules and they don't normally do that. The photons can only work together if there is a conducting material, such as a metal wire, inside the oven. In that case, the photons can accelerate mobile electric charges along the conducting paths and create sparks. Such sparks can cause chemical damage, but nothing worse than the chemical damage caused by scorching food with a flame or broiler. Even if your microwave is full of sparks for some reason, I doubt that the food will be any worse for you than it would be if you cooked it over an open flame or barbecue.

691. What is light? — KB, Winnipeg, MB
Light consists of electromagnetic waves. An electromagnetic wave is a self-sustaining disturbance in the electric and magnetic fields that can exist even in empty space. You have probably seen two electrically charged objects push or pull on one another, such as when a sock clings to a shirt as you pull the two from the clothes dryer. You have probably also seen two magnetically poled objects push or pull on one another, such as when a magnet pulls itself toward a refrigerator door. These electric and magnetic forces are mediated by electric and magnetic fields respectively and, while those fields certainly exist in the space between the sock and shirt or between the magnet and refrigerator, they can also exist all by themselves. In an electromagnetic wave, the electric field creates the magnetic field and the magnetic field creates the electric field so that these two fields go on creating one another indefinitely as the wave travels through space at an enormous speed—the speed of light. Electromagnetic waves are distinguished by their frequencies or wavelengths, characteristics that are familiar to anyone who has watched water waves approaching the beach. But only a certain group of electromagnetic waves are visible to our eyes—those with frequencies between about 4.0*1014 cycles per second and 7.5*1014 cycles per second (wavelengths between about 750 nanometers and 400 nanometers). Outside of this range are infrared light at the low frequency end and ultraviolet light at the high frequency end.

692. How does the auto-focusing system on a camera work? — RM, Lititz, PA
There are several different systems for autofocusing. I think that the three most popular systems are optical contrast, rangefinder overlap, and acoustic distancing. The optical contrast scheme places a sophisticated light sensitive surface in the focal plane of the camera's lens. This sensor recognizes when sharp focus is achieved by looking for the moment of maximum contrast in the image. When the lens is out of focus, the image is fuzzy and has little contrast. But when the lens is focused properly, the image is sharp and the sensor detects the strong spatial variations in darkness and brightness. The camera automatically scans the focus of its lens until it detects maximum image contrast.

The rangefinder overlap system observes the scene in front of the camera through two auxiliary lenses that are separated by a few inches. It uses mirrors to overlap the images from these two lenses and can determine the distance to the objects in the picture by the angles of the mirrors. The camera uses this distance measurement to set the focus of its main lens.

The acoustic distancing system bounces sound waves from the objects in front of the camera to determine how far away they are. The camera then adjusts its main lens for that distance. While this acoustic scheme has the advantage of working even in complete darkness, it's confused by clear surfaces—if you take a picture through a window, it will focus on the window. The optical schemes will focus on the objects rather than the window, but they will only work when there is light coming from the objects. That's why many autofocus cameras that use optical autofocus schemes have built in lights to illuminate the objects during the autofocusing process.

693. What are gas permeable contact lenses made from and what do they use to pigment them? — TG, Tulsa, OK
A gas permeable contact lens is one that allows oxygen to diffuse through it to the cornea of the wear's eye. While conventional hard lenses were made almost entirely of a plastic known as poly(methyl methacrylate) or PMMA, commonly known as Plexiglas or Lucite, gas permeable hard or semirigid lenses are copolymers containing both methacrylate and siloxane molecular units. The polymers used in soft lenses are made only of siloxane molecular units and are commonly known as silicon rubbers. The molecules in silicon rubbers are mobile at remarkably low temperatures, giving silicon rubber its flexibility. In fact, these molecules are so mobile that they must be linked together or "vulcanized" to keep them from flowing as a liquid at room temperature. Even when they have been linked together, portions of these molecules are very mobile, so that gas atoms and molecules can diffuse easily through them. I'm not sure what chemicals are used to color contact lenses, but I expect that the dye molecules are permanently linked to the polymer molecules to keep them in place.

694. What types of sound can humans hear? What types of materials are soundproof? How is the volume of a sound changed? Is the speed of sound the same in all types of media, such as water or air? — JM, Fairfax, VA
In air, sounds are disturbances that consist of compressions and rarefactions—the air molecules are packed either more tightly or less tightly than normal. These regions of too high or too low pressure and density move through the air at about 330 meters per second—the speed of sound and when they pass our ears, we may hear them as sound. As a particular sound passes our ears, the air pressure rises and falls and then rises again, over and over. The number of full cycles—a pressure rise then a pressure fall—that pass our ears each second determines the pitch of the sound we hear. The lowest pitch that our ears are sensitive to is about 20 cycles per second and the highest pitch that we can detect is about 20,000 cycles per second. While other pitches are possible, we simply can't hear them with our ears.

A sound's volume is determined by the extent to which the air pressure fluctuates as the sound passes. A loud sound involves a stronger pressure fluctuation than a soft sound. Soundproof materials are ones that decrease the volume of the sound passing through them by weakening the pressure fluctuations. There are two ways to decrease the volume of sound passing through a material: by absorbing the sound or by reflecting it. Soft materials such as carpet or foam rubber absorb sound by allowing the sound's pressure fluctuations to waste their energies bending the materials. The sound's energy is converted into thermal energy. Hard, dense materials reflect sounds by making the sounds change speed. Sound travels quickly through most solids and liquids—typically about 5 to 10 kilometers per second. Whenever a wave changes speed in passing from one medium to another, part of that wave is reflected. Thus as sound speeds up in entering a hard surface from the air and as that sound slows down when reentering the air, much of the sound reflects.

695. How does the trajectory of a ball change when you give it a spin? — BHL, Stavanger, Norway
As spinning ball tends to curve in flight. That's because the ball deflects the airflow around it in one direction and accelerates in the opposite direction. There are two ways in which the spinning ball deflects the air. First, the spinning ball pulls the air it encounters around with it in one direction and produces an imbalance in the airspeeds on its two sides. The air flowing around the side of the ball that is turning back toward the thrower travels faster than the air flowing around the other side of the ball. Since the faster moving air has converted more of its total energy into kinetic energy, the energy of motion, it has less of its energy in the form of pressure. Thus the air pressure on the side of the ball turning toward the thrower is lower than the air pressure on the other side of the ball. The ball accelerates and curves toward the side turning toward the thrower. This effect is called the Magnus effect.

Second, a ball moving at any reasonable speed leaves behind it a turbulent wake and experiences a type of air resistance we call "pressure drag." When the ball is spinning, this wake forms asymmetrically behind the ball and the pressure drag is not even balanced. The ball pushes the air in the wake to one side and the air pushes back. As a result, the ball accelerates sideways—to the same side as occurs with the Magnus force. In both cases, the ball curves toward the side turning toward the thrower. This second effect is called the wake deflection effect.

The direction in which a thrown ball curves depends on its direction of spin. If the left side of the ball turns back toward you after you have thrown it, the ball will curve toward your left. If the right side turns back toward you, it will curve toward your right. If the bottom turns back toward you, the ball will arc downward faster than it would with gravity alone (for example, topspin in tennis). If the top turns back toward you, the ball will arc upward or will at least not arc downward as much as it would with gravity alone (for example, backspin in golf and hanging fastballs in baseball).

696. I suspect that the amount of water on the earth is constant and therefore cannot be used up. Is this true? If so, would it not follow that we don't need to worry about water conservation so much as water pollution? — AP, Kansas City, MO
While the number of water molecules on earth doesn't change very much, it isn't exactly constant. Water molecules are consumed in some chemical reactions (particularly photosynthesis in plants) and produced in other reactions (particularly the burning of petroleum). However, most of the water on our planet is mixed with salt and is therefore unsuitable for drinking. The amount of fresh water on earth is not constant and it can be used up. That's why both the conservation of fresh water and the control of water pollution are important.

697. Is time constant? — RH, Boise, Idaho
That's a complicated and interesting question. To begin with, consider how we measure time: we generally use repetitive mechanical systems to tick off short intervals of time and then count as those intervals pass by. Thus we measure time in terms of the swinging of a clock's pendulum or the vibration of a quartz crystal or the motion of an atom's electrons around its nucleus. If time were to speed up or slow down, it would affect the mechanical motions in our bodies just as much as it would affect the mechanical motions of our clocks, so we wouldn't notice any change in the ticking of our clocks. If time were somehow to begin passing half as fast as normal and you were to look at your watch, your watch would still appear to tick off seconds at the same rate. So the first answer to your question is that we can't tell if time is constant, so long as any changes in time occur uniformly and instantly throughout the entire universe.

The reason for including the bit about "uniformly and instantly throughout the entire universe" is that we can tell if time changes at one location but not another. For example, if time were to slow down near you but not near me, I would be able to look at your watch and see that it's running slow just as you would be able to look at my watch and see that it's running fast. Alternatively, we could synchronize our watches, wait a while, and then compare our watches again. Since your time is running more slowly than mine, our watches would no longer be synchronized. While this situation sounds unlikely, it does occur. The rate at which time passes depends on where you are and on how fast you are moving, a result described by the Special and General Theories of Relativity. Our universe mingles space and time in a complicated way and also permits gravity to influence the passage of time. In short, the faster you are moving or the nearer you are to a large gravitating object, the more slowly time passes for you.

698. How do circuits work, what are they made of, and who came up with the concept? — RK, New Albany, IN
Circuits themselves are as old as electricity. A circuit is literally a complete loop through which electric current can flow. For example, a flashlight contains a circuit whenever it's turned on—the current flows from the battery's positive terminal, through the switch (which is on), through the filament of the light bulb (which glows), and back to the battery's negative terminal. The battery then gives the current some more energy and sends it around this "circuit" again and again.

But electronic "circuits" are much more modern. Here the word circuit is equivalent to "device," "board," or "chip." Such electronic devices date to somewhere around the beginning of the twentieth century. As radio developed, with tube amplifiers and other electronic components, so did these circuits. Modern electronic systems place many of the components involved in an electronic device on a single sheet of plastic or fiberglass and many of the components on that board may exist on the surface of one or more tiny silicon wafers. These single wafer circuits, called integrated circuits, were invented in 1959 by Texas Instruments and became commercial products at Fairchild Semiconductors in 1965.

699. In plain English that a child can understand, how does a magnet work? — EK, Dale City, VA
There are several way in which objects in our universe can push or pull on one another and one of these ways is through electric or magnetic forces. Two objects that have electric charges are observed to push or pull on one another and two objects that have magnetic poles are also observed to push or pull on one another. That's simply the way our universe works. With electric forces, things are relatively easy—when you pull a sock and shirt out of the dryer, the sock may well stick to the shirt because friction has given the two different electric charges (one is positively charged and the other negatively charged). By playing around with electrically charged objects, you can convince yourself that (1) there are two different types of electric charge—normally called "positive" and "negative"—and (2) that like charges repel while opposite charges attract.

With magnetic forces, there is an annoying complication: magnetic poles (the magnetic equivalent of "charge") always come in equal but opposite pairs. As with electric charges, there are two types of magnetic poles—normally called "north" and "south"—and like poles repel while opposite poles attract. However, you won't be able to find a pure north pole anywhere; it always comes attached to a south pole (and vice versa). So any magnet you find will have at least one north pole and at least one south pole (while they typically have only one of each, they can also have many of each). The forces that these poles exert on one another are fundamental to our universe—I can't explain them in terms of more basic phenomena because they are already basic except at a very abstract level. (In fact, electric and magnetic forces are intimately related to one another and it is actually electric charges that are creating the magnetic poles that you observe in a magnet.) If you play around with several magnets for a while, you should be able to convince yourself about the existence of two different poles and that like poles repel while opposite poles attract. You should also notice that the magnets push one another directly toward or away from them (the forces between poles are parallel to the line separating them) and that the forces become stronger as the poles become nearer (the force is inversely proportional to the square of the distance separating the poles).

As for how a permanent magnet works, it's made from a material that contains ordered electrons. Electrons are intrinsically magnetic and, in a few special materials, that magnetism as organized so that the overall materials are themselves magnetic. Each electron has its own north and south pole, but together they give the material a giant north and south pole.

700. Suppose you have two electric currents, one consisting of electrons and the other of protons, moving in the same direction at the same velocity. Will the magnetic fields that these currents produce have identical magnitudes and directions? The right hand rule describes the direction of the magnetic field in terms of the direction of current, so it appears that it should be independent of the current's charge. — ABD, Petersburg, VA
Current is defined as flowing in the direction of positive charge motion. Because electrons are negatively charged, the current they are carrying is flowing in the direction opposite their motion! In your question, you describe two beams, one of electrons and one of protons, and note that both beams are heading in the same direction at the same speed. The proton beam's current is heading in the same direction as the beam while the electron beam's current is heading in the opposite direction from the beam. Assuming that the two beams have equal numbers of particles per second, they will produce magnetic fields of equal magnitudes. But the magnetic field produced by the electron beam will be directed opposite that of produced by the proton beam!

A beam of hydrogen atoms—each of which consists of one proton and one electron—is a perfect example of this situation. The electrons in that atomic beam produce a magnetic field in one direction while the protons in that atomic beam produce a magnetic field in the opposite direction. The two fields cancel one another perfectly, as they must because a beam of neutral hydrogen atoms can't produce any magnetic field.

701. There is an experiment involving grapes and microwaves that we found on the internet. If a grape is cut in half—with a piece of skin attached between the two halves—and it is then microwaves, sparks are produced. What is happening? — GB, Antioch, CA
This experiment is described in Fun with Grapes - A Case Study. While I haven't tried it yet myself, I believe I know why it works. Grape juice is somewhat able to conduct electricity and the two halves of the grape are connected by a weak conducting path: the skin bridge. When the microwave oven is turned on, the microwaves not only heat the water in the grapes, they also push a few mobile electric charges back and forth through the skin bridge from one side of the grape to the other. This current releases energy as it passes through the narrow bridge and it heats the bridge extremely hot. The bridge soon catches fire and the electric current driven by the microwaves begins to pass through the flame. When current passes through a gas, it tends to ionize that gas (remove electrons from the gas atoms) so that the gas itself begins to conduct electricity. When current flows through atmospheric pressure air, it forms a brilliant arc. In this case, the arc that you see is powered by the microwaves as they push electric charges back and forth from one side of the grape to the other. An excellent set of movies showing this and other microwave oven experiments appears at

702. Why don't microwaves get stuck in the food we put in the microwave oven?
Microwaves are like light—both are electromagnetic waves and both move extremely quickly. While it is possible to trap a light wave briefly between two mirrors, that wave will eventually be absorbed or released. The same is true of a microwave. It's almost impossible to trap a microwave for more than 1 second, even in very exotic enclosures, so you needn't worry about them becoming trapped in food. The food simply absorbs them and turns their energy into thermal energy.

703. How does a hydrogen bomb work? How does it differ from the atomic bomb besides the simple difference of fusion and fission? — KS, Lake Oswego, OR
A hydrogen bomb uses the heat from a fission bomb (a uranium or plutonium bomb, sometimes called an atomic bomb) to cause hydrogen nuclei to collide and fuse, thereby releasing enormous amounts of energy. While a fission bomb can initiate its nuclear reactions at room temperature, fusion reactions won't begin until the nuclei involved have been heated to enormous temperatures. That's because the nuclei are all positively charged and repel one another strongly up until the moment they stick. Only at enormous temperatures (typically hundreds of millions of degrees) will the nuclei collide hard enough to stick and release their nuclear energy. A typical hydrogen bomb (also called a fusion bomb or thermonuclear bomb) uses a fission trigger to initiate fusion in a mixture of deuterium and tritium, the heavy isotopes of hydrogen. These neutron-rich isotopes fuse much more easily than normal hydrogen. Because deuterium and tritium are both gases, and because tritium is unstable and gradually decays into the light isotope of helium, some hydrogen bombs form the tritium during the explosion by exposing lithium nuclei to neutrons from the fission trigger. Thus the "fuel" for many thermonuclear bombs is actually lithium deuteride, which becomes a mixture of tritium and deuterium during the explosion and then becomes various helium nuclei through fusion.

704. Do you think it will ever be possible to build/create different atoms up to carbon or perhaps even gold (the alchemist's dream)? You would have to use fusion, wouldn't you? Would this be a good source of energy? — JB, Norman, OK
As you noted, this process of sticking together smaller atomic nuclei or nuclear fragments to form larger atomic nuclei is called fusion. Many smaller nuclei release energy when they grow via fusion, so long as the resulting nuclei are no larger than 56Fe (the nuclei of a normal iron atom). Above that size, energy is consumed in the process of sticking the nuclei together. So building carbon nuclei would release energy and building gold atoms would require energy. But while it's possible to construct atomic nuclei up to carbon or even gold, it isn't very practical. It's very difficult to bring atomic nuclei close to one another because they are all positively charged and repel one another fiercely. Because the nuclear energy these nuclei release during fusion only emerges at the moment they actually touch, something must push them together for that to occur. The nuclei can be pushed together by (1) nuclear fission reactors, (2) particle accelerators, (3) thermonuclear weapons, (4) giant lasers, or (5) thermal fusion reactors. None of these systems is ready to synthesize large quantities of normal atoms in a cost effective manner (although nuclear fission reactors do produce useful quantities of radioactive isotopes) and none is ready to produce practical energy from fusion processes.

705. How fast does sound travel through the telephone? - T
When your voice travels through the telephone, it doesn't travel as sound. Instead, the microphone of your telephone unit produces an electric current that represents the sound of your voice. From there on until it arrives at the earpiece of your friend's telephone unit, your voice travels as an electromagnetic signal—either an electric current, a radio wave, or a light wave. Only when it reaches the earpiece is the electromagnetic signal used to recreate the sound itself. Since electromagnetic signals travel at or near the speed of light, your voice moves extremely quickly from your telephone unit to your friend's telephone unit. It would be quite easy, for example, for a friend living a few miles away to tell you about a nearby explosion or thunderclap and then have you hear that explosion or thunderclap yourself. Your friend's words would travel much more rapidly through the phone lines than the sound would travel over the countryside.

However, even the speed of light isn't fast enough in some cases. Shortly after the break-up of AT&T, new long-distance carriers began to appear. Some of these companies used geosynchronous satellites to handle the long distance calls. Because these satellites sit about 22,300 miles above the earth's equator, the travel time for radio waves to and from these satellites is a substantial fraction of a second. The delay between when you spoke and when your friend heard your voice was long enough that your friend might have begun talking, too. Those conversations were very awkward because you had to be very deliberate about starting and stopping your speech. You almost had to tell your friend when you were done talking so that they could begin. All modern long-distance calls are handled by surface links so that there is almost no delay, except perhaps when going to the other side of the earth.

706. If time passes more slowly for someone who is moving quickly and enormous speeds are needed to explore distant space, is there any way to counteract this time/speed phenomenon so that those on earth will not die waiting for the "space travelers/explorers" to return? — BC, Ottawa, Canada
Unfortunately, no. Those of us who remained on earth would watch the explorers head off at enormous speeds toward the stars and would be old and gray before they returned. Even if the explorers could move at almost the speed of light, it would take them many years to reach nearby stars and many years to return. Since there is no way that they could travel even as fast as the speed of light, the absolute minimum time it would take for a round trip, from our perspective, would be the round trip distance to the stars divided by the speed of light.

But this brings up one of the peculiar results of special relativity. From our perspective on earth, the explorers are moving quickly as they head toward the stars and their clocks appear to be running slowly to us. But from their perspective, we are moving quickly in the other direction and our clocks appear to be running slowly to them. This apparent paradox is resolved by the fact that the explorers would not agree with us on the ordering of two events occurring at different locations—space and time appear differently to us; they are intermingled. However, when the explorers accelerate in order to turn around and headed back toward us, their perceptions of space and time undergo a radical change. They see our clocks zoom ahead while we continue to see their clocks running fairly slowly. When the explorers finally returned to earth, their clocks indicate that they had been gone only a short time. However our clocks indicate that they had been gone at least as long as the time it would take light to complete the roundtrip. This situation leads to the famous "twin paradox," in which one twin travels through space while the other remains at home. When the explorer twin returns to earth, the explorer twin is still young but the earthbound twin is very old. If near-light-speed travel were to become possible (a very remote possibility), such twin paradoxes would certainly occur.

707. Please explain how the different welding systems work, (Arc, TIG, MIG, and Oxy-Acet) and why some types work with certain metals (steel, aluminum, titanium, and cast iron) and others don't? — DC, Ceder, MN
While I have very little experience welding myself, I can make a number of general observations about welding. All of the welding systems you mention are trying to join several pieces of metal by melting them together. In most cases, one of the pieces of metal is being used to form the joint and is sacrificed completely in the process (typically it's a welding rod made of a special metal that's good at forming a joint). How the melting and joining process proceeds depends on the welding system used.

An arc welder passes an electric current through the air from the pieces to be joined to a welding rod. The rod becomes so hot as the result of this arc that it melts and joins with the other pieces of metal, binding them together permanently. This scheme only works with relatively non-flammable metals such as steel. Aluminum or titanium will burst into flames when the arc starts. To joint these flammable metals, the arc has to be protected by a shroud of an inert gas such as argon or helium. TIG and MIG welding are based on this inert gas approach (the "IG" part of the names). In Tungsten-Inert-Gas (TIG) welding, an arc passes from the pieces being joined to a tungsten electrode. Tungsten has such as high melting point that it survives this arc and another piece of metal, the welding rod, is fed into the arc where it melts to form the joint. In Metal-Inert-Gas (MIG) welding, the arc passes from the pieces to a metal welding rod. This system resembles normal arc welding, in that the welding rod melts to form the joint, however now the arc is shrouded by a flow of inert gas so that there is no oxygen around to support combustion. Flammable metals can be welded with TIG or MIG welding and so can non-flammable metals.

As for oxygen-acetylene welding, here a very hot flame is used to heat the pieces involved to very high temperatures. A welding rod that melts at a slightly lower temperature than the pieces themselves is then used to join the pieces. The advantage to using this system is that it doesn't pass a current through the pieces and doesn't rely on their electric properties. The current of an arc welder could damage thin materials but an oxygen-acetylene flame should not (assuming they are relatively non-flammable metals). I'm sure that the metallurgical characteristics of the joints vary from system to system, but I can't make any useful statements about this. For a more detailed discussion of when and where to use each technique, you'll need a more experienced person than me.

708. Why is it impossible to make a wheel that turns forever, all by itself? — AWG, Karachi, Pakistan
While people are always trying to build perpetual motion machines, they will never succeed. All of these devices are intended to obtain useful energy—what physicists call "work"—from either nowhere or from less useful energy—what physicists often call "heat" or "thermal energy". Obtaining work from nowhere is really impossible; energy is a conserved quantity, meaning that it simply cannot be created or destroyed. For a machine to do work, it must obtain energy from somewhere or something else. So if anyone tries to sell you a car engine that doesn't take any fuel at all—thus creating work out of nowhere—don't buy it! It's a fraud.

As for machines that try to convert thermal energy completely into work, they are also impossible, but for a different reason. While they don't violate the conservation of energy, they do violate the laws of thermodynamics. Thermal energy is disordered energy—it is energy that has been distributed randomly among the individual atoms and molecules in an object so that it cannot be easily reassembled to do useful work. When you burn a candle, all of the energy the candle once had is still in the room, but it's much harder to use. Just as a coffee cup is much more useful before you drop it than after you drop it, so energy is much more useful before you disorder it than after you disorder it. The difficulty with reassembling thermal energy to do useful work is a statistical one: it's unlikely that this energy will spontaneously reassemble itself in a useful manner, just as its unlikely that a dropped coffee cup will spontaneously reassemble itself in a useful manner. The laws of mechanics don't prevent either of those reassemblies from occurring, but both reassemblies are statistically very unlikely to occur. How often have you dropped a broken cup and had it fall together rather than apart? So if someone tries to sell you a car engine that uses the thermal energy in the surrounding air as "fuel"—thus turning thermal energy completely into work—don't buy it! It's also a fraud.

709. Why are there pistons in an engine? — T, Enola, PA
The pistons in a gasoline engine compress the fuel and air mixture before ignition and then extract energy from the burned gases after ignition. When the engine is operating, each piston travels in and out of a cylinder with one closed end many times a second. The piston makes four different strokes during its travels. In the first or "intake" stroke, the piston travels away from the closed end of the cylinder and draws the fuel and air mixture into the cylinder through an opened valve. During the second or "compression" stroke, the piston travels toward the closed end of the cylinder and compresses the fuel and air mixture to high pressure, density, and temperature. The spark plug now ignites the fuel and air mixture and it burns. During the third or "power" stroke, the piston travels away from the closed end of the cylinder and the expanding gases do work on the piston, providing it with the energy that propels the car forward. During the fourth or "exhaust" stroke, the piston travels toward the closed end of the cylinder and pushes the burned gases out of the cylinder through another opened valve.

710. What happens to the temperature of water when ice is added? — OS, Havana, FL
Whenever a glass contains a mixture of both ice and water, the temperature of its contents will be 32° F (0° C). That's because liquid water isn't stable below this temperature and solid ice isn't stable above this temperature. The two can coexist stably only at 32° F.

When you first mix the water and ice, the water is likely to hotter than 32° F and the ice is likely to be colder than 32° F. Heat flows from the water to the ice—from hotter to colder—and soon one of them reaches 32° F. While heat may then continue to flow from the water to the ice, the one that's at 32° F won't change temperature any more. Instead, it will begin to turn into the other form. For example, if the ice reaches 32° F first, it will begin to melt as heat flows into it and turn into water at 32° F. Or if the water reaches 32° F first, it will begin to freeze as heat flows out of it and turn into ice at 32° F. Pretty soon, both the ice and the water will be at 32° F and there they will remain so long as both are still present in the glass.

From a molecular standpoint, ice is an orderly crystalline solid in which the water molecules are neatly arranged in rows, columns, and stacks. Liquid water is a disorganized soup of water molecules that are regularly changing neighbors and moving about, though always in contact with one another. As you add heat to cold ice, its molecules jiggle more and more vigorously against one another until they finally begin to move about as liquid water, a process we call "melting." As you remove heat from warm water, its molecules move about less and less rapidly until they finally begin to cling permanently to one another as solid ice, a process we call "freezing."

711. Why does breathing helium make our voices sound Mickey Mouse-ish? Is there anything we can drink that will have the same effect for a longer period? - AP
The pitch of your voice is largely determined by the dimensions of your larynx. That's why men, with their larger larynxes, generally have lower voices than women. While the sound of your voice originates in the vibrations of your vocal cords, string-shaped objects aren't very good at emitting sound. Just as a violin employs a box to assist its strings in producing sound, you use your larynx to assist your vocal cords in producing sound. Which pitches your larynx produces well depends on its size and on the speed of sound. Both of these factors are important because the air itself vibrates and either decreasing the size of your larynx or allowing sound to move faster from one side of it to the other will raise the pitch of your voice. Because the speed of sound is much higher in helium (965 m/s) than it is in air (331 m/s), the pitch of your voice rises when you breathe in helium gas. However, as soon as the helium has left your lungs and is replaced by air, your voice returns to normal. Apart from breathing gases with high speeds of sound, there isn't anything else that will work. You can't live on pure helium gas, so the only way to sustain this effect would be to breath a helium/oxygen mixture instead of air. Some deep-sea divers do just that and their voices continue to sound "Mickey Mouse-ish" as long as they breathe this mixture.

712. Can we create an invisible wall of electromagnetic fields? - AW
Not really. While you could make an electromagnetic "wall" of laser beams or X-ray beams, it wouldn't really be "invisible" and it wouldn't feel like a solid wall. It would just cause injury if you put your hand through it. For a surface to feel like a wall, it would have to push your hand backward if you tried to move your hand through it. A real wall does just that and it does so with electromagnetic forces—when you touch a wall, electromagnetic forces that the wall's atoms exert on your atoms push your hand back and prevent it from penetrating the wall. So a clear window could be described as an "invisible wall of electromagnetic fields," but that isn't really what you want. A freestanding electromagnetic field, one that doesn't involve atoms yet prevents your hand from penetrating it, just isn't possible.

713. Do sparks generated by Tesla coils shock humans? If not, why not? - AW
A Tesla coil is radio-frequency transformer that produces small currents of very high-energy electric charges. A radio frequency alternating current passes through the primary coil of this transformer and it induces a current in the secondary coil of the transformer. The frequency of the alternating current must be extremely high because there is no iron in the core of the transformer to store energy during a cycle, so that each cycle must be very brief. Because the alternating current flowing out of the secondary coil of the transformer has a very high frequency, it travels over the surface of a conductor, rather than through its center. Thus when you allow that current to pass through you, it goes along your skin and not through your body. As a result, you barely feel its passage except perhaps as surface heating (however, it can cause what is called an "RF burn" in some cases.) Also, the current from a typical Tesla coil is very small so it would barely be noticeable even if it went through your body.

714. Is there a touch sensor that can sense when you touch the body of a car? - AW
The same touch sensors that are used in "touch" lamps or some elevator buttons could be used to sense when you touch a car. A car is essentially insulated from the ground by its rubber wheels, so that when you touch it there is a tendency for electric charge to be transferred between the earth and the car through you. That's why you may receive a shock when you touch a car on a cold winter day. Many electronic devices are capable of detecting this charge transfer (in fact, many of them would be damaged by such sudden and large charge transfers). So building a car touch sensor would be easy. Whether there is a commercial product that does this is another matter, and I am not sure of the answer.

715. How does a silencer work? — AWG, Karachi, Pakistan
When a bullet emerges from the barrel of a gun, the high-pressure gas that is propelling it from behind abruptly enters the atmosphere. This sudden burst of pressurized gas is like that released by an exploding firecracker and produces a loud "pop" sound. A silencer slows down this gas's entry into the atmosphere. Before leaving the gun, the bullet passes through a series of air-filled chambers. The gas behind the bullet must enter each chamber, one at a time, and with each passage, its pressure and energy decrease. By the time the gas emerges from the last chamber, its pressure is low enough that it makes only a weak "whoosh" sound as it enters the atmosphere. This same technique is used by an automobile muffler. However, a silencer is only effective with low-velocity bullets. If the bullet itself travels faster than the speed of sound (331 m/s), it will create shock waves as soon as it enters the atmosphere and will generate its own explosion noise—miniature "sonic booms."

716. If you have a glass of water that is real cold but not frozen, can the addition of one normal ice cube make it all freeze? Can I do this in the kitchen? - D
The answer to both questions is yes. If you begin with very pure, dust-free water in a very clean glass, you should be able to supercool it below its normal freezing temperature of 32° F (0° C). That's because water has difficulty forming the initial seed crystals upon which ice can grow. If you then add an ice crystal to the supercooled water, it should begin to freeze rapidly. While I have never done this myself, it shouldn't be too hard. You should probably use distillated and filtered water and a brand new glass that you've cleaned thoroughly. Cover the water to keep out dust. Cool it carefully through 32° F in the freezer and then add a tiny ice chip. The water should begin to crystallize around that ice chip. A simpler example of this sudden freezing phenomenon is a heat pack—one containing sodium acetate. At room temperature, it contains a supercooled solution of sodium acetate that is unable to freeze spontaneously. When you press a button in the pack, you trigger the crystal formation and the whole pack freezes in seconds. The crystallization process releases enough thermal energy to keep the pack hot for hours. Incidentally, ski resorts regularly seed the water they use to make artificial snow with molecules that initiate crystal growth to avoid forming supercooled liquid. Doing so greatly enhances the amount of snow they make.

717. What source of energy keeps the earth and the planets orbiting the sun or keeps electrons orbiting the nuclei of atoms? - AW
It takes no additional energy to keep those objects orbiting—the earth's inertia keeps it moving around the sun. If the sun weren't there, the earth would continue forward in a straight line at a steady pace forever because that is how free objects behave. It takes no energy or force to keep them moving. But the earth is drawn continuously inward by the sun's gravity and so it travels in an elliptical arc instead of a straight line. Assuming that nothing adds or subtracts energy from the earth and sun, the earth will continue to orbit the sun forever. The same applies for the other planets and for electrons orbiting nuclei in an atom. In the latter case, it is electromagnetic forces that draw the electrons inward, rather than gravity.

718. How does an electric lighter work? - AW
In a piezoelectric lighter, a spring-driven mass strikes a piezoelectric crystal and exposes that crystal to a sudden enormous strain. This strain changes the shapes of the electronic levels in the crystal and produces an imbalance in the electric charges on the crystal's surfaces. One side of the crystal acquires a large positive charge, the other a large negative charge. The potential energies of these imbalanced charges are large enough that they have enormous voltages—typically 10,000 to 50,000 volts. With that much voltage (energy per charge), the charges can leap through the air for about a centimeter or more. If you allow these charges to pass through your hand, they will give you a mild shock—there aren't enough charges moving to give you a dangerous shock.

719. How far can electricity be transferred over wires from a power station before the loss factor is too great? — JD, New York NY
That depends on the electricity's voltage. The transmission lines carrying the electricity are important parts of the overall electric circuit. They waste electric power as they carry current and the amount of power they waste is proportional to the square of the current they carry. The purpose of high voltage transmission lines is to send as small a current as possible across the countryside so that the wires waste as little power as possible. This reduction in current is possible if each electric charge moving in that current carries a large amount of energy—the current must be one that consists of high voltage charges. In short, higher voltage transmission lines employ smaller currents and waste less power than lower voltage transmission lines.

When Thomas Edison set out to electrify New York City, he used direct current of the highest practical household voltage. Nonetheless, his relatively low voltage power transmission lines wasted so much power that he had to scatter generating plants throughout the city so that no home was far from a power plant. But when George Westinghouse and Nicola Tesla realized that using alternating current and transformers to temporarily convert the household power to high voltages and small currents, they were able to send power long distances without wasting electricity. That realization eventually destroyed Edison's direct current electric system and gave us the modern alternating current system. It's now common to send electric power several hundred miles through high voltage transmission lines. At those distances, perhaps half the power is lost en route. I doubt that transmission of power more than 1,000 miles is practical.

720. How does an electric motor work? - BR
An electric motor uses the attractive and repulsive forces between magnetic poles to twist a rotating object (the rotor) around in a circle. Both the rotor and the stationary structure (the stator) are magnetic and their magnetic poles are initially arranged so that the rotor must turn in a particular direction in order to bring its north poles closer to the stator's south poles and vice versa. The rotor thus experiences a twist (what physicists call a torque) and it undergoes an angular acceleration—it begins to rotate. But the magnets of the rotor and stator aren't all permanent magnets. At least some of the magnets are electromagnets. In a typical motor, these electromagnets are designed so that their poles change just as the rotor's north poles have reached the stator's south poles. After the poles change, the rotor finds itself having to continue turning in order to bring its north poles closer to the stator's south poles and it continues to experience a twist in the same direction. The rotor continues to spin in this fashion, always trying to bring its north poles close to the south poles of the stator and its south poles close to the north poles of the stator, but always frustrated by a reversal of the poles just as that goal is in sight.

721. Is it possible to sense when a person touches a car, even if the car is painted? - AW
Yes. I wouldn't try to detect mechanical contact, because you'd have trouble differentiating between forces exerted on the car by a hand and those exerted on it by sound waves. But you can tell whether a conducting object (such as a person) is near the car by looking at the car's electric properties. If you were to send electric charge on and off the car rapidly with a source of high-frequency alternating current, you would find that the amount of charge that flowed on or off the car during each cycle would change as the person's hand approached the car. That's because the charges on the car would push or pull on charges in the person's hand and the charges in the person's hand would move. In effect, the person's hand would make the car "larger" and it would draw more charge from your current source. Even if the person didn't touch the car, the nearness of the hand and car would change the way current flowed on and off the car. Such a change would be easy to detect with laboratory equipment and could probably be made by cheap consumer equipment, too. The only complications would be in not detecting everything—passing cars for example—and in not damaging the device with static discharges. Still, I think all of that could be done.

722. What is the principle of the Trinitron Sony TV system? — JPD, Spiennes, Belgium
To form a color image, a color television illuminates a dense pattern of tiny spots—some red, some green, and some blue. By mixing various amounts of these three primary colors of light, the color television can make us perceive any color. But the television must control the amounts of these three colors at each spot on the screen, a very difficult task. A typical color television does this by shining three separate beams of electrons through a mask with holes in it and onto a screen that's covered with tiny phosphor spots. Because the three beams approach the mask at different angles, they illuminate different portions of the screen after passing through the holes. Thus the "blue" beam only illuminates spots of blue phosphor, the "red" beam illuminates red spots, and the "green" beam illuminates green spots.

However, the Sony Trinitron system uses a line mask rather than one containing holes and the phosphors are coated onto the screen in stripes rather than spots. Again, three separate electron beams are used but they now illuminate specific stripes of phosphor rather than spots of phosphor. The advantage of the stripe approach is that there is more active phosphor on the screen (fewer dark places between spots) so the image is brighter.

723. What is the black holey stuff on the doors of microwave ovens? Is it for looks, protection, or what? - K
The black holey stuff on the window of a microwave oven is a metal shield that keeps the microwaves inside the cooking chamber. Because the holes in this metal sheeting are so much smaller than the wavelengths of the microwaves (about 12 cm), the microwaves respond to the sheeting as though it were solid metal and they reflect almost perfectly. By keeping the microwaves inside the oven, this sheeting speeds cooking and protects you from the microwaves.

724. Why is it bad to put metal in a microwave oven? - OR
It isn't necessarily bad to put metal in a microwave oven, but it can cause cooking problems or other trouble. Microwaves cause currents to flow in metals. In a thick piece of metal, these currents won't cause problems for the metal. However, in thin pieces of metal, the currents may heat the metal hot enough to cause a fire. Metallic decorations on fine porcelain tend to become hot enough to damage the porcelain. But even thick pieces of metal can cause problems because they tend to reflect the microwaves. That may cause cooking problems for the food nearby. For example, a potato wrapped in aluminum foil won't cook at all in a microwave oven because the foil will reflect the microwaves. The currents flowing in the metal can also produce sparks, particularly at sharp points, and these sparks can cause fires. In general, smooth and thick metallic objects such as spoons aren't a problem, but sharp or thin metallic objects such as pins or metal twist-ties are.

725. How does a discotheque laser work and how could I build one? — JPD, Spiennes, Belgium
If you are referring to a system that displays illuminated line drawings on a wall that move with the music, then building one is easy. You need a small isolated speaker—just the electronic device, not a whole speaker unit—that you can connect to the music amplifier. Place an elastic membrane over that speaker—a stretched sheet of thin rubber from a latex glove should work well. Then glue a tiny, front-surface mirror to that rubber membrane, choosing a point that is about midway between the middle of the speaker and its edge. A front surface mirror is one that is shiny on its top, so that light doesn't have to go through glass before reflecting. A broken fragment of mirror, about 3 mm on a side, should work. Finally, shine the beam of a laser pointer onto the mirror and begin to play music through the speaker. The mirror will move with the music and the reflected laser beam will form pretty patterns on the wall.

726. How is chlorine gas used to disinfect water at treatment plants? - KM
Chlorine molecules (Cl2) dissolve easily in water, where they react with water molecules to form hypochlorous acid (HOCl), chlorine ions (Cl-) and hydrogen ions (H+). Hypochlorous acid is a weak acid that partially dissociates into hydrogen ions (H+) and hypochlorite ions (OCl-). Studies have shown that it's predominantly the hypochlorous acid molecules and the hypochlorite ions that disable and kill microorganisms. These molecules and ions diffuse onto and into the microorganisms and oxidize important biological components, such as the protein coats of some viruses, key enzymes in many bacteria, and the genetic materials in both bacteria and viruses. — Thanks to J. Symons for pointing out this mechanism to me and providing me with detailed reference materials.

727. Without gravity in space, what would happen to the recoil if a gun were shot off? — DZ, Illinois
Even in the depths of space, so far from any planet that gravity is virtually absent, a gun will have its normal recoil. When you push on something, it pushes back on you just as hard as you push on it. That rule, known as Newton's third law of motion, is as true in empty space as it is on earth. Thus when the gun pushes the bullet forward, the bullet pushes the gun backward equally hard and you feel the gun itself jump backward as result. This recoil effect is the very basis for rocket propulsion—the rocket pushes its exhaust backward and the exhaust pushes the rocket forward. That's why rockets can work outside the earth's atmosphere and away from any celestial objects—the rocket only has to push on its exhaust in order to obtain a push forward.

728. Why does a can of Diet Coke float on water while a can of regular Coke sinks? Does that have to do with density and Archimedes' law? —AB, Riverside, CA
Because regular coke contains large amounts of dissolved sugar, it is much denser than water or than Diet Coke (which has far less dissolved material). Although even Diet Coke is denser than water, as is the aluminum in the can, a can of Diet Coke contains enough gas bubbles to lower its average density to just below that of water. According to Archimedes' law, an object with an average density less than that of the liquid in which it's submerged will float upward. A can of regular Coke has an average density that's greater than that of water, so it sinks.

729. How does an air pump work and how does the air pocket in a Nike Air or Reebok pump shoe keep its form? — MD, Toronto, CA
A typical bicycle pump uses a piston to squeeze air that it has trapped inside a cylinder. As you push the piston into the cylinder, the trapped air molecules are packed more tightly together and their pressure rises. Moreover, because you are transferring energy to the air by doing mechanical work on it, the air's temperature also rises. Air always accelerates toward regions of lower pressure, so this pressurized air will tend to flow through any opening that leads to lower pressure—such as the inside of an underinflated bicycle tire. A one-way valve at the base of the cylinder allows this pressurized air to flow out of the cylinder through a pipe and enter the bicycle tire. Thus each time you push down on the piston, you pressurize the air inside the cylinder and it accelerates and flows toward the lower pressure inside the bicycle tire. As you pull the piston out of the cylinder, a second one-way valve allows new air to enter the cylinder from outside so that you can repeat this process.

In a pumped air athletic shoe, squeezing a rubber bulb packs together air molecules and increases their pressure. When the pressure is high enough, a one-way valve allows this pressurized air to flow into the underinflated air pocket of the shoe. A second one-way valve allows the bulb to refill with outside air when you stop squeezing the bulb. Once the air pocket has been filled with large numbers of air molecules, these molecules exert substantial outward forces on the inner surfaces of that air pocket. The more molecules there are inside the pocket, the more often they collide with the surfaces and the more force they exert on those surfaces. These outward forces from the air molecules allow the air pocket keeps its shape.

730. How do Eskimos burn fires in their igloos without melting the snow and/or ice that the igloos are built out of? I know they use holes in the top to vent the smoke and some heat, but what about the ambient heat? — AK, Bridgeport, CT
To avoid melting the ice, the Eskimos must keep the ice below its melting temperature. That means that they can't add heat to ice indefinitely. But while a central fire will always deliver some heat to the ice of the igloo, the ice of the igloo will also tend to lose heat to colder air outside. As long as the ice loses heat at least as fast as the fire delivers heat to it, the ice won't become any warmer and it won't melt. If heat loss to the outside is fast enough, it may be possible to have the air inside the igloo warmer than 32° F (0° F) and still have the ice remain colder and frozen. However, I'm sure that the average air temperature in the igloos can't be made much warmer than freezing without causing trouble. Still, the air right around the fire can be quite warm without threatening the walls. The area under the fire must be carefully insulated to avoid melting the underlying ice—which must continue to lose heat as rapidly as it arrives from the fire.

731. What happens to water in space? — DZ, Illinois
That depends on the water's temperature. At extremely low temperatures, ice remains stable indefinitely. That's why comets that are as old as the solar system have been able to hold on to their water despite having almost no gravity. But at more moderate temperatures, ice and water both slowly lose water molecules. These water molecules evaporate (or sublime, in the case of ice) and drift off into space. Because there's no air pressure in space to prevent evaporation from occurring inside the body of water, water will actually boil at any temperature. That's what boiling is: evaporation into steam bubbles located inside the water. Atmospheric pressure normally smashes these bubbles as long as the water temperature is below 212° F (100° C), but in empty space the bubbles form without opposition at any temperature.

732. When ice placed in water melts, does the overall volume of water increase, stay the same, or decrease? — AB, Riverside, CA
The volume decreases. That's because ice at 32° F (0° C) is less dense than water at that same temperature. As the ice melts to form water, the density of its molecules increases and the overall volume of material decreases. This situation, in which the solid form of a material is less dense than the liquid form of that material, is virtually unique in nature and explains why ice floats on water.

733. What is an electron and what keeps its mass and charge together so that when the mass moves, the charge moves with it? — WG, Calgary, Canada
An electron is a fundamental particle that has as two of its attributes, a mass and an electric charge. Because the electron appears to be structureless, it has no size and it wouldn't make sense for its mass to be located at a distance from its charge. With a less fundamental particle such as a proton, the charge and mass can be somewhat spread out and displaced so that the charge and mass can move slightly independently. Still, even in the case of a proton there are effects that keep the mass from getting far away from the charge.

734. How does electricity travel through wires?
When the atoms that make up a metal assemble together, some of their electrons become delocalized—they stop associating with specific atoms and can move throughout the overall metal. Most importantly, these mobile electrons can respond to the presence of electric fields and electric forces by accelerating and traveling through the metal. When you turn on a flashlight, you are creating a system in which positive charges on one terminal of the battery and negative charges on the other terminal can begin to push electrons through the flashlight's wires. The mobile electrons in those wires are negatively charged and they accelerate toward the positive terminal of the battery. New electrons from the negative terminal of the battery replace the departing electrons and soon a steady flow of electrons through the flashlight is established.

735. How do scientists measure the speed of light? — DZ, Illinois
There are many possible methods for measuring the speed of light, but the classic technique is easiest to describe. In this method, a rapidly spinning mirror is used to direct a beam of light down a long pipe toward a stationary mirror at the end of that pipe. The first mirror is spinning in such a way that the beam it reflects sweeps across the pipe and can only strike the second mirror during that brief moment when the first mirror is perfectly aligned to direct the light down the pipe. A scientist then looks into the spinning mirror to observe the flash of light that returns from the second mirror. Because it takes a small but finite amount of time for the light to travel back and forth through the pipe, the spinning mirror will have turned a little between the moment when it sent the beam of light toward the far mirror and the moment when that beam of light returns to the spinning mirror. By studying the angle at which the reflected beam leaves the spinning mirror and by knowing how quickly the mirror is spinning, the scientist can determine the speed of light.

However, something has changed since those sorts of measurements were done: the speed of light is now a defined constant. It isn't measured any more—it's simply defined to be 299,792,458 meters per second. The second is defined in a similar manner—as 9,192,631,770 periods of a particular microwave emission from the cesium-133 atom. Because of these two definitions, an experiment that "measures the speed of light" is now used to determine the length of the meter.

736. I have seen some new 48" fluorescent tubes rated at 25W compared to the standard 40W. I was told I could use these in my existing fixtures without doing anything to the ballast. What effect will replacing a 40W bulb with these 25W bulbs have on my fixtures and ballasts? - ST
I would guess that the lower wattage tubes will work fine in your existing fixtures, but I am not expert enough to be certain. The 25W tube itself is evidently built so that a smaller current flows through it than through a normal 40W tubes when the two are exposed to similar voltages. The ballast's job is to prevent a catastrophic rise in that current by adjusting the voltage across the tube dynamically during each half cycle of the power line and to keep the tube operating even as a half cycle is coming to an end. Although the 25W tube will draw less current than the ballast expects, the ballast should behave pretty well. I would expect that the tube designers have anticipated this situation and have built the tube to operate with the standard ballast. If a reader knows better, please let me know.

737. Why is CD audio better than that of a cassette? — MK, Baltimore, MD
CD audio is recorded in a digital form—as a series of numerical pressure measurements. This digital recording is a very accurate representation of the air pressure fluctuations associated with the original sounds that arrived at the microphones. During playback, these air pressure measurements are read from the CD and the original air pressure fluctuations are recreated by the speakers. While there are imperfections in the whole process of measuring air pressure fluctuations and recreating those fluctuations, the CD itself doesn't introduce any imperfections—the information read from the CD during playback is absolutely identical to the information that was recorded on the CD at the manufacturer's plant.

The same isn't true of analog recording on a cassette tape. Cassette audio is recorded in an analog form—as magnetizations of the tape surface that are proportional to the air pressure fluctuations associated with the original sounds. During playback, these magnetizations of the tape are analyzed and used to recreate the sounds. But the tape itself introduces imperfections in the reproduced sound. The information read from the tape during playback isn't quite the same as the information that was recorded on the tape at the manufacturer's plant. The tape isn't perfect and the sound that's reproduced by a tape player isn't quite the sound that was originally recorded.

738. How does a violin create sound? Is the sound only made by the strings or is it made with the help of the violin's shape and structure? — RH, North York, Ontario
While the vibration of the strings is ultimately responsible for the sounds a violin emits, it is the body of the violin that emits most of that sound. Strings are very poor emitters of sound because they aren't able to push on the air effectively. When the string moves back and forth through the air, the air simply flows around it to the other side. So instead of compressing and rarefying the air, as it must do in order to produce sound waves, the string just stirs the air around. But the bridge of the violin rocks back and forth as the strings' vibrate and it conveys this motion to the belly of the violin. The belly moves in and out, compressing and rarefying the air and doing a fine job of producing sound.

739. How does a laser printer work? What is the toner made of and how does the printer place it on the paper? — PC, Brussels, Belgium
A laser printer uses the light beam from a laser to control the placement of electric charges on a photoconductor surface. A photoconductor is a material that only conducts electricity when exposed to light, so that charges can move through the photoconductor only when the laser beam hits it. The printer uses a corona discharge to place charges on the darkened photoconductor and then uses the laser beam to remove charges from certain places. The end result is a pattern of electric charges that's an image of the final print. The toner particles, which are made of black plastic, are given an electric charge so that they cling to the charge image on the photoconductor. This pattern of toner particles is then transferred to electrically charged paper and fused to that paper with heat and pressure.

740. How dangerous is the radiation from high voltage power lines? - K
Probably not very dangerous. The radiation itself is so weak that it can't cause significant heating in your body (as the microwaves used in diathermy treatment do) and so low frequency that it can't do chemical damage (as the X-rays from a CT scan do). The only possible source of trouble is the small electric and magnetic fields from the power lines and there is still no credible evidence that these affect biological tissue. Moreover, there are sound physical arguments why those fields should not be able to affect biological tissue. Only in rare cases of an organ that is devoted to sensing magnetic fields (e.g., in migratory birds) is there any reasonable interaction between tissue and small magnetic fields.

741. Is it true that Tesla invented a way to send electrical power without the use of power lines? If so, how? - BS
Yes. Tesla found that the alternating electromagnetic fields around a large high frequency transformer could propel currents through wires or lamps that were located at a moderate distance from the transformer. But this technique of using the alternating fields near a transformer to provide power aren't very practical—there is too much power wasted through radiation or in heating things that aren't meant to be heated.

742. What is white noise? - AT
Acoustic "white noise" is a collection of random sounds that together have the same volume at every frequency or pitch. It's defined more accurately as having the same amount of power in each unit of its bandwidth, so that the acoustic power between 20 and 21 cycles per second is the same as the acoustic power between 500 and 501 cycles per second.

743. How does a TV or VCR remote control work? Is it infrared light or a laser? How does the TV or VCR know what to do with the light it receives from the remote? — FC, Lafayette, CA
The remote unit communicates with the TV or VCR via infrared light, which it produces with one or more light emitting diodes (LED). The most remarkable feature of this communication is that the TV or VCR is able to distinguish the tiny amount of light emitted by the LED from all the background light in the room. This selectivity is made possible by blinking the LED rapidly at one of two different frequencies. Since it's unlikely that any other source of light in the room will blink several hundred thousand times per second and at just the right frequency, the TV or VCR can tell that it's observing light from the remote. The remote sends information to the TV or VCR by switching back and forth between the two different frequencies. For example, it may use the higher frequency to send a "1" bit and the lower frequency to send a "0" bit. The remote sends a long string of these 1's and 0's, and the TV or VCR detects and analyzes this string of bits to determine (1) whether it's directed toward the TV or VCR (an address component in the information) and (2) what it should do as the result of this transmission (a data component in the information). Assuming that the string of bits was intended for the TV or VCR, its digital controller (a simple computer) takes whatever action the data component of the transmission requested.

744. How was Newton able to prove inertia with gravity and friction still being present? Why didn't people think he was crazy? Did he have some type of vacuum or something? - JP
Actually, it was Galileo who first realized that objects have this tendency to continue moving at a steady rate in a straight-line path—what we call "inertia." He deduced this fact by studying the motions of balls on ramps. He noted that a ball rolling down a slight incline steadily picked up speed while a ball rolling up a slight incline steadily lost speed. From these observations he realized that a ball rolling along a level surface would roll at a steady speed indefinitely, where it not for friction and air resistance. He was aware that friction, air resistance, and gravity were disturbing the natural motions of objects and had figured out a way to see beyond them. But it wasn't until Newton took up this sort of study that the idea of forces and their effects was properly developed. Overall, it took almost two thousand years, from Aristotle to Newton, for the incorrect idea that objects tend to remain stationary when free of forces to be replaced with the correct idea that objects tend to continue at constant velocity when free of forces.

745. When friction is made by two atoms rubbing — it makes heat. But how and why? — GN, Marine City, MI
When two surfaces slide across one another, some of the mechanical energy in those surfaces is converted to thermal energy (or heat). That's because the surfaces are microscopically rough and their atoms collide as the surfaces slide pass one another. Each time a collision occurs, the atoms that collide begin to vibrate more vigorously than before. In this process, the surfaces lose some of their overall mechanical energy but the atoms gain some randomly distributed local vibrational energy—more thermal energy. Those surface atoms become hotter. As the sliding continues, large regions of the surfaces become hotter and the surfaces lose much of their energy. If you don't push them to keep them sliding across one another, they'll come to a stop as all their mechanical energy is converted into thermal energy.

746. Where does the wax from a burning candle go? Also, why do beeswax candles burn virtually completely, leaving no wax behind at all? — SC, Rhode Island
The wax molecules in the candle react with oxygen in the candle flame and are converted into water molecules and carbon dioxide molecules. That reaction is associated with combustion and it releases energy so that the candle produces light and heat. The molecules formed by this combustion drift off into the air.

Normal candle wax (paraffin wax) consists of relatively large hydrocarbon molecules. Each molecule in paraffin is a chain of between 30 and 50 carbon atoms that are surrounded by hydrogen atoms. Because its molecules are fairly long and they stick together reasonably well, paraffin is a firm, crystalline solid. If the chains were shorter, say 20 to 30 carbon atoms long, the material would be softer—it would be a liquid-like wax known as petroleum jelly. If the chains were much longer, say 2000 to 3000 carbon atoms long, the material would be firmer—it would be a solid known as polyethylene. Still shorter chains are used in machine oil, diesel fuel, unrefined gasoline, and finally petroleum gases such as propane and methane. The shorter the chain, the softer, thinner, and more volatile the hydrocarbon is at any given temperature. All of these hydrocarbon molecules can burn completely, leaving only water molecules and carbon dioxide. In a candle, the heat of the flame vaporizes the wax molecules—they become a gas—and they then burn completely in the flame itself. As long as the wax doesn't drip away from the flame, the flame will consume it all completely and leave no ash or wax. Although the structure of the molecules in beeswax is slightly different from that in paraffin, beeswax also vaporizes from the heat of the flame and then burns completely.

747. What is an electric field and how does it affect us? — MT, Brampton, Ontario
Electrically charged particles exert forces on one another. For example, a negatively charged particle attracts a nearby positively charged particle and repels another negatively charged one. These attractions and repulsions are mediated by electric fields that are created by those charges. By this statement, I mean that the negatively charged particle creates an electric field around itself and this electric field is what ultimately exerts forces on the other two charges—attracting the nearby positively charged particle and repelling the negatively charged one. Whenever an electrically charged particle finds itself in an electric field, it experiences a force. The direction of that force depends on its electric charge (either positive or negative) and on the direction of the electric field (which may have somewhat different directions at different points in space). The strength of that force depends on the amount of electric charge on the particle and on the strength of the electric field (which can vary from nothing at all to extremely strong).

But while electric fields always exist around charged objects and exert forces on any other charged objects that enter them, electric fields can also exist far away from charges. Electromagnetic waves contain electric and magnetic fields (the magnetic equivalents of electric fields) and these two fields sustain one another as the wave travels. Although electromagnetic waves are created and destroyed with the help of charged particles, they can travel alone and without any nearby charged particles to assist them.

While electric fields exert forces on the charged particles in our bodies, the response of those charges isn't likely to injure us. When you are exposed to an electric field, there is a subtle rearrangement of electric charges on the surface of your body that then creates its own electric field. The result is that there is essentially no electric field inside you. Only when you are exposed to extremely strong electric fields, and spark and currents begin to flow through you, is there any significant effect to you.

748. How are the paints made that artists (like Rembrandt and Monet) used in the past? — SB, Oedenrode, The Nederlands
These paints consisted principally of a pigment and a drying oil binder. The pigment was usually a colored powder that didn't dissolve in the oil. Historically, these pigments were materials collected from nature. The drying oil binder was usually linseed oil, obtained from the seed of the flax plant and a byproduct of the linen industry. Like most organic oils, linseed oil is a triglyceride—it consists of a glycerin molecule with three fatty acid chains attached to it. But while in typical animal or tropical plant oils the carbon atom chains of the fatty acids are completely decorated with hydrogen atoms (saturated fats) or almost completely decorated (monounsaturated fats), the carbon atom chains in linseed oil are missing a significant number of hydrogen atoms (polyunsaturated fats). The polyunsaturated character of linseed oil makes it vulnerable to a chemical reaction in which the chains stick permanently to one another—a reaction call polymerization. With time and exposure to air, the molecules in linseed oil bind together forever to form a real plastic! This "drying" process takes weeks, months, or years, depending on the chemicals present in the paint. It can be accelerated by the addition of catalysts—chemicals that assist the polymerization process but that don't become part of the final molecular structure of the plastic.

749. How do rotary telephones work? — JG, DeSoto, Kansas
As your finger turns the dial of the telephone, you wind a spring and store energy in that spring. When you remove your finger, the spring unwinds and its stored energy drives the dialing mechanism. This mechanism consists of a cogged wheel and a switch, as well as a centrifugal governor. As the dial unwinds, the cogged wheel turns and it's cogs close and open a switch one time for each number on the dial. For example, if you dial a "6", the switch closes briefly 6 times. For a "0", the switch closes 10 times. Each time the switch closes during this action, it "hangs up" the telephone briefly. The switching system at the telephone company recognizes these brief hang-ups as signals for establishing the connection. The centrifugal governor controls the rate at which the dial unwinds and makes sure that the pulses coming from the telephone occur at a uniform rate.

750. Warner Brothers has been misleading children! The coyote and the anvil hit the ground at the same time!
You're exactly right. Occasionally one of those cartoons shows the coyote falling with the anvil directly above his head and the distance between them remaining constant, which is what should happen (ignoring air resistance). But more often, the coyote falls much faster than the anvil, hits the ground first, and is then pounded by the anvil. It sure would be neat to live in a cartoon—the laws of physics just wouldn't apply.

751. When accelerating, can you decelerate by going in a direction that is not opposite (your velocity)? For example, going north can you decelerate by going east?
Decelerating is a very specific acceleration—always in the direction opposite your velocity. If you were heading north and accelerated toward the east, your velocity would soon point toward the northeast. It would have some northward aspect because you were initially heading north and hadn't yet accelerated toward the south. It would have some eastward aspect because you had initially been heading neither eastward nor westward and had since accelerated toward the east.

On the other hand, if you were heading north and then turned toward the east, you would have lost your northward velocity and obtained an eastward velocity. This "turning" would have involved a southward acceleration (to get rid of the northward velocity) and an eastward acceleration (to acquire an eastward velocity).

752. You said that from the moment the ball leaves your hand (after you threw it upward), it accelerates downward even though you threw it upward. However you then said that the ground (gravity) pushed on your foot to make you accelerate, so why would you also not be accelerating in the opposite direction, like the ball? Why would you not accelerate in the direction in which you were pushed?
I got ahead of myself by using forces I had not yet introduced. I was using friction to push me horizontally across the floor! Here is the complete story:

When I tossed the ball upward and it was rising, gravity was pulling downward on it and it was accelerating downward. But when I obtained a force from the ground, it was not gravity that exerted that force on me; it was friction! As we will discuss in a few days, whenever you try to slide your foot across the floor toward the left, friction pushes your foot toward the right. In class, I traveled toward the right because I was being pushed by friction toward the right. I was actually accelerating in the direction I was pushed, just as you expect.

753. In today's lecture, you stated that a person accelerating downward OR UPWARD does not feel the effects of gravity. How do you explain the g-forces felt by astronauts at escape velocity? - TH
In the lecture, I said that a person who is falling does not feel the effects of gravity, even when they are traveling upward. But when they are falling, they are accelerating downward at a very specific rate—the acceleration due to gravity, which is 9.8 meters/second2 at the earth's surface. When an astronaut is accelerating upward during a launch, they are not falling and they do feel weight. In fact, because they are accelerating upward, they feel particularly heavy.

754. What is heat? — PM, Princeton, NJ
Heat is thermal energy that's flowing from one object to another because of a temperature difference between those two objects. Whenever an object contains thermal energy—which it always does—the atoms and molecules in that object are jittering about microscopically. Each atom or molecule isn't completely stationary; instead it is vibrating back and forth, and pushing or pulling on its neighbors. The object's thermal energy is the sum of the tiny kinetic and potential energies of those atoms and molecules as they move back and forth (kinetic energy), and push or pull on one another (potential energy). The hotter an object is, the more thermal energy each of its atoms has, on average, so this thermal energy tends to flow to a colder object when you touch the two objects together. When that thermal energy is flowing from the hotter object to the colder object, we call it "heat."

755. How does a refrigerator work? - SK
A refrigerator uses a material called a "working fluid" to transfer heat from the food inside the refrigerator to the air around the refrigerator. This working fluid moves through the refrigerator's three main components—the compressor, the condenser, and the evaporator—over and over again, in a continuous cycle. I'll begin as the fluid enters the refrigerator's compressor, which is usually located on the bottom of the refrigerator where it's exposed to the room air. The working fluid enters the compressor as a low-pressure gas at roughly room temperature. The compressor squeezes the molecules of that gas closer together, increasing the gas's density and pressure. Since squeezing a gas involves physical work (a force exerted on an object as that object moves in the direction of the force), the compressor transfers energy to the working fluid and that fluid becomes hotter as a result.. The working fluid leaves the compressor as a high-pressure gas that's well above room temperature. The working fluid then enters the condenser, which is typically a snake-like pipe on the back of the refrigerator. Since the fluid is hotter than the room air, heat flows out of the fluid and into the room air. The fluid then begins to condense into a liquid and it gives up additional thermal energy as it condenses. This additional thermal energy also flows as heat into the room air.

The working fluid leaves the condenser as a high-pressure liquid at roughly room temperature. It then flows into the refrigerator, then through a narrowing in the pipe, and then into the evaporator, which is another snake-like pipe that's wrapped around the freezing compartment (in a non-frostfree refrigerator) or hidden in the back of the food compartment (in a frostfree refrigerator). When the fluid goes through the narrowing in the pipe, it's pressure drops and it enters the evaporator as a low-pressure liquid at roughly room temperature. It immediately begins to evaporate and expands into a gas. In doing so, it uses its thermal energy to separate its molecules from one another and it becomes very cold. Heat flows from the food to this cold gas. The working fluid leaves the evaporator as a low-pressure gas a little below room temperature and heads off toward the compressor to begin the cycle again. Overall, heat has been extracted from the food and delivered to the room air. The compressor consumed electric energy during this process and that energy has become thermal energy in the room air.

756. If heat rises, how come snow accumulates on mountains? Why is it colder up there instead of down here? — HG, Grand Prairie, TX
On a local scale, hot air does rise through cold air. That's because when hot air and cold air are at the same temperatures, the hot air has fewer air molecules per liter than the cold air and so each liter of hot air is lighter than each liter of cold air. In short, hot air is less dense than cold air and it floats upward in cold air. But when hot air rises a long way through the atmosphere, something begins to happen to the hot air. It cools off! That's because the air pressure decreases with altitude. The air pressure that's around us on the ground is only present because the air down here must support the air overhead. The air down here must push upward on the air overhead and it does this by developing a high pressure. But as you move upward in the atmosphere, there's less air overhead and therefore less air pressure around you.

So as the hot air rises upward, the air pressure around it gradually diminishes and the hot air expands. It has to expand because whenever its pressure is higher than the surrounding pressure, its molecules experience outward forces that cause them to spread out. But this expansion process uses some of the hot air's thermal energy—the hot air must push the surrounding air out of the way as it expands. With less thermal energy in it, the hot air becomes cooler. Dry air loses about 10° C for every kilometer it rises, while moist air loses about 6° or 7° C per kilometer. This cooling effect explains why air at higher altitudes, such as the air on mountains, is colder than the air at lower altitudes, such as the air in valleys.

Furthermore, whenever cold air descends through the atmosphere, it is compressed and its temperature rises! This warming process also increases the air's water-carrying ability so that it becomes relatively dry. That effect explains the special "Katabatic" winds that blow warm and dry out of the mountains—including the Santa Ana winds near Los Angeles, the Chinook in the Rocky Mountains, the Foehn in the Alps, and the Zonda in Argentina.

757. Is there any way to make a homemade fog machine, like they use in clubs? — JW, Westport, CT
While it's pretty clear that fog machines fill the air with tiny water droplets, I'm not sure how all of them work. Some probably use high-frequency sound waves to break up water into tiny droplets and then blow these droplets into the room with a fan. That technique is used in some room humidifiers and you can see a stream of fog emerging from them as they operate. An easier way to make fog is to mix water and liquid nitrogen. While liquid nitrogen is harder to find, all you have to do is put them together and they'll start making fog. The boiling nitrogen shatters the water into tiny droplets, which flow out of the mixture in a layer of cold nitrogen gas.

758. How do steam generators produce electricity? — KA, North Platte, NE
In a steam generating plant, water is boiled in a confined container (a "boiler") to produce very high-pressure steam. This steam is allowed to flow through a turbine to the low-pressure region beyond the turbine. A turbine resembles a fan, but one that is turned by the gas that flows through it rather than by a motor. The steam flows through the blades of the turbine and exerts forces on those blades to keep the turbine rotating. The steam loses energy as it twists the turbine around in a circle and this energy is transferred to the rotating turbine. The low-pressure steam is recovered from the end of the turbine. It is then condensed back into liquid water with the help of a cooling tower and then returned to the boiler for reuse.

The rotating turbine is connected to the rotating portion of a generator. This rotating component is an electromagnet and, as it spins, its magnetic field passes across a set of stationary wire coils. Whenever the magnetic field through a coil of wire changes, any current flowing through that coil experiences forces that may add or subtract energy from it. In this case, the rotating magnet transfers energy to the current passing through the wire coils and "generates" electricity. The current in these stationary wires carries away energy from the generator and it is this energy that eventually arrives in your home through the power lines. Overall, the energy flows from the boiler, to the steam, to the turbine, to the generator, to the current, and to your home.

759. Why do metal objects spark/arc in the microwave? Why don't the metal walls of the microwave spark? - JR
Like all electromagnetic waves, microwaves are composed of electric and magnetic fields. Since an electric field exerts forces on charged particles, a microwave pushes electrons back and forth through any metals it encounters. It is this motion of electrons back and forth through the metal walls of the microwave oven that allow that metal to reflect the microwaves and keep them inside the oven. If you leave a spoon in you cup of coffee as you heat it in the microwave, electrons will move back and forth through the spoon. This motion of charge will cause no problems so long as (1) the spoon can tolerate this flow of charge without overheating and (2) the spoon doesn't allow the charges at its ends to leap into the air as a spark. To keep the spoon from overheating, it must be a good conductor of electricity. Since most spoons are pretty thick, the modest currents flowing through them in the microwave will leave little energy inside them and they won't overheat. But a thin twist-tie or small bit of aluminum foil may well overheat and begin to burn. To keep the spoon from sparking, it should have smooth ends. Electrons are more likely to leave the end of a metal surface at a sharp point, so avoiding points is important. Most spoons are smooth enough that no sparks will occur. But a fork, a sharp piece of foil, or a twist-tie may well begin to emit electrons into the air as those electrons pile up at one end of the wire while the microwave oven is on. Like a spoon, the walls of the oven are good conductors of electricity and they have no sharp points. While electrons move back and forth in these walls, they simply reflect the microwaves without becoming very hot and without emitting any sparks. You'll note that the light bulb for the microwave is always outside the cooking chamber because it contains small bits of metal that would have trouble inside a microwave oven.

760. What are the two chemical in glow sticks? — JW, Westport, CT
I believe that the glow sticks contain luminol and hydrogen peroxide, which mix when you crack the glass ampoule and begin to emit light. There are several other chemicals present in the sticks to assist and control the process, but the principal reaction is one in which the hydrogen peroxide oxidizes ("burns") the luminol molecule. The result is a product molecule that is initially in an excited state—its electrons have more energy than they need—and it emits a particle of bluish-violet light. Since our eyes aren't particularly sensitive to that bluish-violet light, it's often converted into more visible light with the help of a fluorescent dye. The green light sticks probably contain sodium fluorescein molecules, each of which can absorb a photon of bluish-violet light and reemit some of its energy as a photon of green light. Other dyes, probably rhodamines, are used to make red or orange light sticks.

761. Can you explain once again how the bowling ball and the tennis ball drop at the same time. Are weight and mass proportional? If mass is the resistance to acceleration and weight is a gravitational force pulling down on the ball, doesn't the weight of the bowling ball make it fall faster? Or does the bowling ball's increased mass in a way cancel out the bowling ball's increased weight? - HC
Weight and mass are proportional to one another and the bowling ball's increased mass does effectively cancel out its increased weight. Let's suppose that the bowling ball is 100 times as massive as the tennis ball—meaning that it takes 100 times as much force to make the bowling ball accelerate at a certain rate as it does to make the tennis ball accelerate at that same rate. Because weight is proportional to mass, the bowling ball also weighs 100 times as much as the tennis ball. So if the only force on each ball is its weight, each ball will accelerate at the same rate. The bowling ball will experience 100 times the force but it will be 100 times as hard to accelerate. The two factors of 100 will cancel and it will accelerate together with the tennis ball.

762. If forces are always equal but opposite, how can a hammer drive a nail into a wall? Don't the forces on the nail cancel?
Although forces always appear in equal but oppositely directed pairs, the two forces in each pair act on different objects. The nail and hammer experience one of these force pairs—the hammer pushes on the nail just as hard as the nail pushes on the hammer. Because the nail's force on the hammer is the only force that the hammer experiences, the hammer accelerates away from the nail and the wall. The nail and wall experience the other force pair—the wall pushes on the nail just as hard as the nail pushes on the wall. The nail thus experiences two horizontal forces: the hammer pushes it toward the wall and the wall pushes it away from the wall. As long as all the forces are gentle, the two forces on the nail cancel and it doesn't accelerate at all. But if you hit the nail hard with the hammer, the wall can't exert enough support force on the nail to prevent it from enter the wall. The two forces on the nail no longer cancel and it accelerates into the wall.

763. If the net force on an object is zero and it has no acceleration, then what causes it to have velocity? Doesn't a force give it velocity? And doesn't this make the gravitational and support forces unequal? - EH
The great insights of Galileo and Newton were that an object doesn't need a force on it to have a non-zero velocity. Objects tend to coast along at constant velocity when they are free of forces, or when the net force on them is zero. Inertia keeps them going even though nothing pushes on them. While it takes an acceleration and thus a non-zero net force to get an object moving in the first place, it will continue to move even if the net force on it drops to zero. So while I was lifting the bowling ball upward at constant velocity, the net force on the bowling ball was truly zero—it was coasting upward because its weight and the support force from my hand were canceling one another. However, to start the bowling ball moving upward, I had to push upward on it harder than gravity pushed downward. For a short time, the bowling ball experienced an upward net force and it accelerated upward. After that, I stopped pushing extra hard and let the bowling ball coast upward at constant velocity.

764. What would it be like if Newton's third law weren't true? Can we imagine that?
Many strange things would happen. For example, suppose that you pushed on your neighbor and your neighbor didn't push back—you wouldn't feel any force pushing against your hand so you wouldn't even notice that you were pushing on your neighbor. Your neighbor would feel you pushing on them and they would accelerate away from you.

Among the many consequences of such a change would be that energy wouldn't be conserved—you would be able to create energy out of nowhere. To see how that would be possible, imagine lifting a heavy object and suppose that as you pushed upward on it, it didn't push downward on you. As you lifted it upward, you would do work on it—you would exert an upward force on it and it would move upward. But it wouldn't do negative work on you—it would exert no force on you as your hands lifted it upward. As a result, its energy would increase but your energy wouldn't decrease. Energy would be created. In fact, you wouldn't even notice that you were lifting it because it wouldn't push on you as you lifted it.

765. Would a small-mass hammer that accelerated rapidly exert more horizontal force on a nail than a large-mass hammer that didn't accelerate very much?
Yes. Since the only horizontal force acting on the hammer is that exerted on it by the nail, the hammer's acceleration is entirely determined by that force. The force on the hammer is equal to the hammer's mass times the hammer's acceleration (Newton's second law). If both hammers experienced the same acceleration, then the large-mass hammer would have to be experiencing the larger force from the nail and would therefore be exerting the larger force on the nail. But because the small-mass hammer is experience a larger acceleration, the force that the nail is exerting on it may be quite large. If the small-mass hammer's acceleration is large enough, the force on it may exceed the force on the large-mass hammer.

766. You said that if I push on a friend they will push back (even if they are asleep). But if I push hard enough, they will fall to the ground, whereas I will not. Therefore, I don't see how the reaction is equal. Can you please explain this? - JK
Newton's third law only observes that the forces two objects exert on one another are equal in amount but opposite in direction. The law doesn't make any statement about the consequences of those forces on the objects involved. Moreover, it doesn't say that those forces are the only forces on the objects. When you push on an awake friend, your friend will obtain additional forces from the ground or a nearby wall, and will manage to avoid falling over. Even though you push your friend away from you, your friend will see to it that the ground pushes them toward you. As a result, they will probably stay in one place. But when your friend is asleep, they won't be able obtain the additional forces necessary to compensate for the force you exert on them and they may accelerate away from you or fall over.

767. In "Empire Strikes Back", when Luke learns that Darth Vader is his father, he falls/jumps off a platform in Cloud City without his hand. Given the fact that objects reach terminal velocity, which would have a faster terminal velocity and which would hit the ground first if in the movie that fell from a height of 1000 meters?
Luke would probably reach the ground before his hand. An object reaches a terminal velocity as it fall because the upward force of air resistance becomes stronger as the object's downward speed increases and this upward force eventually stops the object from accelerating downward. The object's downward speed at the point when it stops accelerating is its terminal velocity. Since air resistance is what sets this terminal velocity, an object that experiences a great deal of air resistance relative to its weight will have a smaller terminal velocity than an object that experiences relatively little air resistance relative to its weight. Because Luke is much larger than his hand, he has lots of weight relative to his surface area. Since surface area largely determines air resistance, he experiences relatively little air resistance relative to his weight. His hand has less weight relative to its surface area and it experiences a lot of air resistance relative to its weight. So Luke's terminal velocity is larger than that of his hand. He reaches the ground first. This tendency for large objects to descend faster than small objects explains why small animals, such as insects, can fall from incredible heights without injury. They reach their terminal velocities quickly and descend rather slowly to the ground.

768. Some friends and I are having a debate. They maintain that if a person sleeps on an unheated waterbed, heat might be drawn from their body to the point that hypothermia would occur. Is it possible for a waterbed to do this? — JS, College Park, MD
The answer depends on how cold you allow room temperature to become. Without a heater, the water temperature in the bed will be very close to room temperature. When you then lie on the bed, you will be in contact with a surface that's at room temperature and heat will flow out of you and into the water. Your heat will warm the water and it will tend to float upward and remain at the top surface of the waterbed, forming an insulating layer that will slow your heat loss. However, heat will continue to diffuse into the water as a whole and you will continue to lose heat. As long as the water isn't too cold, your metabolism will be able to replace the lost heat and you'll stay warm. But if the room and waterbed are very cold, your temperature will begin to drop. I'm not sure how cold the water would have to be for this to happen, but if the room and water were almost ice cold, you'd probably have trouble.

769. How does a thermostat regulate temperature? — TF, Auburn, WA
A typical thermostat turns on the furnace whenever the temperature falls below a certain temperature and turns the furnace off whenever the temperature rises above another temperature. Those two temperatures are slightly separated so that the furnace doesn't turn on and off too rapidly. In a typical home thermostat, a bimetallic coil tips a small mercury-filled glass bottle. The bimetallic coil is made from two different metal strips that have been sandwiched together and then rolled into a coil. As the temperature changes, the two metals expand differently and the coil winds or unwinds. As it does, it tips the glass bottle and the mercury rolls from one end of the bottle to the other. When the mercury falls to one end, it allows an electric current to flow between two wires and the furnace turns on. When the mercury falls to the other end of the bottle, the current stops flowing and the furnace turns off. So the winding and unwinding of the coil controls the furnace and the home temperature tends to hover at the point where the bottle of mercury is almost perfectly level. When you adjust the set point of the thermostat, you tilt the whole coil and bottle so that the average temperature in your home must shift in order for the bottle to be almost level.

770. How does a dehumidifier know when to turn on and off? The one I bought from Sears doesn't use the "wet-bulb/dry-bulb" method (of which I could use a better understanding, too). How does its on-off switch work? — JS, Amherst, NY
Most humidity sensing switches or "humidistats" use the expansion or contraction of certain materials to measure humidity. The more humid the air is, the more water molecules there will be in those materials and their shapes and sizes will be affected. For example, human hair becomes longer when wet and it makes an excellent humidity sensor. On a dry day, a hair will contain relatively few water molecules and its length will be shorter. On a humid day, the hair will contain more water molecules and its length will be longer.

A wet-bulb/dry-bulb system measures humidity by looking at the temperature drop that occurs when water evaporates. As water evaporates from the bulb of the wet thermometer and the bulb's temperature drop, the rate at which water molecules leave the bulb's surface decreases. The bulb temperature drops until the rate at which water molecules leave the bulb is equal to the rate at which water molecules return to the bulb from the air. At that point, there is no net evaporation going on. In humid air, water molecules return to the bulb more often so that this balance is reached at a higher temperature than in dry air. The wet bulb temperature is thus warmer on a humid day than it is on a dry day.

771. How do an ammeter and voltmeter work? Why must the former be connected in series while the latter goes in parallel? — SK, New Haven, CT
The answer is somewhat different for older electromechanical meters than for modern electronic meters. I'll start with the electromechanical ones and then briefly describe the electronic ones. An electromechanical meter has a coil of wire that pivots in a nearly friction-free bearing and has a needle attached to it. This coil also has a spring attached to it and that spring tends to restore the coil and needle to their zero orientation. Because the spring opposes any rotation of the coil and needle, the orientation of the needle depends on any other torque (twist) experienced by the coil of wire—the more torque the spring-loaded coil experiences, the farther the coil and needle will turn away from the zero orientation. The needle's angle of deflection is proportional to the extra torque on the coil.

The extra torque exerted on the spring-load coil comes from magnetic forces. There is a permanent magnet surrounding the coil, so that when current flows through the coil it experiences a torque. Because a current-carrying coil is magnetic, the coil's magnetic poles and the permanent magnet's magnetic poles exert forces on one another and the coil experiences a torque. This magnetic torque is exactly proportional to the current flowing through the coil. Because the torque on the coil is proportional to the current and the needle's angle of deflection is proportional to this torque, the needle's angle of deflection is exactly proportional to the current in the wire.

To use such a meter as a current meter (an ammeter), you must allow the current flowing through your circuit to pass through the meter. You must open the circuit and insert this ammeter in series with the rest of the circuit. That way, the current flowing through the circuit will also flow through the meter and its needle will move to indicate how much current is flowing.

To use such a meter as a voltage meter (a voltmeter), some current is divert from the circuit to the meter through an electric resistor and then returned to the circuit. The amount of current that follows this bypass and flows through the electric resistor is proportional to the voltage difference across that resistor (a natural phenomenon described by Ohm's law). The voltmeter system thus diverts from the circuit an amount of current that is exactly proportional to the voltage difference between the place at which current enters the voltmeter and where it returns to the circuit. The needle's movement thus reflects this voltage difference.

In an electronic voltmeter, sensitive electronic components directly measure the voltage difference between two wires. Virtually no current flows between those two wires, so that the meter simply makes a measurement of the charge differences on the two wires. An electron ammeter uses an electronic voltmeter to measure the tiny voltage difference across a wire that is carrying the current. Since the wire also obeys Ohm's law, this voltage difference is proportional to the current passing through the wire.

772. Does an electric blanket produce enough EMF to affect the body and possible increase the risk of cancer? - FL
The electromagnetic fields (EMF) produced by the currents in an electric blanket are very weak and it takes a pretty sensitive electronic device to detect them. You body is not nearly so sensitive and I still haven't seen any credible explanation for how these fields could cause any injury to biological tissue. I strongly suspect that all the concern about EMF is just hysteria brought about by a few epidemiological flukes or mistakes.

773. On really cold winter days at temperatures well below zero, I've noticed that sunlight is brighter and whiter than on days that are a little below freezing. Why does this happen? — CP, Madison, WI
The colder the air is, the less humidity it can hold. That's because at low temperature, water molecules in the air are much more likely to land on a surface and stick than they are to break free from a surface and enter the air. Thus cold air is relatively free of water molecules. Water molecules in the air tend to bind together briefly and form tiny particles that scatter light. The sky is blue because of such scattering from tiny particles. With less water in the air, there is less scattering of sunlight. As a result, the sky is a darker blue, almost black, and the sunlight that reaches you directly from the sun retains a larger fraction of its blue light. The sun appears less red and more blue-white than on a warmer, more humid day.

774. Why do we see colors when light strikes atoms? — GN, Marine City, MI
When white light strikes a molecule, that molecule may absorb some of the light. Light interacts with molecules as particles called "photons" and whether a particular photon is absorbed depends on the structure of the molecule and the color of the photon. Each molecule has the ability to absorb only certain colors of light. For example, a particular molecule may absorb only red photons. As a result, your eye will see only green and blue light photons coming from that molecule when it's exposed to white light and you will perceive that molecule as having a blue-green color known as cyan. In general, the colors that you see coming from molecules that are illuminated by white light are the colors of light that the molecules don't absorb.

775. How do neon lamps work? — TF, Auburn, WA
A neon lamp consists of a neon-filled tube with an electrode (a metal wire) at each end. When you put enough electrons on one of the electrodes and remove enough electrons from the other, electrons will begin to leap off the first electrode and accelerate toward the other electrode. Because the density of neon atoms in the tube is relatively low, only about 1/1000th that of air molecules in normal air, the electrons can travel long distances without colliding with a neon atom. As the electrons accelerate, their kinetic energies increase. However, these electrons occasionally collide with neon atoms and, when they do, they can give up some of their kinetic energies to those atoms. The neon atoms then end up with excess energy and they often emit this energy as light. The color of this light is determined by the structure of a neon atom and tends to be the familiar red of a neon sign.

776. Could you see a laser beam in outer space since it can't reflect off of anything? — RM, Rochester, NY
No. The reason that you can see a very intense laser beam as it passes through the air is that light can scatter off of dust particles and air molecules. When it does, some of the laser light is sent toward your eyes and you see the light coming toward you from the laser beam's path. But if there is no air in the path of the laser beam, the light will travel without scattering and you won't see the path at all.

777. What happens to gas in a gas mask? — TF, Auburn, WA
Most gas masks remove toxic molecules from the air by allowing those molecules to react with or stick to a surface inside the mask. Molecules are generally too small to remove from the air with simple filters, so they must be removed by chemical processes. Highly reactive molecules, such as chlorine, fluorine, and ozone, naturally attack and bind with many chemicals and are easily removed by a mask containing those chemicals. Other molecules aren't so reactive and must be collected in a more complicated manner. Sometimes the gas mask will contain a reactive chemical that seeks out specific toxic molecules in the air and binds chemically to those molecules. But some mask simply use activated carbon, which just sticks molecules to its surface. The molecules don't stick very tightly to the carbon surface, so they can be driven off by baking the carbon. But the carbon is finely divided so that it has an enormous amount of surface area and can accumulate a great many molecules before it becomes "full." Finally, some gas masks contain catalysts that decompose certain toxic molecules, chopping them up before they enter your lungs.

778. Why is the element mercury a liquid at room temperature when none of its neighbors on the periodic table are? — BZ, Trenton, NJ
The answer to that question lies at least partly in the electronic structure of the mercury atom. The mercury atom is the largest member of the third row of transition metals, meaning that it is the atom at which the 5d shell of electrons is finally filled completely. Whenever a shell of electrons is filled, that shell can no longer assist in forming chemical bonds. While the d shell electrons normally help hold transition metal atoms together, making these metals strong and hard to melt, the filling of the 5d shell makes it hard for mercury atoms to stick to one another. In contrast to metals like tungsten and tantalum, which melt only at very high temperatures, mercury is a liquid at room temperature. Actually, the zinc atom is the atom at which the 3d shell is filled and the cadmium atom is the atom at which the 4d shell is filled. While those two metals are solid at room temperature, they have very low melting points.

779. With Newton's first law, the word "tends" seems a bit ambivalent. Does this word suggest there are exceptions to the rule?
The statement of inertia contains the word "tends" (an object in motion tends to continue in motion and object at rest tends to remain at rest) because it doesn't deal with the presence or absence of forces. If forces were outlawed, then the word "tends" could be dropped from the statement.

However, Newton's first law is not ambivalent and does not contain the word "tends." It states directly that an object that's free of outside forces moves at constant velocity. No ifs, ands, or buts. If I have inserted the word "tends" into this law in class, it was a mistake on my part.

780. In Exercise #9 on pg. 33: If you are riding on an escalator, with a suitcase, doesn't the escalator supply the upward force? Doesn't this also mean that the forces of the suitcase and escalator cancel one another to produce a net force of zero?
First, let's suppose that the suitcase is resting directly on the escalator and you are not touching it (I had intend that you hold the suitcase in your hand). Because the suitcase is traveling at constant velocity, the net force on it must be zero. Since the suitcase has a downward weight, the escalator must be pushing upward on the suitcase with a force exactly equal in magnitude to the suitcase's weight. As you suggest, the force of the suitcase's weight and the support force of the escalator cancel one another to produce a net force of zero on the suitcase. Now, if you are holding the suitcase, it's your job to exert this upward force on the suitcase. Once again, that upward force is equal in magnitude to the weight of the suitcase.

781. If air in a rigid 80 cubic foot scuba tank is pressurized to 3000 psi, giving the diver a certain amount of breathing time, then why does bottom time decrease with depth? I know about external pressure, but how does the pressure affect air inside the tank? - RJ
The deeper a scuba diver goes, the greater the water pressure and the more the water presses in on the diver's chest. To be able to breathe, the air in the diver's mouth must have roughly the same pressure as the water around the diver's chest. That way, the diver will be able to use chest muscles to breathe the air into the diver's lungs. But the pressure of the air in the diver's mouth is proportional to its density and thus to the number of air molecules contained in each liter of air. At great depths, the diver must breathe dense, high-pressure air and this air contains a great many air molecules per liter. Since the scuba tank contains only so many air molecules, these molecules are consumed more rapidly at great depths than they are at shallow depths. The scuba regulator automatically controls the density of air entering the diver's mouth so that the air pressure is equal to the surrounding water pressure. That way, the air is easy to breathe. The deeper the diver goes, the more air molecules the regulator releases into each of the diver's breaths and the faster the air in the scuba tank is consumed.

782. How do you determine the volume of water passing through a weir? - R
If the speed of the water were uniform as it passes through the opening, you could measure that speed and multiply it by the cross-section of the weir to obtain the volume of water passing through the weir each second. However, since the flow is faster near the center of the flow, it's difficult to calculate the volume flowing each second. Your best bet is probably to divide the opening into a number of regions and then to measure the water's velocity at the center of each region. Multiply each velocity by the cross-sectional area of that region and then sum up all the products to obtain the overall volume flow per second.

783. How do flashing lights, chasing lights, and any type of Christmas lights work? - N
Years ago, many strings of Christmas lights consisted of about 20 or 30 light bulbs in series. In this series, electric current passed from one bulb to the next and deposited a small fraction of its energy in each bulb. The result was that each bulb glowed brightly so long as every bulb was working. If a single bulb burned out, the entire string went dark because no current could flow through the open circuit. If you replaced one of the bulbs in a working string with a special blinker bulb, the whole string would blink. The blinker bulb contained a tiny bimetallic switch thermostat that turned it off whenever the temperature rose above a certain point. At first, the bulb would glow and the whole string would glow with it. Then the thermostat would overheat and turn the bulb and string off. Then the thermostat would cool off enough to turn the bulb and string back on. This pattern would repeat endlessly.

But modern electronics has replaced the blinker bulbs with computers and transistor switches. Transistorized switches determine which bulbs or groups of bulbs receive current and glow at any given time and carefully timed switching can make patterns of light that appear to move or "chase." As for the problem with one failed bulb spoiling the string, a reader has informed me that the bulbs are now designed with a fail-safe feature. If a bulb's filament breaks, the sudden surge in voltage across that bulb activates this fail-safe mechanism. Wires inside the bulb connect to allow current to bypass that bulb completely. The remaining bulbs in the string glow a little more brightly than normal and their lives are shortened slightly as a result.

784. Is it true that you can get lead poisoning in your home more easily from hot water than from cold water? — WH, Erial, NJ
Yes, assuming that your home has either lead pipes or copper pipes that were joined with lead-containing solders. That's because lead compounds are more soluble in hot water than they are in cold water. The amount of lead that was permitted in pipe solders has diminished over the years until now, when pipe solders can't contain any lead at all. While very little lead actually leaches out of the solder joints and enters the water, the effect is slightly more significant in hot water pipes than in cold water pipes. That's why it's recommended that you not use water from hot water pipes in cooking.

785. How does electricity get from the generating station to the outlet in my living room? — JJ, Arlington, MA
The generating station uses a large generator to transfer energy from a giant turbine to an electric current flowing through a coil of wire. Current from this generating coil then flows through the primary coil of a huge transformer, where it transfers its energy to the magnetic core of the transformer. The current then returns to the generator to obtain more energy.

The magnetic core of the transformer transfers its energy to a second current—one that is passing through the secondary coil of the transformer. Because this current consists of far fewer electric charges per second, each charge receives a very large amount of energy. This large energy per charge gives the current a high voltage and it flows very easily through a high voltage transmission line. Because the amount of power that a wire loses is proportional to the square of the current passing through it, this high-voltage, low-current electricity wastes very little power in the transmission line on its way across country to your city. When the current reaches your city, it passes through another transformer and its energy is transferred to a third current. The cross country current then returns through the transmission line to the original power station to obtain more energy from the first transformer.

This third current involves more charges per second, so each charge carries less energy and the voltage is lower. This medium voltage electricity travels to your neighborhood before passing through a final transformer. This final transformer is probably either a gray metal can on a utility pole or a green box on a nearby lawn. In passing through the final transformer, the current transfers its energy to a current which then enters your home. This last current delivers energy to your appliances and lights and then returns to the final transformer to obtain more energy.

786. How much does it cost to run a regular 60 to 100 watt light bulb per minute or per hour? — JM, Smithfield, ME
Electricity typically costs about 7 cents per kilowatt-hour. Over the course of an hour, a 100-watt light bulb will use 100 watt-hours or 0.1 kilowatt-hours, at a cost of about 0.7 cents. That's about 0.012 cents per minute.

787. In our busy trial court we have preserved the original cassette tapes since 1989. They are kept in a relatively constant room temperature environment in our modern courthouse. Should we take any further precautions to extend the life of these tapes, considering the possibility that they may need to be replayed one day, such as in the retrial of a death penalty case that is reversed a decade after trial? I've heard of the practice of unwinding and rewinding tapes for this purpose, but haven't attempted it yet. The time involved is daunting! What is your opinion? — JD, Bryan, TX
A magnetic recording tape is usually a Mylar ribbon, coated with a thin layer of plastic that's impregnated with tiny permanent magnets. As long as it's store away from heat and moisture, the Mylar film itself shouldn't age. However, the layer of permanent magnets can change slightly with time. When a tape is left tightly wound on its reel for a long time, the magnetic layers can begin to affect one another—the magnetic fields from one layer of tape can alter the magnetization of the layers above and below it. The result is that sounds from one layer of tape can gradually transfer themselves weakly to the adjacent layers, creating faint echo effects. The solution to this problem is to unwind and rewind the tape, so that the layers shift slightly relative to one another. But while these echoes may be annoying in a recording of classical music, they probably aren't important in a recording of a noisy courtroom. Unless I hear otherwise from someone reading this note, I wouldn't worry about unwinding and rewinding your tapes. The slight imperfections that will result from transfers between layers shouldn't affect their utility in later trials. Properly stored, I'd expect the tapes to outlive everyone involved with the trials, even without any unwinding and rewinding.

788. Please explain the concepts of magnetism pertaining to ferromagnetism, diamagnetism, and paramagnetism. - SC
A ferromagnetic material is one that contains intrinsic magnetic order. Iron, for example, is a ferromagnetic material—meaning that if you were to examine a microscopic region of the iron, you would find that it was highly magnetic. The magnetism in a ferromagnetic material is often hidden by a domain structure, in which microscopic magnetic regions or "domains" all point in random directions to give the material no apparent magnetism. Only when you expose the ferromagnetic material to a magnetic field does its magnetic character suddenly reveal itself. A ferromagnetic material becomes strongly magnetic when it's exposed to a magnetic field.

A diamagnetic material is one in which the electrons begin moving when it's place in a magnetic field. These moving electric charges create a second magnetic field that partially cancels the original field. A diamagnetic magnetic field partially shields itself from magnetism when it's exposed to a magnetic field.

A paramagnetic material is one in which individual magnetic electrons respond magnetically to any external magnetic field. It becomes weakly magnetic when it's exposed to a magnetic field. Unlike a ferromagnetic material, a paramagnetic material has no intrinsic magnetic order before it's exposed to an external field.

789. Is heating milk by microwave advisable? - I
Microwave cooking leaves no permanent mark on the food. It causes virtually no chemical damage and absolutely no radioactivity. The only drawback with heating milk by microwave is that the heating may be uneven and may denature some protein molecules in regions of the milk that become excessively hot. Since most protein molecules are disassembled by your digestion anyway, this treatment probably has no effects worth worrying about. Even with infant formula, my only concern would be the hot spots. If you carefully shake the milk after heating, so that its temperature is uniform, it should be just fine. I suspect that companies warn you not to heat milk in a microwave because they are worried that you will either not shake the milk to distribute its temperature evenly or that you will overcook it until it boils and the bottle explodes.

790. How do the 2" diagonal color LCD screens used in some of the new digital video cameras work? — M, Waynesboro, MS
Like most liquid crystal displays (LCD), these devices use liquid crystals to alter the polarization of light and determine how much of that light will emerge from each point on the display. Liquid crystals are large molecules that orient themselves spontaneously within a liquid—much the way toothpicks tend to orient themselves parallel to one another when you pour them into box. The liquid crystals used in an LCD display are sensitive to electric fields so that their orientations and their optical properties can be affected electronically. The liquid crystals in the display occupy a thin layer between transparent electrodes and two polarizing plastic sheets. Light from a fluorescent lamp passes through a polarizing sheet, an electrode, the liquid crystal layer, another electrode, and another polarizing sheet. The orientation of the liquid crystal determines whether light from the first polarizing sheet will be able to pass through the second polarizing sheet. When electric charges are placed on the two electrodes, the liquid crystal's orientation changes and so does light's ability to pass through the pair of polarizing sheets.

To create a full color image, the display has many rows of electrodes on each side of the liquid crystals and a pattern of colored filters added to the sandwich. In "active" displays, there are also thin-film transistors that aid in the placement of charges on the electrodes. Overall, the display is able to select the electric charges on each side of every spot or "pixel" on the screen and can thus control the brightness of every pixel.

791. What would things look like if I could see wavelengths of the spectrum other than just visible light (e.g., X-rays, radio waves, ultraviolet, infrared, gamma rays, etc.)? — SH, Hurricane, UT
As you looked around, you would see a general glow of radio waves, microwaves, and infrared light coming from every surface. That's because objects near room temperature emit thermal energy as these long-wavelength forms of light. While we don't normally see such thermal radiation unless an object is hot enough for some of it to be in the visible range, your new vision would allow you to see everything glow. The warmer an object is, the brighter its emission and the shorter the wavelengths of that emission. People would glow particularly brightly because of their warm skin.

You would also see special sources of radio waves, microwaves, and infrared light. Radio antennas, cellular telephones, and microwave communication dishes would be dazzlingly bright and infrared remote controls would light up when you pressed their buttons.

You would see ultraviolet light in sunlight and from the black lights in dance halls. But there wouldn't be much other ultraviolet light around to see, particularly indoors. X-rays and gamma rays would be rare and you might only see them if you walked into a hospital or a dentist's office. Gamma rays would be even rarer, visible mostly in hospitals.

792. Why do colors fade in the sun? - RD
While light travels as electromagnetic waves, it's emitted and absorbed as particles called "photons." Each photon carries with it a tiny bit of energy. The amount of energy in a photon depends on the wavelength of the light associated with it. While a photon of red light contains too little energy to cause chemical processes to occur in most molecules, a particle of violet or ultraviolet light contains enough energy to cause significant chemical damage to a typical molecule. Since sunlight contains a substantial amount of violet and ultraviolet lights, it can cause a fair amount of chemistry to occur in the molecules that absorb it. That's why colors often fade in sunlight. Many colored molecules are relatively fragile and are damaged by photons of ultraviolet light. The portion of a dye molecule that gives it its color is called a "chromophore" and is usually the most fragile part of the molecule. Destroying its chromophore will often leave a dye molecule colorless. Exposure to sunlight was the traditional way to bleach fabrics and make them white.

793. Radioactive elements' half-lives are fixed and they decay at a constant rate. Their decay rates have been determined thanks in part to our nuclear weapons research. Under what circumstances can a radioactive element have its decay rate changed? Can the element's radioactivity be destroyed (cancelled) by applying high temperatures? If so, how high would the temperature have to go to achieve this? — RD, Humble, TX
Since radioactivity is a feature of atomic nuclei, the only way to alter radioactivity is to alter atomic nuclei. But there aren't many ways to change atomic nuclei. Of various atomic and subatomic particles, only a neutron can enter a nucleus easily and cause it to rearrange. However, it's more common for a neutron to increase radioactivity than to destroy it, so that's not a good approach. Furthermore, the only practical way to obtain neutrons is with radioactivity.

Heating a collection of nuclei can cause them to collide and rearrange. However, this process is also fraught with problems. The products of the fusion and fission events that occur when nuclei collide will probably be radioactive themselves, so that it's unlikely that heating radioactive materials will make them less radioactive. Instead, it's likely that heating radioactive materials will make them more radioactive. Furthermore, the temperatures at which nuclei will begin to collide are extraordinarily high. Even the smallest nuclei repel one another fiercely so that they need temperatures of 100 million degrees C or more to begin colliding effectively. Larger nuclei, such as those common in nuclear wastes, won't collide until their temperatures exceed 1 billion degrees C. The only way to reach these temperatures is with nuclear weapons and they certainly don't reduce the radioactivity of nearby materials. In short, the only way to get rid of radioactivity is by waiting patiently.

794. Can you explain how the telephone wiring in my home works for the telephone? My touch-tone phone has 4 wires, but I understand that only 2 wires are used. Does the phone use the other 2 wires for the light on the phone pad, etc.? — DS, Larkspur, CA
Your telephone performs all of its functions using only those 2 wires. The 2 extra wires are virtually never used by a single-line telephone. The only exception that I'm aware of is the old "Princess Telephone," which had a special light powered by the extra pair of wires. In most telephones, even the power for the lighted keys comes from the 2 main wires. While the telephone is off the hook, the telephone company sends a constant DC current through those two wires. This current powers the telephone's electronics and its lights. When you talk, the microphone causes the telephone's electric impedance to fluctuate up and down and this variation causes sound to be reproduced in your friend's earpiece. Pressing the dialing buttons causes similar fluctuations in impedance and the telephone company uses these tones to make the proper connections. When the telephone company rings your telephone, they send a higher voltage AC current through the two wires and the telephone's bell rings.

795. How does an infrared sensor faucet work? — DD, Sacramento, CA
The sensor has two lenses: one that emits a beam of infrared light and the other that looks for a reflection of that light. As long as there is nothing beneath the faucet, there is very little infrared light reflected back toward the sensor and the sensor prevents any water from flowing out of the faucet. But when you hold your hands under the faucet, the infrared light reflects from your hands and some of it returns to the sensor. The sensor detects this light and opens an electronic valve to permit water to flow out of the faucet. The lenses are aimed so that only objects under the faucet itself will reflect the infrared light back toward the lens. A more distance object may reflect some of the infrared light, but the light won't pass through the sensor at the proper angle and won't be detected.

796. How fast is the earth moving through space? Does this movement affect our perception of time? — GR, Grabil, IN
Because there is no preferred reference frame for the universe, we can only talk about the earth's speed in reference to other objects. For example, the earth is moving at about 5 kilometers per second relative to the sun and about 30,000 kilometers per second relative to the center of the galaxy. These speeds do affect our perceptions of time, so that times passes at a different rate for us than for someone closer to the sun or to the galactic center. However, gravitational wells also affect the perception of time, so that the effects are complicated. The earth is also receding extremely rapidly from objects at the far side of the universe; so fast that time passage is dramatically affected. Those distant objects appear to be aging very slowly and their light is shifted substantially toward the red.

797. If energy is always conserved, why does a pendulum eventually stop swinging if you leave it alone?
The pendulum experiences friction and air resistance, both of which extract energy from the pendulum. Friction turns that energy into thermal energy and air resistance transfers the energy to the air.

798. If there were no friction or air resistance, would the bowling ball pendulum continue in motion forever?
Yes. If the pendulum had no way to convert its energy into thermal energy (e.g., via friction) and no way to transfer that energy elsewhere (e.g., via air resistance), it would continue to swing forever. While its energy would transform from gravitational potential energy (at the ends of each swing) to kinetic energy (at the middle of each swing) and back again, over and over, the total amount of energy it has won't change.

799. In class, you sat motionless on a cart with a ball in your lap. You said that your momentum was zero. You then threw the ball in one direction and you began moving in the other direction. You said that your momentum was still zero. How can your momentum be zero if you are moving?
In both cases, I was referring to the total momentum of the ball and me. The total momentum of the ball and me was zero before I threw the ball and it was still zero after I threw the ball. However, before I threw the ball nothing was moving and after I threw the ball the two of us were moving in opposite directions. It was our total momentum that was zero after the throw, not our individual momenta. While the ball and I each had a nonzero momentum after the throw, our momenta were equal in amount but opposite in direction—the ball's momentum was exactly opposite mine. If you were to add our momenta together, they would sum to zero. Since momentum is conserved and we couldn't exchange momentum with anything around us, the ball and I began and ended with the same total amount of momentum: zero.

800. Piling sandbags in the back of a truck would increase friction between the wheels and the ground, but wouldn't it also increase the truck's inertia, making it harder to stop on an icy road?
Adding sandbags to the back of a pickup truck increases the truck's traction and adds to the truck's mass. Fortunately, the truck's traction increases more dramatically than its mass and it becomes easier to start and stop the truck, rather than the reverse. That's because even a modest amount of sand can double the force pressing the rear wheels against the road and thus double the frictional forces the wheels can experience. That same amount of sand won't double the total mass of the truck.

801. How do fletchings stabilize an arrow in flight after it is shot from a bow? — SH, Newton, TX
Like all isolated objects, the arrow naturally pivots about its own center of mass, a point located near its geometric center. If the arrow had no fletchings (or fins) it would tend to rotate wildly in flight. But the fletchings experience substantial aerodynamic forces whenever the arrow isn't flying point first and these aerodynamic forces twist the arrow back toward its proper orientation. Thus whenever the arrow begins to rotate so that its point isn't first, the air pushes hard on the fletchings and returns the arrow to its point-first orientation. The same effect keeps airplanes and birds flying nose (or beak) forward.

802. I have a thermometer made of a column of fluid containing seven spheres of fluid that rise and fall according to the temperature (commonly known as a Galileo thermometer). How does this work? — LS, Conroe, TX
A Galileo thermometer combines Archimedes' principle with the fact that liquids generally expand faster with increasing temperature than solids do. Each sphere in the thermometer has an average density (a mass divided by volume) that is very close to that of the fluid in the thermometer. As stated in Archimedes' principle, if the sphere's average density is less than that of the fluid, the sphere floats and if the sphere's average density is more than that of the fluid, it sinks. But the fluid's density changes relatively quickly with temperature, becoming less with each additional degree. Thus as the temperature of the thermometer rises, the spheres have more and more trouble floating. Each sphere's density is carefully adjusted so that it begins to sink as soon as the thermometer's temperature exceeds a certain value. At that value, the expanding fluid's density becomes less than the average density of the sphere and the sphere no longer floats. The spheres also expand with increasing temperature, but not as much as the fluid.

Here is a picture of a combined Galileo thermometer and simple barometer. In addition to measuring the temperature with floating spheres, this device measures the outside air pressure with a column of dark liquid. It has a trapped volume of air that pushes the liquid (visible at the bottom of the unit) up a vertical pipe when the outside air pressure drops. The owner of this unit would like to know its history and origin, so if you have any information about it, please let me know.

803. At times a very thin invisible layer of ice forms on road surfaces. The road surface appears dry and does not have the telltale reflections of ice. Many people refer to this as "black ice." How is this ice formed? What are the crystal properties that make it invisible? - BK
Black ice is a layer of ice that is almost free of internal defects or air bubbles and that does not have a smooth surface. The absence of internal defects or air bubbles is what makes it transparent rather than white. Snow and crushed ice appear white because they contain countless tiny surfaces. Whenever light changes speed, as it does in going from ice to air or air to ice, some of that light reflects. Since snow and crushed ice contain many ice/air interfaces, they reflect light extensively and appear white. In contrast, black ice contains no internal ice/air interfaces and doesn't reflect any light from inside. Any light that makes it into the black ice goes all the way to the roadway. If the roadway reflects any of this light, it again passes unscathed through the black ice. The only evidence that the black ice exists at all comes from its surface, but here again the ice offers little that you can see. Since true black ice is microscopically rough, the small amount of light that reflects as it enters the ice from the air is reflected randomly in all directions. So little of that reflected light travels in any one direction that you can barely see it at all. Overall, black ice reflects so little light that you see only the roadway itself. While I am not sure, I think that it forms when moisture in the air condenses to dew on the roadway and then freezes into ice. Whatever process forms it must leave it almost without holes and therefore invisible.

804. How do stalactites and stalagmites form in caves? — GS, Conroe, TX
They form when various minerals come out of solution in water and crystallize on the surfaces of a cave. To understand how this process occurs, we must look at the interface between the water and the cave surface. Whenever water is in contact with a mineral surface, there is a chance that an atom of the surface will suddenly leave the surface and dissolve in the water. If there are atoms already dissolved in the water, there is also a chance that one of them will suddenly come out of solution in the water and attach to the surface. Atoms leave and return to cave surfaces all the time as water drips from the ceiling of a cave to its floor.

What is important for the growth of stalactites and stalagmites is that more atoms stick to the cave surfaces than leave those surfaces. That is exactly what happens and it does so because the water has already picked up more than enough dissolved atoms before it reaches the stalactite. Either because of temperature changes or because of evaporation, the water that runs across the cave roof and down the sides of a stalactite deposits more atoms on the stalactite's surface than it removes. The same goes for the stalagmite after the water drips down to the cave floor. As the atoms build up on the cave surfaces, the stalactites grow down and the stalagmites grow up.

805. How does reverse osmosis work? - MC
Normal osmosis in water is a process in which pure water flows through a semi-permeable membrane to dilute a concentrated solution on the other side. It is driven by statistics—it's much more likely for a water molecule on the fresh water side to pass through the membrane than it is for a water molecule on the concentrated solution side to pass through the membrane. There are simply more water molecules trying to cross the membrane from the fresh water side! In fact, water molecules will continue to flow from the fresh water side to the concentrated solution side until the solution has been highly diluted or an accumulation of pressure on the solution side slows the passage of water and brings it to a halt.

Reverse osmosis occurs when the pressure on the solution side is raised so high that the movement of water reverses directions. If you squeeze the concentrated solution hard enough, you can drive additional water molecules from that solution through the semi-permeable membrane and into the fresh water on the other side. The raised pressure on the solution changes the statistics, making it more likely for water molecules to go from the solution side to the fresh water side. This technique is used to purify water in homes and to desalinate water in desert countries.

806. In steam generation, wouldn't it be more economical to heat a small boiler and feed it just enough water for it to maintain its optimal steam generating temperature than to heat a huge boiler as is normally done? — MF, Gillette, WY
Not really. Once you have heated the water to its steam generating temperature, all of the heat you add goes into converting water into steam. The presence of more or less water just doesn't make any difference. The extra water requires no extra heat while the boiler is making steam. And having that extra water does act as a buffer in case you add too much or too little heat for a short while. That's probably why most boilers have a bit more water than they need over any short period of time. Furthermore, it's not always easy to add water to a boiler when the boiler's pressure is very high.

807. How does a magnetic train work? How can I make an experiment with it for a school project? — AASE, Quito, Ecuador
There are many techniques for supporting a train on magnetic forces, but the simplest and most promising involves electrodynamic levitation. In this technique, the train has a strong magnet under it and it rides on an aluminum track. The train leaves the station on rubber wheels and then begins to fly on a cushion of magnetic forces when its speed is high enough. Its moving magnet induces electric currents in the aluminum track and these currents are themselves magnetic. The train and track repel one another so strongly with magnetic forces that the train hovers tens of centimeters above the track.

To demonstration this effect, you can lower a very strong magnet above a rapidly spinning aluminum disk. In my class, I spin a sturdy aluminum disk with a motor and lower a 5 cm diameter disk magnet onto its surface. I hold the magnet firmly with a strap made of duct tape, so that the magnet won't fly across the room or flip over as it descends. Instead of touching the spinning disk, the magnet floats about 2 cm above it. If you try this experiment, don't spin the aluminum disk too fast or it will tear itself apart. It should spin about as fast as an electric fan on high speed. Also, be careful with the magnet, because it will experience magnetic drag forces as well as the magnetic lift force. If you don't hold tight, it will be yanked out of your hand.

For a simpler experiment that anyone can do, float an aluminum pie plate in a basin of water and circle one pole of a strong magnet just above its surface. The pie plate will begin to spin with the magnet. You are again inducing currents in the aluminum, making it magnetic. While the forces here are too weak to lift the magnet in your hand, they are enough to cause the pie plate to begin spinning, even though you never actually touch it. This technique is used in many electric motors. That's physics for you—the same principles just keep showing up in seemingly different machines.

808. How does a light switch work? — AB, Tulsa, OK
A light switch controls the flow of electricity through a circuit—a complete, unbroken loop through which electric charges can move. When the light switch is on, these electric charges can move in an endless loop. This loop starts with a trip to the power company—actually to the power transformer near your home—where the charges pick up electric energy. They then flow through wires to the light switch, then to the light bulb where they deliver their electric energy, and finally back to the power company to obtain more energy. The same charges complete this loop over and over again. The loop is called a circuit.

But when you turn off the light switch, you open or break the circuit. One of the wires connecting the power company to the light bulb suddenly has a gap in it and the current of electric charges can no longer flow. The switch itself actually contains two separated wires and a mechanical device that connects them only when the switch is in its on position. The precise structure of the mechanical switching device differs from switch to switch, but the behavior is always the same: the switch disconnects the two wires—and thus breaks the circuit—whenever you turn the switch off.

809. How does an electronic dimmer work? I know that a regular household dimmer works through resistance coils, but I read that electronic dimmers actually clip the A.C. cycle. Is this why you read the voltage output of an electronic dimmer the voltage remains the same even when it is dimmed down? Why can electronic dimmers dim fluorescents and arc lamps, but resistive dimmers cause those lamps to flicker? — KG, New York, NY
Electronic dimmers do clip the AC cycle. They use transistor-like devices called triacs to switch on the current to a lamp part way into each half-cycle. By shortening the time that power is delivered to the lamp, the dimmer reduces the total energy delivered to the lamp during each half-cycle and the lamp dims. But while a triac turns on easily, the only way to turn it off is to get rid of any voltage drop across it. The dimmer uses the alternating current itself to turn off the triac—the voltage of the power line naturally goes to zero at the end of each half-cycle and the triac turns off. The triac then waits until the dimmer restarts it, sometime into the next half-cycle.

Since the dimmer messes up the waveform of the electric current flowing through the lamp circuit, what you measure with a voltage meter depends on how that meter works. Since many AC voltmeters just measure peak voltage and assume that they are looking at a pure sinusoidal current, they don't give you an accurate sense for what is really happening to the voltage of this clipped waveform as a function of time. Unless an electronic dimmer is turned way down, the peak voltage it delivers will be close to the normal power line peak, a fact which tricks the voltage meter into reading a high value and which allows a properly designed fluorescent lamp to continue operating normally but at a dimmer level.

810. How much water power do you need to turn on a light bulb? How much wind power does it take to turn on a light bulb? Can artificial light make a solar paneled car run? If so, how bright? — BB, Stafford Springs, CT
If you are trying to light a 60 watt bulb, you must deliver 60 watts of electric power to it (unless you are willing to have it glow relatively dimly). So the answers to your questions are 60 watts of waterpower and 60 watts of windpower. But you are probably more interested in how much water or wind is needed to run those power sources. An efficient water generator that produces 60 watts of power lowers about 6 liters (or one and a half gallons) of water about 1 meter (or 3 feet) each second. An efficient wind generator that produces 60 watts of power stops about 1 cubic meter (or 32 cubic feet) of air moving at 36 km/h (or 21 mph) each second. Finally, a solar powered vehicle needs at least several hundred watts of power to operate. Since solar panels are only about 20% energy efficient and artificial light sources are also only about 10 to 50% energy efficient, it would take thousands of watts of artificial lighting to operate a solar powered car. Not very practical.

811. I have to do an experiment for school on the electromagnetic properties of iron, steel, and aluminum. The only problem is that I am not too sure what I should be testing. Any ideas? — CP, Nassau, Bahamas
Iron and steel (not stainless) are ferromagnetic metals, meaning that they are intrinsically magnetic. While this magnetism is normally hidden by the formation of millions of tiny, randomly oriented magnetic domains, it becomes apparent when you hold a magnet near the iron or steel: they are attracted! Aluminum has no intrinsic magnetism and is not attracted to a magnet. There are far more non-magnetic metals than magnetic ones. Why don't you try to see which metals will stick to a magnet. Only the ferromagnetic ones will. Even common stainless steel is non-ferromagnetic.

812. What is analog? I hear about digital audio being better than analog, but nobody defines what analog is. — DG, Houston, TX
In analog audio, the air pressure fluctuations of sound at the microphone are represented by a continuously variable physical quantity such as an electric current, a voltage, or a magnetization. Thus as the air pressure at a tape recorder's microphone rises during one moment of a song, an electric current in the recorder will rise and a region of a magnetic tape surface will become particularly strongly magnetized in a particular direction. Overall, each value of air pressure is converted to a particular value of the physical quantity.

The problem with analog recording is that when the sound is recreated, any defect in the physical quantity representing air pressure will lead to an imperfection in the reproduced sound. For example, if the magnetization of the recording tape has changed slightly due to how it was stored, the sound that the tape recorder produces won't be exactly the same as the sound that the microphone heard. Digital recording avoids this problem by recording the information as bits. The physical quantity such as magnetization is representing bits (which take only two possible values) rather than the air pressure itself (which can take a broad range of values). Minor changes in the physical quantity representing these bits won't change the bits. Thus imperfections in the recording or playback process won't affect the sound quality.

813. If I want to create a radio controlled device, how do I make sure it does not create interference with other devices or receive interference. How does digital RF work and does it stop interference problems? — KG, New York, NY
Radio interference occurs whenever two nearby radio transmitters are simultaneously emitting radio waves that overlap in space and frequency. The receivers for these two waves can't tell them apart and end up receiving both at once. This interference is familiar with AM radio, where you can sometime hear two broadcasts at the same time. With FM radio, the receivers are clever enough to distinguish one radio wave from another, but they can't determine which broadcast they're supposed to follow. Instead, they lock onto whichever wave is strongest and will often flip back and forth from one station to the other as their signal strengths fluctuate.

The only way to avoid interference completely is to choose a radio frequency that no one else nearby is using. That way your transmission is certain to be stronger than any other at the same frequency and your receiver will follow only your broadcast. If you have no choice but to share a particular frequency, then you must use some encoding scheme such as digital transmission so that your receiver can tell when it's receiving a broadcast from your transmitter and not from some other transmitter. Your receiver looks for your personal encoding scheme and won't respond to that of some other transmitter. However, if that other transmitter is strong enough, it will probably prevent your receiver from detecting your transmission. That trick of overwhelming a receiver with a second transmission is the principle behind jamming of a radio transmission.

814. How can I check the magnetron in a home microwave oven? I have checked the HV (high voltage) transformer, the rectifier, and capacitor and all are OK. Does the magnetron output decrease with age? The oven has a hum that is much louder than normal. — AA, Ontario, CA
While I have only a little experience repairing microwave ovens, I can make reasonable guesses. The loud hum you hear is probably an indication that something is overloading the power transformer. That suggests that the diode, capacitor, or magnetron are bad. If you have checked the first two carefully, at full operating voltage, and found no problems, then I would suspect the magnetron. I have been told by a reader that magnetrons usually fail by shorting out, the result of electromigration of the filament material. The tube would then draw excessive currents from the high voltage transformer. That has probably happened in your case. Still, free advice like mine is only worth what you've paid for it. I'd suggest you consult a local repairperson, who has test equipment that can pinpoint the problem in seconds.

815. How does a dishwasher machine work? — WW, Bochum, Germany
A dishwasher is really a number of simple machines that work together to clean dishes. These machines are controlled by a mechanical or electronic timer and include an electrically operated water valve, a water level sensor, one or two water pumps, a thermostat, an electric heating element, one or more rotating spray nozzles, and a fan.

The cycle begins when the timer sends electric current through a coil of wire in the water valve, making that coil magnetic and pulling the water valve into its open position. Water flows then flows from the high pressure in the water line to the atmospheric pressure in the cleaning chamber. When the water sensor detects that the dishwasher is adequately filled, it shuts off current to the valve and the valve closes.

The thermostat measures the water temperature and may delay the start of the cycle if the water is too cool. If so, it directs electric current through the heating element, where that current's energy is converted into thermal energy and transferred to the water. When the water is hot enough, the cycle continues.

During the cleaning cycle, one or more pumps operate. They add energy to the water and increase its pressure. This high-pressure water flows slowly to the rotating nozzles and then accelerates to high speeds as it enters the narrow openings and sprays out into the low-pressure cleaning chamber. As the high-speed water collides with the dishes and slows down, its pressure rises again and begins to exert substantial forces on the food particles. The food particles are pushed off the dishes and fall into the bottom of the dishwasher. Soap added to the cleaning water forms tiny spherical objects called micelles that trap and carry away fats that would otherwise not mix with water. At the end of the cycle, the water, food particles, and fat-filled soap micelles are pumped down the drain.

The cleaning cycle may repeat with fresh water and is then followed by a rinse. A soap-like surfactant may be added to the rinse water to lower its surface tension and prevent it from beading up on the dishes. When the pumps have removed the last of the rinse water, a fan begins to blow air over the dishes. The heating element may heat this air to assist evaporation. The water molecules leave the surfaces of the dishes and become gaseous water vapor. The dishes are left clean and dry.

816. How does a rotary phone switching system distinguish between the off-hook signal and the dialing signals, one through ten? - B
It doesn't. When you dial a rotary phone, it briefly hangs itself up one time for every number on the dial. Thus if you dial a "5", it hangs itself up briefly 5 times. In fact, you can dial the phone by tapping the switchhook briefly one time for every number. For example, if you want to dial a "5", tap the switchhook (hang up the phone) briefly 5 times very quickly. It takes some skill, but you can "dial" just fine without ever touching the dial. It used to be that people installed key locks on the rotary dial to prevent unauthorized use of the telephone. Unfortunately, this action didn't prevent someone with a nimble hand from dialing with the switchhook.

817. When I read of scientists discovering galaxies "on the edge of the universe," perhaps 15 billion light years away, I wonder if they are including the distance the objects must have traveled in the time it took for the light to reach their telescopes. Very distant objects are said to be receding from any other point in space at a higher rate than closer objects. If a galaxy is discovered 15 billion light years away today, the light left that galaxy 15 billion years ago while receding at a high rate. Where is it today, really? Twice as far away? — DK, Missouri City, TX
This seemingly simple question has a surprisingly complicated answer. You might expect that if the earth and one of these distant galaxies had been very near one another at the creation of the universe and had both been moving away from one another at almost the speed of light, that after 15 billion years each would have moved almost 15 billion light years in opposite directions and would thus be separated by almost 30 billion light years. That's not the case. That simple view ignores the important effects of special relativity on rapidly moving objects.

To understand these effects, suppose that there was an observer who was stationary at the creation and watched the earth and galaxy head off in opposite directions at almost the speed of light. From that observer's perspective, the two objects are heading away from one another at almost twice the speed of light. After 15 billion years, this observer sees the galaxy as almost 30 billion light years away from the earth.

Now suppose that there was another observer who was on the earth at the creation. From this person's perspective, the galaxy recedes from the earth at almost the speed of light, but no more. Nothing can move faster than speed of light! After 15 billion years, this observer sees galaxy as almost 15 billion light years away from the earth.

These two observations don't seem to agree. The problem lies in how the two observers perceive time and space. According to special relativity, observers who are moving relative to one another don't perceive time and space in the same way. Their perceptions will be so different that they will not even agree about just when 15 billion years has passed.

With this long introduction, here is the answer to your question: no distant galaxy in the observable universe can ever be farther from us than the distance light has traveled since the creation of the universe. Since that creation was about 15 billion years ago, the most distant possible galaxy is almost 15 billion light years away.

818. Why doesn't a helium balloon pop when it reaches the ceiling?
The buoyant force lifts the helium balloon upward—the denser air flows downward to fill the space vacated as the balloon is squeezed upward. When the balloon finally reaches the ceiling, the ceiling exerts a downward force on the balloon and prevents it from rising further. But the force the ceiling exerts on the balloon's skin is gentle enough and spread out enough that it doesn't injure the rubber. The balloon simply comes to a stop and remains suspended until enough helium diffuses out of the balloon to cause it to descend.

819. How does air pressure affect the distance a soccer ball can be kicked? — SR, Pittsburgh, PA
In general, the greater the air pressure, the greater the air resistance. As the soccer ball moves through the air, the air in front of it experiences a rise in air pressure and pushes the ball in the direction opposite its motion. While there are various other changes in air pressure around the ball's surface, this rising pressure in front of the ball remains largely unbalanced and it slows the ball down. The higher the air pressure was to start with, the greater its rise in front of the ball and the stronger the backward push of air resistance. Thus if you were to play soccer in the Rocky Mountains, where the air pressure is much less, you'd be able to kick the ball significantly farther.

820. How does a heat lamp work and could it be harmful to the eyes of pets from extended exposure? — DM, Osceola, IA
A heat lamp is much like a normal incandescent lamp, except that the heat lamp's large filament operates at a much lower temperature. Because of this lower temperature, the filament emits relatively little visible light. Instead, it emits mostly invisible infrared light. While you can't see infrared light, you can feel it as heat. Looking at a heat lamp is no more dangerous than looking at the glowing coals in a fireplace. Their thermal radiation heats your skin and the surfaces of your eyes, and is likely to make you uncomfortable enough to turn away before it causes real damage. In contrast, ultraviolet light from a sunlamp can injure your skin and eyes without causing any immediate pain—it's only much later that you feel the sunburn on your skin and corneas. That's why a heat lamp is relatively safe while a sunlamp is not.

821. I have read recently that achieving absolute zero is impossible. Why is this the case? What will happen to objects at this temperature (i.e., solid, liquid, and gas)? — BC, Ottawa, Ontario
Absolute zero can't be reached for the same reason that any perfect order is impossible. It's just too unlikely to ever happen. For an object to reach absolute zero, every single bit of thermal energy and every aspect of disorder must leave the object. If the object is a crystalline material, then its crystal structure must become absolutely perfect. This sort of perfection is essentially impossible. Reducing the temperature of an object towards absolute zero requires great effort and ends up creating a great disorder elsewhere. The closer the approach to absolute zero, the more disorder is created elsewhere. To reach absolute zero, you'd have to create infinite disorder elsewhere. For something to think about, imagine trying to make you lawn absolute perfect. The more perfect you tried to make it, the more gardeners you'd need and the more food, money, and services would be consumed. The lawn would grow more and more perfect but everything else would grow more disordered. And still you would never have a truly perfect lawn.

822. Can I soften small quantities of tap water by merely adding table salt to it? Any idea how much salt to add for tape water that is medium to very hard? I want enough to use in a steam iron regularly? — HD, Kintnersville, PA
There are two issues here. First, hard water is water that contains dissolved calcium, magnesium, and iron salts. The metal ions in these salts interfere with soaps and detergents, causing soaps to form soap scum and preventing detergents from effectively carrying away fats and oils. The standard way to soften water is to exchange sodium ions for the calcium, magnesium, and iron ions because sodium ions don't have such bad effects on soaps and detergents. Adding salt to hard water, as you propose to do, won't exchange sodium ions for the other ions. It will only add more metal ions to the water and the water will remain hard.

Second, a steam iron shouldn't use hard water because when hard water boils away as steam, it leaves behind all the calcium, magnesium, and iron salts as unsightly scale. Again, adding salt to your hard water will simply leave more scale on the insides of your iron or on your clothes. You need demineralized water, not soft water, for your iron. The best way to demineralize water is to distill it.

823. How long will the magnetic data last on a VCR tape before it becomes no longer useable as read data? — KR, Urbana, IL
As long as the tape is kept cool and dry, its magnetization should remain stable for years. However, there is the problem of magnetic imprinting from one layer of tape to the adjacent layers on a spool. With time, one layer transfers some of its magnetization to those adjacent layers. In a videotape, this imprinting leads to a gradual appearance of noise in the video images. As long as you're willing to tolerate a little video "snow," this imprinting shouldn't be too much of a problem. You can reduce its severity by occasionally winding and rewinding the tapes. But I don't see any real reason why a tape won't be reasonably useable for decades.

824. I am interested in finding out if and what materials affect magnetic fields. — HLD, Jacksonville, FL
Magnetic fields are associated with lines of magnetic flux, invisible structures that stretch between north and south magnetic poles or that curve around on themselves to form complete loops. Unless a material has its own north or south magnetic poles, it can't terminate the magnetic flux lines and can have only small effects on magnetic fields. The few materials that do affect magnetic fields substantially are ones such as iron or steel that are intrinsically magnetic and that can easily develop strong north and south magnetic poles. These magnetic materials can significantly shift the paths of the magnetic flux lines. If you put an iron or steel box in a magnetic field, the flux lines will tend to travel through the walls of the magnetic box. As a result, there will be few magnetic flux lines inside the box and almost no magnetic field. This effect is used to shield sensitive equipment such as the picture tubes in televisions from magnetic fields.

825. Don't microwaves penetrate metal at all? — DR, Tampa, FL
If the metal is a good conductor, then the microwaves don't penetrate more than a fraction of a millimeter. That's because the microwave electric fields push on the metal's mobile electrons and those electrons immediately rearrange in such a way that they cancel the microwave fields inside the metal. Only the skin of the metal responds to the fields and it shields the rest of the metal from the microwaves.

826. How can we clean the microwave oven? - PTW
Since the cooking chamber of a microwave oven doesn't get hot, there is no way to make a "self-cleaning" microwave oven. Instead, you have to clean it by hand with a sponge and perhaps a little soapy water. As long as you get the soap or any other cleaning agents out, you can clean the cooking chamber just as you'd clean the top of a stove.

827. If the condenser in a microwave is bad, what is the most likely reaction the microwave generator will exhibit? — IF, Bakersfield, CA
According to a reader, most microwave oven capacitors have fuses in them so that when they fail, they usually become open (they lose all of their ability to store separated charge and behave as a simple open circuit). You'd need a capacitor checker to find this open circuit within the capacitor.

828. Is it possible to eat a microwave while you eat food that was cooked in the microwave oven? - PTW
Not one that came from the microwave oven. Microwaves are all around us and are completely innocuous. Your body emits weak microwaves all the time, as part of its thermal radiation! Like light, microwaves don't remain still in objects so you can't eat one that was put in the food by the oven.

829. Where is the best place to put a microwave oven? Is it dangerous to place it on the refrigerator? - PTW
You can put a microwave oven anywhere that it's stable and where it has adequate ventilation. A microwave oven has a fan and vents through which it gets rid of its excess heat. You mustn't block the vents or the oven will overheat.

830. How does ultrasound detect cracks or imperfections in metal? Is this to do with density or is it just reflecting off surfaces? — PA, Essex, UK
Like all waves, ultrasound reflects whenever it passes from one material to another and experiences a change in speed (or more accurately, a change in impedance). Any inhomogeneity in a metal is likely to change the speed of sound in that metal and will cause some amount of sound reflection. With the proper instruments emitting sound and detecting the reflected sound, it's possible to image the imperfections. The same technique is used in medical ultrasound to image organs or fetuses, and even to image the insides of the earth.

831. How do Oven Cooking Bags work? I know they are made of heat resistant nylon resin, but can you explain what that means? — HY, Halifax, Nova Scotia
There are two broad classes of plastics: (1) thermoplastics that can melt, at least in principle, and (2) thermosets that can't melt under any circumstances. Thermoplastics consist of very long but separable molecules and common thermoplastics include polyethylene (milk containers), polystyrene (Styrofoam cups), Nylon (hosiery), and cellulose (cotton and wood fiber). Thermosets consist of very long molecules that have been permanently cross-linked to one another to form one giant molecule. Common thermosets include cross-linked alpha-helix protein (hair) and vulcanized rubber (car tires).

Most common plastic items are made from thermoplastics because these meltable plastics can reshaped easily. But different thermoplastics melt at different temperatures, depending on how strongly their long molecules cling to one another. The plastic in an Oven Cooking Bag is almost certainly a thermoplastic form of Nylon, but one that melts at such a high temperature that it doesn't change shape in the oven. It's possible that the Nylon has been cross-linked to form a thermoset, so that it can't melt at all, but I wouldn't expect this to be the case.

832. How can we measure magnetic fields or magnetic potentials of solvent atoms that reside interstitially inside solid solutes? — DR, Tampa, FL
You can measure the magnetic fields in which certain atoms reside with the help of nuclear magnetic resonance (NMR). This technique examines the magnetic environment of the atom's nucleus by determining how much energy it takes to change the orientation of the nucleus. Since the nucleus is itself magnetic, it tends to align with any magnetic field—like a compass. The stronger that magnetic field, the harder it is to flip the nucleus into the wrong direction.

833. What happens when matter and anti-matter collide? Do they just destroy each other? I thought that matter couldn't be created or destroyed? - S
As Einstein's famous formula points out, mass and energy are equivalent in many respects. In most situations, mass is conserved and so is energy. But at the deepest level, it's actually the sum of those two quantities that's conserved. When matter and anti-matter collide, they often annihilate one another and their mass/energy is converted into other forms. For example, when an electron and an anti-electron (a positron) collide, they can annihilate to produce two or more photons of light. There is no fundamental law that prevents matter from being created or destroyed but there is a fundamental law that mass/energy must be conserved. In this case, the masses of the electron and positron become energy in the massless photons. Overall, mass/energy has been conserved but what was originally mass has become energy. The fact that when matter and anti-matter annihilate, the product is usually energy, makes this mixture attractive as a possible super-rocket fuel. But don't hold your breath; anti-matter is incredibly difficult to make or store.

834. How does the light emission of Wint-O-Green Lifesavers work? If you bite them, they give off light, but what are the chemicals involved and how does it work? — KA, Davis, CA
This phenomenon is the result of tiny electric sparks that occur when sucrose crystals in the Lifesaver crack as they are exposed to severe stresses. A separation of electric charge occurs between the two sides of the fracture tip and an electric discharge occurs through the air separating those two sides. The light that you see is produced by this electric discharge.

To understand how this charge separation occurs, we must look at how crystals respond to stress. Many crystalline materials are microscopically asymmetric, meaning that their molecules form orderly arrangements that aren't entirely symmetric. To visualize such an arrangement, consider a collection of shoes: an orderly arrangement of left shoes can't be symmetric because a left shoe isn't its own mirror image—you can't built a fully symmetric system out of asymmetric pieces. Like left shoes, sucrose molecules (the molecules in table sugar) are asymmetric so that a crystal of sucrose is also asymmetric.

Whenever you squeeze a crystal, exposing it to stress, its electric charges rearrange somewhat. In a symmetric crystal, this microscopic rearrangement doesn't have any overall consequences. But in an asymmetric crystal such as sucrose, the microscopic rearrangement can produce a large overall rearrangement of electric charges and huge voltages can appear between different parts of the crystal. The most familiar such case is in the spark lighters for gas grills, where a stressed asymmetric crystal creates large sparks. In a Wint-O-Green Lifesaver, the large build-ups of charge cause small sparks that produce the light you see.

835. If the Fermi level is the highest energy level used by an electron, how can electrons shift to conduction levels that are at energies above the Fermi level? — PH
The Fermi level is the highest energy level occupied when all the electrons have as little energy as possible. That situation occurs only when all the electrons are paired two to a level and the levels are filled all the way from the lowest energy level up to the Fermi level. At any reasonable temperature and in the presence of light or other energy sources, some of the electrons will have been shifted out of their normal levels and into levels above the Fermi level. The Fermi level doesn't change when these shifts occur—it's defined before the electrons shift.

836. How do you calculate the change in water pressure as the diameter of the hose changes? - JH
When water flows through a hose, it has three main forms for its energy: kinetic energy, gravitational potential energy, and an energy associated with its pressure—which I'll call pressure potential energy. Since energy is conserved, the water's energy can't change as it flows through the hose (we'll ignore frictional forces here, although they really are pretty important in a hose). Let's assume that the hose is horizontal, so that the water's gravitational potential energy can't change. When the water enters a narrowing in the hose, the water must speed up to avoid delaying the water behind it. This increase in speed is associated with an increase in kinetic energy. Since the water's energy can't change, the increase in kinetic energy must be accompanied by a decrease in pressure. If the water then enters a widening in the hose, it slows down, its kinetic energy drops, and its pressure rises to conserve energy! If the hose then rises upward, so that the water's gravitational potential energy rises, the water's pressure must drop to conserve energy. In general, one form of energy can become another but the sum of those three forms can't change.

837. How does 240-volt electricity work in house wiring? If each "hot" wire in a circuit from the central wiring panel is at 120 volts with respect to neutral/ground, how are devices that use 240 volts wired? — GK, Ottawa, Ontario
Most homes receive power through three wires: two power wires and one neutral wire. Each power wire is at 120 volts AC with respect to the neutral wire, meaning that its electric potential fluctuates up and down with respect to the neutral wire and behaves as though, on average, it were 120 volts away from the potential of the neutral wire. But the fluctuations of the two power wires are opposite one another—when one power wire is at a positive voltage relative to the neutral wire, the other power wire is at a negative voltage relative to the neutral wire. If you compare the two power wires to one another, you'll find that they behave as though, on average, they are 240 volts away from one another. Thus home appliances that need 240 volts are powered by the two power wires, rather than one power wire and one neutral wire.

838. What is a kVA? Can you convert watts to kVA? - M
kVA is the product of kilovolts (kV) times amperes (A) and is a measure of power. In fact, if you multiply the voltage in volts delivered to an electric heater by the current in amperes sent through that heater, you will obtain the electric power in watts consumed by the heater. Thus the heater's power consumption in watts is the same as the product of its voltage times its current, or its kVA. However, there are many devices that don't behave like an electric heater. The heater is purely resistive, while many other devices such as motors are both resistive and reactive. Reactive devices don't obey Ohm's law and may not draw their peak currents at times of peak voltage. Therefore, the power in watts consumed by a reactive device isn't the same as the product of its current times its voltage, or its kVA.

839. What is the difference between a single-phase electric motor and a three phase motor? Does that make one of them more efficient, better, or longer lasting than the other? — EJ, Houston, TX
To keep the center component or "rotor" of an electric motor spinning, the magnetic poles of the electromagnets surrounding the rotor must rotate around it. That way, the rotor will be perpetually chasing the rotating magnetic poles. With single-phase electric power, producing that rotating magnetic environment isn't easy. Many single-phase motors use capacitors to provide time-delayed electric power to some of their electromagnets. These electromagnets then produce magnetic poles that turn on and off at times that are delayed relative to the poles of the other electromagnets. The result is magnetic poles that seem to rotate around the rotor and that start it turning. While the capacitor is often unnecessary once the rotor has reached its normal operating speed, the starting process is clearly rather complicated in a single phase motor.

In a three phase motor, the complicated time structure of the currents flowing through the three power wires makes it easy to produce the required rotating magnetic environment. With the electromagnets surrounding the rotor powered by three-phase electricity, the motor turns easily and without any starting capacitor. In general, three phase motors start more easily and are somewhat more energy efficient during operation than single phase motors.

840. How do remote garage door openers work? — JD, Greenville, SC
The communication from the remote to the opener is done with radio waves. When you push the button on the remote, it produces a brief burst of radio waves at a specific frequency and with a selected pattern of pulses. A radio receiver in the opener is continuously looking for a transmission at that same frequency and with that same pattern of pulses. While other garage door openers may use radio waves of the same frequency, it's extremely unlikely that they will make use of the same pattern of pulses. This pattern of pulses is the security code that prevents unauthorized opening of your garage door. These security codes have grown longer and more sophisticated over the years. Early garage door openers had no security code at all and could be opened by almost any radio transmission at the right frequency. You could drive around neighborhoods with a remote and open garage doors right and left. But now the security codes are complicated enough that opening someone else's garage door is almost impossible.

841. How does a fax machine send written words over telephone wires? — AM, Halifax, CA
The fax machine uses a row of optical sensors to detect dark and light spots on the original document. It scans the document one line at a time and enters the pattern of dark and light spots into a digital controller or simple computer. The controller or computer than encodes this pattern, together with enough information to correct minor transmission errors if they occur, as a series of numbers. The numbers are then sent through the telephone system in much the same way that computer information is sent through the telephone wires by a modem. The numbers becomes specific patterns of tones and volumes. While the electric currents flowing through the telephone system are meant to represent voice sounds, they can do a moderately good job of representing numbers instead. Because of various limitations on the currents that the phone wires can carry well, the fax system can only so much information each second. The receiving fax machine analyzes the tones and volumes it receives over the telephone wires and recreates the pattern of dark and light spots. It then uses one of several printing techniques to reproduce that pattern on a piece of paper. It recreates the document one line at a time.

842. You mentioned that time perception is different for different locations in the universe. Were could we find a place where one day is equal to one thousand years of time on earth? — AWG, Karachi, Pakistan
The perception of time is different for observers who are in motion relative to one another. The issue is not how far away they, it's how fast they are moving relative to one another. If you were to observe a person who is traveling past the earth at almost the speed of light, you would notice that their watch is running extremely slowly. It might be as though you'd have to wait one thousand years for their watch to show that a day has passed for them. Yet paradoxically, they would make the same observation about you! You would see them aging slowly and they would see you aging slowly! The resolution to this apparent paradox lies in the differences in the perceptions of space that these differences in the perceptions of time. In this short answer, I can hardly begin to resolve the paradox. I'll simply point out that the mixing of space and time associated relativity are caused by relative motion not by relative position.

843. How do balloon pilots navigate around countries that forbid overflights? — LS, Ashland, OR
The speeds and directions of the winds vary considerably with altitude. For example, while surface winds near the sea blow toward shore on a hot summer day, high-altitude winds blow away from the shore at the same time, completing a huge circulation loop. Unlike a sailboat, which is at the mercy of the surface winds, a balloonist can adjust the balloon's altitude to search for winds heading in the desired direction. The balloonist makes these altitude adjustments by changing the balloon's weight and volume so that it sinks or rises.

844. How does desiccant absorb and hold water? — JP, Houston, TX
Water molecules from the air are continuously colliding with surfaces and sometimes one of those water molecules will stay attached to a surface for some amount of time. That water molecule forms a weak chemical bond with the surface and remains there until thermal energy knocks it back into the air. As a result of this occasional sticking, most surfaces have a thin layer of water molecules on them. Desiccants are materials that tend to keep those water molecules for a relatively long time and that have lots of surface area on which those water molecules can stick. However, the strongest desiccants react chemically with water molecules so that those water molecules essentially never leave.

845. What effect does ice have on potholes? - AH
Water and ice are major contributors to potholes. When water flows into cracks in the road and then freezes, it tears the roadway apart. That's because ice takes up more room than the water from which it's formed—ice is less dense than water. Since the water expands as it freezes, it enlarges the cracks that contain it and gradually breaks up the roadway.

846. Do regular magnets lose their magnetism or do they stay magnetized always? What about electric magnets, like the ones used in wrecking yards? — KM, Delta, British Columbia
Permanent magnets are made from materials with two important magnetic characteristics. First, these materials are intrinsically magnetic, meaning that some of the electrons in these materials retain their natural magnetism. While electrons are always magnetic, that magnetism is lost in most materials because of complete cancellations—each magnetic electron is paired with another magnetic electron so that they cancel one another perfectly. However, there are some materials in which the cancellation is imperfect and these materials (including iron, cobalt, nickel, and many steels) are the basis for most permanent magnets.

Second, the materials used in permanent magnets have internal structures that make the magnetic electrons align along particular directions. Once the electrons are aligned along one of those directions, they stay aligned and the material exhibits strong magnetic characteristics. It becomes a "permanent magnet."

A permanent magnet remains its magnetization as long as nothing spoils the alignments of its magnetic electrons. These electrons can be knocked out of alignment by vibrations, heat, or other magnets. If you hit a permanent magnet with a hammer or heat it in the oven, you will change and perhaps destroy its magnetization. This magnetization can be recovered by exposing the permanent magnet to the magnetic influences of an electric current. In fact, permanent magnets are originally magnetized by placing them near electric currents that align their magnetic electrons. Moreover, even a material that doesn't have the internal structures needed to keep its electrons aligned along a particular direction will become magnetized temporarily by placing it near an electric current. That's how a wrecking yard magnet works-an electric current temporarily turns a large piece of iron into a strong magnet.

847. Magnets stick to metal, but can you make a magnet repel metal? - M
Yes, but not in the way you're thinking of. When you bring a magnet near a piece of steel, the intrinsic magnetic character of that steel causes it to become magnetic in such a way that it attracts the magnet. There is no way for the steel, or another similar metal, to become magnetic in such a way that it would repel the magnet.

However, if the metal is already magnetized it can repel an approaching magnet. A more interesting case is when a magnet approaches a normally non-magnetic metal at high speeds; in which case electric currents begin to flow through the metal and these currents do repel the approaching magnet.

848. How does a crystal radio work?
A crystal radio uses a crystal diode to detect tiny fluctuating currents in its antenna system. When a radio wave passes across an antenna, the wave's electric field pushes electric charges up and down the antenna. The crystal diode acts as a one-way gate that allows some of this moving charge to flow onto another wire and then prevents it from returning to the antenna. Since the charge can't return to the antenna, it flows elsewhere—passing through a sensitive earphone and creating sound. An AM radio station encodes sound as changes in the intensity (or amplitude) of the radio wave. As the radio wave's intensity fluctuates, the amount of electric charge flowing through the earpiece of the crystal radio also fluctuates and you hear sound.

849. In cooking, what are some examples of absorbing microwaves, transferring microwaves, and reflecting microwaves? - K
In a microwave oven, water-containing foods absorbs microwaves. The microwaves disappear as they pass through the food and the food becomes hotter. Microwaves are transferred from the small antenna near the magnetron to the cooking chamber by sending those microwaves through a metal pipe. This rectangular pipe is typically a few inches wide and an inch or so tall, and is called a "wave guide." Finally, the walls of the cooking chamber reflect the microwaves. When a microwave encounters a metal surface, it pushes electric charges back and forth in the metal and this moving charge causes the microwave to reflect.

850. In your explanation of why microwaves don't penetrate the oven door, you said it is because the holes in the screen are smaller than the wavelength of a microwave. Wouldn't it be the amplitude of the wave and not its wavelength? - P
When a microwave tries to pass through the holes in the metal screen, electric charges in that screen begin to move. The microwave's electric field fluctuates back and forth rapidly and the charges reverse directions rapidly as a result. If the electric current made up of these charges has enough time to travel all the way around each hole before it reverses directions, it will be as though the screen were made of solid metal and the screen will be able to completely reflect the microwave.

Like any electromagnetic wave, a microwave has a wavelength (the spatial distance between adjacent wave crests) and a period (the temporal spacing between adjacent wave crests). The electric current that a microwave propels through a metal travels about one microwave wavelength during one microwave period. Therefore, the current can work its way around a hole in the metal only if the hole is significantly smaller than the microwave wavelength. The amplitude of the microwave doesn't matter—increasing the amplitude of the microwave just makes more current flow.

851. Does blowing on or waving a developing Polaroid picture actually speed up its development process? — PS, Columbus, OH
The speed of the development process is determined by the diffusion of molecules within the developing film and by the rates at which they react with one another. Both processes, diffusion and chemical reactions are temperature sensitive. If blowing on or waving the film manages to increase its temperature, then diffusion and reactions will both speed up and the development time will decrease.

852. I have read about how black holes can emit X-rays and radiation. If they absorb light, why do they emit these other things? — BA, Fairbury, IL
A black hole is surrounded by an imaginary surface called the event horizon. Nothing at all can escape from within this surface-not light, not X-rays...nothing! However, as matter falls into the black hole, and before it reaches the event horizon, the matter can emit any type of radiation it likes. The X-rays and radiation emitted "from a black hole" are actually coming from the area surrounding the event horizon, not from within that surface. As matter pours into a black hole, it often heats up so hot that it emits incredible amounts of radiation of all types so that black holes appear as very bright objects.

853. Is the jet stream flowing in the same direction in the southern hemisphere as it is in the northern hemisphere? — LS, Ashland, OR
The jet streams flow eastward in both hemispheres. Their directions of flow are determined by the Coriolis effect, in which high-altitude winds that are heading away from the equator veer eastward because of their angular momentum on the spinning earth.

854. How do you demagnetize a magnet?
A permanent magnet was magnetized when it was first made out of metal. It did have microscopic regions of magnetic order—magnetic domains—but those regions all pointed in