How Everything Works
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MLA Citation: Bloomfield, Louis A. "How Everything Works" How Everything Works 19 Oct 2017. Page 121 of 160. 19 Oct 2017 <http://www.howeverythingworks.org/prints.php?topic=all&page=121>.
1201. How does lightning damage electrical appliances that are properly grounded and have their power switches in the off position? Doesn't that eliminate a path for the electricity? — RDU, Atlanta, Georgia
When lightning strikes a power line, it pours enormous amounts of electric charge onto that wire. These like charges repel one another and they quickly spread out all over the wire. If this wire enters your home, the charges traveling along it will flow into any appliance that's plugged in, whether it's turned on or not. But if the appliance is turned off, this charge will reach the open switch and it will come to a stop, at least temporarily.

What matters then is just how much charge enters the appliance. The open switch would normally block the passage of electricity, which is why the appliance doesn't operate while it's turned off. But as charge accumulates on one side of the switch, the voltage at that point rises higher and higher. When the voltage becomes high enough, as it easily does after a lightning strike, the charges can leap into the air and travel to the other side of the switch even though the two sides don't touch one another. Another view of this disaster is that the like charges on one side of the switch repel one another so vigorously that some of them are pushed through the air to the other side of the switch. As a result of this movement of charges through the air—an electric arc—current passes through the appliance as though it were turned on. If this current exceeds what the appliance can tolerate, the appliance will be destroyed. Even grounding the appliance may not help—charges can flow uncontrollably through the appliance and, while some charges take paths to ground, others flow through sensitive components and destroy them.


1202. You claim that the metal walls of the cooking chamber in a microwave oven protect us from the microwaves. How can they protect us from microwaves when they aren't even able to keep sound contained? You can hear popcorn popping through the walls. — RB, Beltsville, MD
The fact that sound waves can pass through the cooking chamber's metal walls doesn't mean that microwaves can. These two types of waves are very different and the chamber's walls handle them very differently.

Any type of wave will partially reflect from a surface if passing through that surface causes the wave's speed to change or, more generally, introduces a change in the "impedance" the wave experiences. Impedance is a quantity that relates various parts of a wave to one another—it relates pressure to velocity in sound and it relates the electric field to the magnetic field in a microwave. Since both sound waves and microwaves change speeds and impedances when they encounter the cooking chamber's metal walls, they both partially reflect. The sound that you hear when popcorn pops inside the oven is slightly muffled because the sound is having some trouble escaping from the cooking chamber. However, the impedance change for the microwaves is so enormous that the reflection is complete. No microwaves at all escape from the cooking chamber! The same effect occurs when you hold a large mirror up in front of your face. You can hear what's happening on the other side of the mirror because some sound can pass through the mirror. But light is completely reflected and you can't see through the mirror at all.


1203. Could the traffic flow on freeways be modeled as a one-dimensional gas? You can see waves of motion among the cars and these waves travel faster as the cars pack more tightly. — RH, Escondido, California
There are many similarities between the cars traveling on a freeway and the molecules in a gas. As you point out, disturbances at one point in the traffic cause ripples of motion to spread backward through the cars—similar to what happens in a gas. However, normal gas molecules only interact with one another when they actually touch, while cars interact at much larger distances—unlike gas molecules, cars don't do so well when they collide with one another. To avoid collisions, the drivers watch what's happening far ahead of them and react accordingly. In that sense, traffic's behavior resembles that of a non-neutral plasma—a gas of charged particles that all have the same electric charge and therefore repel one another even at large distances. If you were to send such a plasma through a narrow pipe, its particles would jostle back and forth as they tried to stay as far as possible from one another. Ripples of motion would pass through the plasma and this motion would be very similar to that of cars on a freeway.

1204. If you take a compressed metal spring and place it in a container of metal dissolving acid, what happens to the energy in the spring assuming the entire spring dissolves at one time? — BR, Mount Pleasant, SC
That energy becomes thermal energy in the metal/acid solution. Before the spring dissolves, the energy it stores is actually found in the forces between adjacent metal atoms. The crystals in the metal are slightly distorted, bringing the atoms in these crystals a little too close or a little too far from one another. Since each of these displaced atoms has a little extra potential energy, it is a little more chemically reactive than normal. When the acid attacks one of these atoms and pulls it away from the crystal, the atom comes away a little more easily than normal because it brings with it a little extra energy. This extra energy enters the solution, making the solution a little warmer than it would have become had the spring not been compressed.

1205. If a given volume of water is placed in a container and frozen, will it weigh more, less, or remain the same relative to when it was in a liquid state? — RL, Denver, Colorado
Freezing water has virtually no effect on its weight—as long as the same number of water molecules remain in the container, the overall weight of the container and water/ice won't change significantly. But water does expand as it freezes, so the container will become more full as the ice forms. Water's expansion upon freezing makes ice less dense—less mass per volume—than liquid water. This decrease in density explains why ice floats on water and why pipes often break as the water inside them freezes.

However, you'll notice that I said "freezing the water has virtually no effect on its weight." In reality, the water does lose a tiny fraction of its weight. That's because to freeze the water, you must remove some of the water's energy. As Einstein pointed out with his famous formula E=mc2, energy and mass are related to one another and since mass acquires weight when it's near the earth, so does energy. Because the thermal energy in liquid water has a tiny weight, when you remove some of this thermal energy from the water, the water loses some of its weight. But don't expect to measure this weight loss with a common scale—the weight change is on the order of one part in a trillion, a factor that's presently beyond the precision of even the most advanced research measuring devices.


1206. When a rear-wheel drive truck goes up a hill, do its rear wheels gain traction because of a transfer of weight to its rear wheels? I think it depends on the center of gravity, right? — DA, Issaquah, Washington
The traction a wheel experience depends largely on how hard it's being pushed into the roadway. When the truck is on level pavement, the roadway prevents the wheel from sinking into it by pushing upward on the wheel with a force called a support force. Because a wheel's traction is roughly proportional to the support force it's experiencing, the harder the wheel is pushed into the roadway, the more traction that wheel has.

Since a truck has its heavy engine in front, the front wheels bear more of its weight than the rear wheels and they experience more traction than the rear wheels. But as the truck tilts upward on the hill, the weight of its engine is born more and more by the rear wheels. In physics terms, the truck's center of gravity, which is almost over the front wheels while the truck is level, shifts to be more and more over the rear wheels as the truck tilts upward.

However, the extra weight that the rear wheels are supporting as the truck tilts doesn't improve their traction. That's because this extra weight isn't being supported entirely by support forces—much of it is being supported instead by friction between the rear wheels and the roadway. In fact, the support forces exerted by the roadway on the rear wheels to keep them from sinking into the pavement actually become weaker as the truck tilts uphill, so the truck loses traction as the tilt increases. Since traction is responsible for the friction that is also supporting the truck, the truck is in danger of slipping down the road. There is clearly a limit to how steep the roadway can get before the truck begins to slide.

1207. What was the difficulty in developing the blue LED? — JM, Hoboken, NJ
A light emitting diode (an LED) produces light when a current of electrons passes through the junction between its two pieces of semiconductor—from a n type semiconductor cathode to an p type semiconductor anode. The LED's light is actually produced in the anode when an electron that has just crossed the p-n junction and is orbiting a positively charged region (called a "hole") drops into the hole to fill it. In filling the hole, the electron releases energy and that energy becomes light through a process called fluorescence.

The energy in a particle of light (a photon) is related the color of that light—with blue photons having more energy than red photons. Here is where the difficulty in making blue LED's comes in: to produce a blue photon, the electron in an LED must give up lots of energy as it fills the hole in the anode. This need for a large energy release places a severe demand on the semiconductors from which the blue LED is made. These semiconductors need an unusually large band gap—the energy spacing between two types of paths that electrons can follow in the semiconductor. It wasn't until recently that good quality semiconductors with the appropriate electrical characteristics were available for this task.


1208. When I warm more than one cup of coffee or milk together in a microwave oven, some of them warm more than others. Why does this happen? Is there something wrong with our microwave oven? — ON, Istanbul, Turkey
When the microwaves bounce around inside the oven's cooking chamber, they experience an effect called interference. Interference occurs when similar waves, or portions of the same wave, follow different paths to the same region in space. As they pass through that region, their crests and troughs ride up on top of one another and they interfere. Sometimes the crests of one wave ride on the crests of the other wave, creating enormous crests—an effect called constructive interference. However, it is also possible for the crests of one wave to ride on the troughs of the other wave, so that they cancel one another out—an effect called destructive interference.

These interference effects are quite visible in wave waves, but they also make themselves apparent in microwaves. In your oven, they lead to regions of the cooking chamber that heat quickly (regions where the microwaves experience constructive interference) and regions that don't heat well at all (regions where they experience destructive interference). Because these fast and slow cooking regions can't be avoided, many microwave ovens incorporate turntables to keep the food moving through the various regions inside the oven. Some ovens use rotating metal paddles to stir that microwaves around inside the cooking chamber, so that the fast and slow cooking regions move about.

Your experience with uneven heating of coffee or milk is an example of this interference problem. The solution is to move the cups occasionally while they are being heated.


1209. How does gravity bend visible light? — AHM, Pasadena, California
According to the concept of inertia, established by Galileo and Newton several hundred years ago, an object that's not experiencing any pushes or pulls will continue to move in a straight line at a steady pace—in short, it travels at a constant velocity. This observation can also be stated simply as an object in motion continues in motion and an object at rest remains at rest.

When Newton formulated his theory of gravity, he viewed gravity as exerting forces on objects—it pulled them toward one another so that they no longer followed their straight inertial paths. That's why a ball arcs through the air, gradually turning toward the ground as the earth's gravity pulls it downward. This interpretation of gravity was very successful and remains extremely useful to this day.

However, there is a second interpretation of gravity: the one offered by Einstein in the general theory of relativity. According to this interpretation, concentrations of mass/energy warp space-time so that objects that are following inertial paths—called geodesics—no longer travel in simple straight lines. In effect, a ball arcs through the air because it is following a curved geodesic path and not because it is experiencing a force. While this exotic interpretation for gravity isn't all that useful for slow moving objects like balls—Newtonian gravity is much more practical in that case—it's important when dealing with fast moving objects like light. Light also follows geodesics, but because it travels so quickly its geodesics tend to be rather straight. Even light passing just above the surface of the sun bends only just enough to measure. Still, one of the most important confirmations of general relativity came during a total solar eclipse when light from a star was found to bend slightly as it passed by the sun's obscured surface.

Finally, I should say that you can also interpret the bending of light in terms of Newtonian gravity—that because light contains energy, it acquires a weight when gravity is present and this weight causes its path to bend. However, this Newtonian observation omits so much of the intrigue and beauty that comes with the bending of space-time that I prefer the more modern interpretation.


1210. We know that high speeds cause time to distort. We also have found wormholes in space that connect two distant points. Therefore, by entering a wormhole we can travel through time. How can we create a wormhole and control its destination point? — JB, Union, New Hampshire
Near some large concentration of mass/energy, the equations of general relativity do admit solutions that have two open ends and that could be interpreted as being wormholes. However, there is no widely accepted interpretation of these solutions and no evidence that such solutions are actually realized in our universe. While there are some physicists and astrophysicists who remain hopeful that wormholes will ultimately be found, the only ones I've ever heard about are in science fiction stories.

Even if such exotic structures do exist, there is also no evidence that people could traverse the severely distorted space-time between the two open ends without being destroyed and without having an infinite amount of time pass in the rest of the universe while they were en route. If all of these issues aren't enough to discourage you, let me add that the possibility of engineering wormholes to connect specific regions of space-time is extraordinarily remote. Working with a wormhole would be at least as difficult as working with a black hole and I, for one, hope never to encounter such a destructive and dangerous object.


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