How Everything Works
How Everything Works How Everything Works

Site Map
At UVa:
How Everything Works  
Page 131 of 160 (1595 Questions and Answers)

1301. How does a halogen bulb work and is it really better than a regular bulb?
A halogen bulb uses a chemical trick to prolong the life of its filament. In a regular bulb, the filament slowly thins as tungsten atoms evaporate from the white-hot surface. These lost atoms are carried upward by the inert gases inside the bulb and gradually darken the bulb's upper surface. In a halogen bulb, the gases surrounding the filament are chemically active and don't just deposit the lost atoms at the top of the bulb. Instead, they react with those tungsten atoms to form volatile compounds. These compounds float around inside the bulb until they collide with the filament again. The extreme heat of the filament then breaks the compounds apart and the tungsten atoms stick to the filament.

This tungsten recycling process dramatically slows the filament's decay. Although the filament gradually develops thin spots that eventually cause it to fail, the filament can operate at a higher temperature and still last two or three times as long as the filament of a regular bulb. The hotter filament of a halogen bulb emits relatively more blue light and relatively less infrared light than a regular bulb, giving it a whiter appearance and making it more energy efficient.

1302. If the thermometer works on the concept of liquids expanding when heated, how can the glass not expand as well. I mean, the glass expands, maybe the thermometer gets longer, or the hole in the middle where the liquid is, gets smaller or larger or something but the glass must also expand, so why does the thermometer work or does it? — RP, Hotchkiss, Colorado
You're right about the glass expanding along with the liquid inside it. But liquids normally expand more than solids as their temperatures increase. That's because the atoms and molecules in a liquid have more freedom to move around than those in a solid and they respond to increasing temperatures by forming less and less tightly packed arrangements. Since the liquid in a thermometer expands more than the glass container around it, the liquid level rises as the thermometer's temperature increases.

1303. What is the force produced when two cars crash? — DT, Nicosia, Cyprus
There are two forces present when the cars collide: each car pushes on the other car so each car experiences a separate force. As for the strength of these two forces, all I can say is that they are exactly equal in amount but opposite in direction. That relationship between the forces is Newton's third law of motion, the law dealing with action and reaction. In accordance with this law of motion, no matter how big or small the cars are, they will always exert equal but oppositely directed forces on one another.

The amount of each force is determined by how fast the cars approach one another before they hit and by how stiff their surfaces and frames are. If the cars are approaching rapidly and are extremely stiff and rigid, they will exert enormous forces on one another when they collide and will do so for a very short period of time. During that time, the cars will accelerate violently and their velocities will change radically. If you happened to be in one of the cars, you would also accelerate violently in response to severe forces and would find the experience highly unpleasant.

If, on the other hand, the cars are soft and squishy, they will exert much weaker forces on another and they will accelerate much more gently for a long period of time. That will be true even if they were approaching one another rapidly before impact. When the collision period is over, the cars will again have changed velocities significantly but the weaker forces will have made those changes much more gradual. If you have to be in a collision, chose the soft squishy cars over the stiff ones—the accelerations and forces are much weaker and less injurious. That's why cars have crumple zones and airbags: they are trying to act squishy so that you don't get hurt as much.

1304. I understand that light waves cause electrically charged particles in matter to vibrate so that these particles can absorb and reemit light, even in transparent materials. But doesn't that explanation contradict quantum theory, which states that only specific photons corresponding to allowed electronic transitions can be absorbed? — GS, Akron, OH
When a light wave passes through matter, the charged particles in that matter do respond—the light wave contains an electric field that pushes on electrically charged particles. But how a particular charged particle responds to the light wave depends on the frequency of the light wave and on the quantum states available to the charged particle. While the charged particle will begin to vibrate back and forth at the light wave's frequency and will begin to take energy from the light wave, the charged particle can only retain this energy permanently if doing so will promote it to another permanent quantum state. Since light energy comes in discrete quanta known as photons and the energy of a photon depends on the light's frequency, it's quite possible that the charged particle will be unable to absorb the light permanently. In that case, the charged particle will soon reemit the light.

In effect, the charged particle "plays" with the photon of light, trying to see if it can absorb that photon. As it plays, the charged particle begins to shift into a new quantum state—a "virtual" state. This virtual state may or may not be permanently allowed. If it is, it's called a real state and the charged particle may remain in it indefinitely. In that case, the charged particle can truly absorb the photon and may never reemit it at all. But if the virtual state turns out not to be a permanently allowed quantum state, the charged particle can't remain in it long and must quickly return to its original state. In doing so, this charged particle reemits the photon it was playing with. The closer the photon is to one that it can absorb permanently, meaning the closer the virtual quantum state is to one of the real quantum states, the longer the charged particle can play with the photon before recognizing that it must give the photon up.

A colored material is one in which the charged particles can permanently absorb certain photons of visible light. Because this material only absorbs certain photons of light, it separates the components of white light and gives that material a colored appearance.

A transparent material is one in which the charged particles can't permanently absorb any photons of visible light. While these charged particles all try to absorb the visible light photons, they find that there are no permanent quantum states available to them when they do. Instead, they play with the photons briefly and then let them continue on their way. This playing process slows the light down. In general blue light slows down more than red light in a transparent material because blue light photons contain more energy than red light photons. The charged particles in the transparent material do have real permanent states available to them, but to reach those states, the charged particles would have to absorb high-energy photons of ultraviolet light. While blue photons don't have as much energy as ultraviolet photons, they have more energy than red photons do. As a result, the charged particles in a transparent material can play with a blue photon longer than they can play with a red photon—the virtual state produced by a blue photon is closer to the real states than is the virtual state produced by a red photon. Because of this effect, the speed at which blue light passes through a transparent material is significantly less than the speed at which red light passes through that material.

Finally, about quantum states: you can think of the real states of one of these charged particles the way you think about the possible pitches of a guitar string. While you can jiggle the guitar string back and forth at any frequency you like with your fingers, it will only vibrate naturally at certain specific frequencies. You can hear these frequencies by plucking the string. If you whistle at the string and choose one of these specific frequencies for your pitch, you can set the string vibrating. In effect, the string is absorbing the sound wave from your whistle. But if you whistle at some other frequency, the string will only play briefly with your sound wave and then send it on its way. The string playing with your sound waves is just like a charged particle in a transparent material playing with a light wave. The physics of these two situations is remarkably similar.

1305. I have read that very old panes of glass become thicker at the bottoms than the tops. Doesn't that show that glass flows? — MJ
While it is sometimes noted that old cathedral glass is now thicker at the bottom than at the top, such cases appear to be the result of how the glass was made, not of flow. Medieval glass was made by blowing a giant glass bubble on the end of a blowpipe or "punty" and this bubble was cut open at the end and spun into a huge disk. When the disk cooled, it was cut off the punty and diced into windowpanes. These panes naturally varied in thickness because of the stretching that occurred while spinning the bubble into a disk. Evidently, the panes were usually put in thick end down.

Modern studies of glass show that below the glass transition temperature, which is well above room temperature, molecular rearrangement effectively vanishes altogether. The glass stops behaving like a viscous liquid and becomes a solid. Its heat capacity and other characteristics are consistent with its being a solid as well.

1306. The earth's surface is moving at something like 950 mph as it rotates. Why don't we notice this when we are in an airplane? — DT, Nicosia, Cyprus
It's true that the earth's surface is moving eastward rapidly relative to the earth's center of mass. However, that motion is very difficult to detect. When you are standing on the ground, you move with it and so does everything around you, including the air. While you are actually traveling around in a huge circle once a day, for all practical purposes we can imagine that you are traveling eastward in a straight line at a constant speed of 950 mph relative to the earth's center of mass. Ignoring the slight curvature of your motion, you are in what is known as an inertial frame of reference, meaning a viewpoint that is not accelerating but is simply coasting steadily through space.

You'll notice that I keep saying "relative to the earth's center of mass" when I discuss motion. I do that because there is no special "absolute" frame of reference. Any inertial frame is as good as any other frame and your current inertial frame is just as good as anyone else's. In fact, you are quite justified in declaring that your frame of reference is stationary and that everyone else's frames of reference are moving. After all, you don't detect any motion around you so why not declare that your frame is officially stationary. Since the air is also stationary in that frame of reference, flying about in the air doesn't make things any more complicated. You are flying through stationary air in your old stationary frame of reference. The only way in which the 950 mph speed appears now is in comparing your frame of reference to the rest of the earth: in your frame of reference, the earth's center of mass is moving westward at 950 mph.

1307. What holds the atoms in a molecule together?
The atoms in a molecule are usually held together by the sharing or exchange of some of their electrons. When two atoms share a pair of electrons, they form a covalent bond that lowers the overall energy of the atoms and sticks the atoms together. About half of this energy reduction comes from an increase in the negatively charged electron density between the atoms' positively charged nuclei and about half comes from a quantum mechanical effect—giving the two electrons more room to move gives them longer wavelengths and lowers their kinetic energies.

When two atoms exchange an electron, they form an ionic bond that again lowers the overall energy of the atoms and sticks them together. Although moving the electron from one atom to the other requires some energy, the two atomic ions that are formed by the transfer have opposite charges and attract one another strongly. The reduction in energy that accompanies their attraction can easily exceed the energy needed to transfer the electron so that the two atoms become permanently stuck to one another.

1308. Why is it that when you stand in front of a flat mirror, your image is reversed horizontally (left-right) but remains the same vertically (up-down)? — CC, Martinsville, NJ
A mirror doesn't really flip your image horizontally or vertically. After all, the image of your head is still on top and the image of your left hand is still on the left. What the mirror does flip is which way your image is facing. For example, if you were facing north, then your image is facing south. This front-back reversal makes your image fundamentally different from you in the same way a left shoe is fundamentally different from a right shoe. No matter how you arrange those two shoes, they'll always be reversed in one direction. Similarly, no matter how you arrange yourself and your image, they'll always be reversed in one direction.

While you're looking at your image, the reversed direction is the forward-backward direction. But it's natural to imagine yourself in the place of your image. To do this you imagine turning around to face in the direction that your image is facing. When you turn in this manner, you mentally eliminate the forward-backward reversal but introduce a new reversal in its place: a left-right reversal. If you were to imagine standing on your head instead, you would still eliminate the forward-backward reversal but would now introduce an up-down reversal. Since it's hard to imagine standing on your head in order to face in the direction your image is facing, you tend to think only about turning around. It's this imagined turning around that leads you to say that your image is reversed horizontally.

1309. I enjoy watching the pole-vaulters at the Olympics, especially Daly Thompson. Could you explain the physics of the pole vault for me? — ZG, Bullcreek, West Australia
The pole vault is all about energy and energy storage. Lifting a person upward takes energy because there is an energy associated with altitude—gravitational potential energy. Lifting a person 5 or 6 meters upward takes a considerable amount of energy and that energy has to come from somewhere. In the case of a pole-vaulter, most of the lifting energy comes from the pole. But the pole also had to get the energy from somewhere and that somewhere is the vaulter himself. Here is the story as it unfolds:

When the pole-vaulter stands ready to begin his jump, he is motionless on the ground and he has no kinetic energy (energy of motion), minimal gravitational potential energy (energy of height), and no elastic energy in his pole. All he has is chemical potential energy in his body, energy that he got by eating food. Now he begins to run down the path toward the jump. As he does so, he converts chemical potential energy into kinetic energy. By the time he plants his pole at the jump, his kinetic energy is quite large.

But once he plants the pole, the pole begins to bend. As it does, he slows down and his kinetic energy is partially transferred to the pole, where it becomes elastic potential energy. The pole then begins to lift the vaulter upward, returning its stored energy to him as gravitational potential energy. By the time the vaulter clears the bar, 5 or 6 meters above the ground, almost all of the energy in the situation is in the form of gravitational potential energy. The vaulter has only just enough kinetic energy to carry him past the bar before he falls. On his way down, his gravitational potential energy becomes kinetic energy and he hits the pit at high speed. The pit's padding extracts his kinetic energy from him gently and converts that energy into thermal energy. This thermal energy then floats off into the air as heat.

One interesting point about jumping technique involves body shape. The vaulter bends his body as he passes over the bar so that his average height (his center of gravity) never actually gets above the bar. Since his gravitational potential energy depends on his average height, rather than the height of his highest part, this technique allows him to use less overall energy to clear the bar.

1310. I'm grateful for your work and the availability of your site. Though I think that your ignorant condemnation of the work of other professionals about whose work you know absolutely nothing is contemptuous. Once again the arrogance of the established order and refusal to open-minded investigation. I would not have this opinion if you had used the careful, open-minded, systematic investigation that you espouse before you let your ego expose your ignorance. Carolyne Myss, with a verifiable accuracy rate of 93% percent, should not be called a quack. I wonder if, in your answers on this site, you could attain that rate of accuracy. I sincerely doubt it. In fact, with your over-blown ego, you could really benefit from her work. So stick to the information about lightning and CDs and stay away from that which you obviously are quite ignorant! — Unsigned
This comment, which responds to a previous posting on this site, points out one of the most important differences between physical science and pseudo-science: the fact that pseudo-science isn't troubled by its lack of self-consistency.

Physical science, particularly physics itself, is completely self-consistent. By that I mean that the same set of physical rules applies to every possible situation in the universe and that this set of rules never leads to paradoxical results. Despite its complicated behavior, the universe is orderly and predictable. It's precisely this order and predictability that is the basis for the whole field of physics.

In contrast, pseudo-science is eclectic—it draws from physics and magic as it sees fit. It uses the laws of physics when it finds those laws useful and it ignores the laws of physics when they conflict with its interests. But the laws of physics only make sense if they apply universally—if there were even one situation in which a law of physics didn't apply, physics would lose its self-consistency and predictive power. That's just what happens with pseudo-science when it begins to ignore the laws of physics on occasion. Moreover, the new rules that pseudo-science introduces to replace the ones it ignores make the trouble even worse. Overall, pseudo-science is inconsistent and can't be counted on to predict anything.

Pseudo-science might argue that the laws of physics are correct as far as they go, but that they're incomplete. No doubt the laws of physics are incomplete; physicists have frequently discovered improvements to the laws of physics that have allowed them to make even more accurate predictions of the universe's behavior. But in the years since the discoveries of relativity and quantum physics, the pace of such discoveries has slowed and what remains to be understood is at a very deep and subtle level. It's extraordinarily unlikely that the laws of physics as they're currently understood are wrong at a level that would allow a person to bend a spoon with their thoughts alone or predict the order of a deck of cards without assistance. Just because I haven't dropped a particular book doesn't prevent me from predicting that it will fall when I let go of it. I understand the laws that govern its motion and I know that having it fly upward would violate those laws. Similarly, I don't have to watch someone try to bend a spoon with their thoughts to know that it can't be done legitimately. Again, I understand the laws that govern the spoon's condition and I know that having it bend without an identifiable force acting on it would violate those laws. I also don't have to watch someone try to predict cards to know that it, too, can't be done legitimately. Without a clear physical mechanism for transporting information from the cards to the person, a mechanism that must involve forces or exchanges of particles, there is no way for the person to predict the cards.
The How Everything Works Home Page
The Complete Collection of Questions (160 pages, from oldest to newest):
Previous 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 Next 
Copyright 1997-2018 © Louis A. Bloomfield, All Rights Reserved
Privacy Policy