How Everything Works
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MLA Citation: Bloomfield, Louis A. "How Everything Works" How Everything Works 21 Jan 2018. Page 22 of 160. 21 Jan 2018 <http://www.howeverythingworks.org/prints.php?topic=all&page=22>.
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.

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