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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?
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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.

499. How are tessellations used in roofing, tiles, and quilts?
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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?
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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?
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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?
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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).

505. Are divining rods and their abilities to locate ground water fact or myth?
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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.

509. How does a UPC scanner work?
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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.

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
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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).

548. Can a compound have triple bonds? If so, please give an example. — BA, IL
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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?
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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.

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