How Everything Works
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Page 107 of 160 (1595 Questions and Answers)

1061. What makes a three-way touch lamp work? What makes a three-way light bulb work? - CY
A three-way touch lamp is much like a simple touch lamp—it detects your touch by applying a high frequency alternating charge to the lamp's surfaces and uses this fluctuating charge to measure the lamp's electric capacitance—the ease with which charge can moved on or off the lamp's surfaces. When you touch the lamp, the lamp's capacitance changes and the lamp's electronics detect this change.

In a three-way touch lamp, the lamp's electronics control 4 different light levels alternately: dim, medium, bright, and off. How these light levels are obtained depends on the lamp. If the lamp uses a three-way light bulb, which contains two separate filaments, then it can obtain the 3 brightness levels by turning on one or both of the filaments. It uses just the small filament for dim, just the large filament for medium, and both filaments for bright. That's exactly what a normal three-way lamp does.

But if the lamp uses a normal bulb and obtains three light levels from it, then it uses the same technique as a dimmer switch. In this technique, an electronic switching device called a triac is used to limit the times during which electric current can flow through the bulb and deliver power to it. In the bright setting, the triac permits current to flow through the bulb at all times and the bulb appears as bright as possible. But in the dim or medium settings, the triac prevents current from flowing at certain times. The triac takes advantage of the fact that the power flowing through a household lamp is alternating current—current that reverses directions 120 times a second (in the United States) for a total of 60 full cycles of reversal, over and back, each second (60 Hz). At the beginning of each current reversal, the electronic devices that control the triac start a timer. This timer allows those devices to wait a certain amount of time before they trigger the triac and allow it to begin carrying current to the light bulb. Once triggered, the triac will allow current to flow through the bulb until the next reversal of current in the power line. Thus the amount of energy that reaches the bulb during each half-cycle of the power line depends on how long the electronic devices wait before triggering the triac. The longer they wait, the less energy will reach the bulb and the dimmer it will glow. In the bright setting, the triac is triggered immediately after each current reversal so that power always flows to the bulb and it glows brightly. But in the medium and dim settings, the triac is triggered well into the half-cycle that follows the reversal. A normal dimmer gives you complete control over this delay, but a three-way touch switch only provides three preset delays. The medium setting has a medium delay while the dim setting has a long delay.

1062. Please explain ideal mechanical advantage and actual mechanical advantage. How can I demonstrate these two ideas? — S
Mechanical advantage is any process that allows you to exchange force for distance (or torque for angle) while performing a particular task. The amount of mechanical work you must do (i.e., the amount of energy you must supply) to perform that task won't change, but the relationship of force and distance (or torque and angle) will. For example, you can increase the altitude of a wooden block by 1 meter either by lifting it straight upward 1 meter or by pushing it several meters uphill along a ramp. In the first case, you'll have to exert a large upward force on the block but you won't have to move it very far to complete the task. In the second case, you'll have to exert a much smaller uphill force on the block but you'll have to move it a long way along the ramp. If you multiply the force you exert on the block times the distance that block travels while rising 1 meter, you'll find that it's exactly the same in either case. You've simply calculated the work required to raise the block 1 meter and that work won't change, regardless of how you perform the task! That's the crucial issue with mechanical advantage—it doesn't let you avoid doing the work, it just lets you do that work with a small (or larger) force exerted over a longer (or shorter) distance. In a situation involving rotation, mechanical advantage lets you do the same work with a smaller (or larger) torque exerted over a larger (or smaller) angle. In all of these cases, you're doing the same amount of work but you're making it more palatable by adjusting the balance between force and distance or between torque and angle.

As for actual mechanical advantage, it's simply a recognition that any mechanical system involves imperfections. The work that you do with the help of a machine doesn't all go toward your goal. Instead, you end up doing some work against sliding friction or air resistance and that work is lost to thermal energy. For example, when you slide a block up a ramp, friction with the ramp wastes some of your energy. If you multiply the uphill force you exert on the block while pushing it up the hill times the distance it travels along the ramp, you'll find that you must do somewhat more work while raising the block 1 meter than you would have done by simply lifting the block directly upward that 1 meter. So ideal mechanical advantage assumes no change in the work you do while actual mechanical advantage recognizes that you're going to end up doing extra work whenever you employ a machine to obtain mechanical advantage.

1063. What is a VU meter on tape deck? How does it differ from a dB meter? I know that the best recording is achieved when the needle hovers around the zero and that the sound distorts above zero and is barely audible the lower into the negative numbers you go, but what are the meanings of the plus and minus readings? — GF, California
VU and dB meters both measure the audio power involved in recording and they both use logarithmic scales to report that power. Because of these logarithmic scales, a factor of 10 increase in power produces an increase of 10 in both the VU reading and the dB reading. For example, -20 dB is 10 times the power of -30 dB. In both measures, the zero is chosen as the highest acceptable power—the highest power for which distortion is acceptable.

Where VU and dB differ is in how they measure audio power. VU is short for "volume units" and it is a measure of average audio power. A VU meter responds relatively slowly and considers the sound volume over a period of time. Its zero is set to the level at which there is 1% total harmonic distortion in the recorded signal. dB is short for "decibels" and it is a measure of instantaneous audio power. A dB meter responds very rapidly and considers the audio power at each instant. Its zero is set to the level at which there is 3% total harmonic distortion. Because of these differences in zero definitions, the dB meter's zero is roughly at the VU meter's +8. Nonetheless, both meters are important and both should be kept at or below zero to avoid significant distortion in a recording. In certain situations, such as when there are sudden loud sounds or with instruments that are very rich in harmonics, it's possible to have the dB meter read above zero even though the VU meter remains below zero.

1064. How can you make a hologram? — JM, Kettering, OH
The classic technique for making a hologram begins with splitting the light from a laser into two parts. Part of the laser light is used to illuminate a scene while the other part is used to illuminate a piece of film placed in front of the scene. Actually, the film is exposed to light from two sources: (1) the second part of the laser beam and (2) a portion of the first part of the laser beam that the objects reflect toward the film. Lights from these two sources don't simply add when they reach the film; they interfere with one another. Laser light is unusual in that it is coherent light—a giant wave consisting of numerous identical particles of light. When the wave from the laser and the wave reflected from the objects meet at the film, they interfere. When the crest of one wave joins the crest of the other wave, the two waves form an extra large crest—constructive interference. But when the crest of one wave joins the trough of the other wave, the two waves cancel and produce essentially nothing—destructive interference. Because of this interference, the film ends up recording not only the intensity information that we associate with normal photography; it also records phase information that is an important aspect of waves. This phase information indicates where crests and troughs in the wave occurred. Because the hologram contains both kinds of information, it allows a viewer to see things that they would not see in a simple photograph.

To make a hologram, you should take a laser and split its light into two unequal portions with the help of a laser beam-splitter (or even a glass slide). The laser should operate at only a single wavelength, so that its light is highly coherent, and it should have a coherence length much longer than any distance in the scene—two requirements that are met by most common continuous-wave lasers, including laser pointers and basic helium-neon lasers. Send the stronger portion of the laser beam through a diverging lens and allow it to illuminate a scene that is otherwise in complete darkness. Light reflected from this scene should reach the film holder in which the hologram will be made. Send the weaker portion of the laser beam through another diverging lens and allow it to illuminate the film holder from the scene side. For best results, the light reflected from the scene on the film holder should be about as bright as light from this second beam.

Now place fine-grained black and white film in the film holder. Be sure that the film is sensitive to the laser light—some black and white films aren't sensitive to red light. Allow light to strike the film for long enough to expose it. Finally, develop the film and observe the developed film while it's illuminated from behind with laser light that has been spread out by a diverging lens. You should see the original scene as a three-dimensional image.

Unfortunately, there is one detail I've omitted until now. To make sure that the phase information is properly recorded, you must be sure that nothing moves by even a fraction of a wavelength of laser light during the entire exposure period. That's a very demanding requirement. Vibrations are everywhere and they will spoil the hologram. If you want this technique to work, you'll have to isolate everything—the laser, the optics, the scene, and the film—from vibrations. In a laboratory, this vibration isolation is done by floating a massive optics table on a cushion of air. All of the objects involved in making the hologram are rigidly attached to this table so that they can't move. As an alternative, you can put all the objects for the hologram on as rigid and massive a surface as you can find and support that surface on a thick layer of foam rubber. Make the holograms at night when there is little traffic of any sort around and be sure that nothing is jiggling about nearby that might shake the floor even a little bit. If you're careful, you ought to be able to create a hologram with such an arrangement.

1065. How does a toilet work? — JJ, Stafford VA
A toilet is actually a very clever device that makes use of a siphon to extract the water from its bowl. A siphon is an inverted U-shaped pipe that can transfers water from a higher reservoir to a lower reservoir by lifting that water upward from the higher reservoir and then lowering it into the lower reservoir. In fact, the water is simply seeking its level, just as it would if you connected the two reservoirs with a pipe at their bottoms. In that case, the water in the higher reservoir would flow out of it and into the lower reservoir, propelled by the higher water pressure at the bottom of the higher reservoir. In the case of a siphon, it's still the higher water pressure in the higher reservoir that causes the water to flow toward the lower reservoir, but in the siphon the water must temporarily flow above the water levels in either reservoir on its way to the lower reservoir. The water is able to rise upward a short distance with the help of air pressure, which provides the temporary push needed to lift the water up and over to the lower reservoir. At the top of the siphon, there is a partial vacuum—a region of space with a pressure that's less than atmospheric pressure. The same kind of partial vacuum exists in a drinking straw when you suck on it and is what allows atmospheric pressure to push the beverage up toward your mouth.

In the toilet, the bowl is the higher reservoir and the sewer is the lower reservoir. The pipe that connects the bowl to the sewer rises once it leaves your view and then descends toward the sewer. Normally, that rising portion of the pipe isn't filled water—water only fills enough of the pipe to prevent sewer gases from flowing out into the room. As a result of this incomplete filling, the siphon doesn't transfer any water. But when you flush the toilet, a deluge of water from a storage tank rapidly fills the bowl and floods the siphon tube. The siphon then begins to function. It transfers water from the higher reservoir (the toilet bowl) to the lower reservoir (the sewer) and it doesn't stop until the bowl is basically empty. At that point, the siphon stops working because air enters the U-shaped tube with a familiar sound and water again accumulates in the bowl. When the storage tank has refilled with water, the toilet is ready for action again.

1066. How does wing shape affect flight?
During flight, an airplane wing obtains an upward lift force by making the air flowing over its top surface travel faster than air flowing under its bottom surface. When the air over its top speeds up, that air's pressure drops. Since the pressure of the slower moving air under the wing is larger than the pressure of the faster moving air over the wing, there is a net upward force on the wing due to this pressure imbalance and the wing is lifted upward. A wing also experiences drag forces—or air resistance—that tend to slow the plane down. But as long as an airplane wing doesn't cause the airstreams flowing around it to separate from its surface, it will experience relatively little pressure drag force; the most important drag force for a large, fast-moving object.

The details of the airplane wing's surfaces have relatively subtle affects on the wing's performance. While most wings are asymmetric, with broadly curved top surfaces and relatively flat bottom surfaces, that isn't essential. It's quite possible to use wings that are symmetric, with the same curvature on their tops as on their bottoms. But a symmetric wing won't obtain an upward lift force unless it's tilted upward, while an asymmetric wing can obtain lift even when it's horizontal. A broader, more highly curved wing can also obtain more lift at a lower speed, as required for slow moving propeller planes. So wing shapes are often dictated by the desired flight angle and speed of a particular airplane and its wings.

1067. How is infrared light produced?
There are many ways of producing infrared light. First, any warm surface emits infrared light. For example, a heat lamp or an electric space heater emits enormous amounts of it. That's because the thermal radiation of a warm object lies mostly in the invisible infrared portion of the electromagnetic spectrum.

Second, many light-emitting electronic devices emit infrared light. For example, the light emitting diodes in a television remote control unit emit infrared light. In this case, the infrared light is emitted by electrons that are shifting from one group of quantum levels in a semiconductor to another group—from conduction levels to valence levels. This emission isn't thermal radiation; it doesn't involve heat.

Lastly, some infrared light is produced by lasers. In this case, excited atoms or atomic-like systems amplify passing infrared light to produce enormous numbers of identical light particles—identical photons. Infrared industrial lasers are commonly used to machine everything from greeting cards to steel plates.

1068. How does a steam whistle work? — DR
As far as I know, a steam whistle is just a whistle that's blown by steam rather than air. The principle behind a whistle is straightforward: the air inside the whistle is driven into intense vibration by the stream of gas blown across a slot-shaped opening. This stream of gas is directed at the sharp edge on the far side of the opening and might or might not actually enter the whistle. If air happens to be flowing out of the slot-shaped opening as the stream flows across the slot, the outgoing air will deflect the stream outward and that stream won't enter the whistle. But if air happens to be flowing into the slot as the stream crosses the slot, the stream will be deflected into the whistle. This situation leads to an amplifying effect: if any air is flowing into the slot, the whole stream of gas will flow into the slot. If any air is flowing out of the slot, the whole stream of gas will flow out of the slot.

Now air inside the whistle is never perfectly still—it's always sloshing back and forth at least a tiny bit, much like water sloshes in a basin. As a result, there is always a little motion of air in or out of the slot. When the stream of gas begins to blow across the slot, it amplifies any tiny motions of air inside the whistle so that they become more and more vigorous. Soon the air inside the whistle is vibrating intensely and the resulting pressure fluctuations radiate outward from the whistle as sound.

This same principle is active in many other musical devices, including pipe organs and flutes. In a steam whistle, the stream of gas that drives this vibration is steam rather than air. Water is heated in a boiler until it forms moderately high-pressure steam and then the steam is released through a valve to a large whistle, which sounds loudly.

1069. How did the sniperscopes used in World War II work? They seem to have used an invisible light to illuminate the target and the sniper then looked through the scope and was able to see the target. — ND, Florence, Montana
These sniperscopes used infrared light to illuminate their targets and then detected this infrared light with the help of an infrared-sensitive photocathode. Producing infrared light is easy; any incandescent bulb produces large amounts of it. The sniperscope simply filtered out the visible light from an incandescent bulb, leaving only the invisible infrared light to illuminate the target.

Understanding the photocathode system requires an examination of the interactions of light and metal. Whenever a particle of light—a photon—strikes a metal surface, there is the possibility that the photon will eject an electron from that metal surface. However, each type of metal requires a certain minimum photon energy before it will release an electron. Because infrared light photons carry very little energy, they can only eject electrons from very special metals. The sniperscope contained a very thin layer of one such infrared-sensitive metal.

Actually, this metal layer was deposited on a transparent glass window that formed the front end of a vacuum tube. Light from the scene in front of the sniper passed through a converging lens that formed a real image of the scene on the metal layer. The metal layer was so thin that light striking its front surface through the glass window caused electrons to emerge from its back surface. Electrons ejected from the back of the metal layer were accelerated by a high voltage that was applied between this metal photocathode layer and a phosphor-coated anode layer located very nearby. Each electron acquired so much energy during its brief flight that it caused the phosphors on the anode to glow brightly when it hit them. The electron flight path was short so that electrons emitted by a certain spot on the photocathode would hit a corresponding spot on the phosphor anode and the sniper would see a clear image of the scene in front of the sniperscope.

Because one infrared photon striking the photocathode could lead to the release of dozens of photons from the phosphors on the anode, this sniperscope provided a modest amount of "image intensification." But modern starlight scopes go far beyond this level of amplification. Like the old sniperscope, these modern devices also use a photocathode to turn a pattern of light from the real image of a lens into a pattern of free electrons. But the starlight scope then amplifies these electrons by sending them through narrow channels that have highly charged walls. As the electrons bounce their ways through the channels, they knock out hundreds, then thousands, then even millions of other electrons so that each original photon can release more than a million electrons from the amplifying system. When these electrons strike the phosphor-coated anode, the image they produce is bright and visible, so that the person looking at the anode can effectively see when each photon of light strikes the photocathode and initiates one of these electron cascades. With such incredible light sensitivity, there is no longer any need to actively illuminate the target with infrared light—even starlight is enough illumination to make the target visible through the starlight scope's image intensification system.

1070. Why does food become soggy after heating in the microwave oven, particularly pastry?
A normal oven heats foods by exposing them to hot air and thermal radiation. It cooks the foods from the outside in. As a result, a normal oven tends to make the surfaces of food dry and crispy because it heats those outer surfaces first and drives the water out of them. A microwave oven heats the food by heating the water in that food. It cooks foods from the inside out. As a result, a microwave oven tends to drive water out of the middle of the food and into the outer layers of that food. The outer layers are essentially "steamed" and steaming makes everything soggy.
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