What is the most effective way to electronically measure the level of charge of …

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What is the most effective way to electronically measure the level of charge of a lead acid battery? — RS

The voltage of any battery—the amount of energy it gives to each positive charge that it transfers from its negative terminal to its positive terminal—increases slightly when the battery is fully charged. That’s because when the battery is fully charged and its chemicals are highly ordered, the laws of thermodynamics that encourage the development of disorder act to increase the battery’s disorder through effects that also increase the battery’s voltage. But as the battery discharges, these thermodynamic effects fade and the battery’s voltage diminishes slightly. So the easiest way to determine the battery’s charging status electronically is to look at the voltage rise across the battery when little or no current is flowing through it. The higher the voltage, the more fully charged the battery is.

How fast can maglev trains travel?

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How fast can maglev trains travel? — AP

As long as the track is straight enough that the train doesn’t experience severe accelerations up, down, left, or right, there is no limit to how fast it can go. In fact, the levitation process becomes more and more energy efficient as the speed increases. However, the moving train does experience a pressure drag force (a type of air resistance) that increases roughly as the square of the train’s speed. The power needed to overcome this drag force increases as the cube of the train’s speed, making it impractical to propel the train forward above a certain speed.

Why does a body at rest remain at rest and a body in motion remain in motion, in…

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Why does a body at rest remain at rest and a body in motion remain in motion, in the absence of unbalanced force? — AW, Karachi, Pakistan

That observation, known as Newton’s first law of motion, is one of the fundamental characteristics of the universe. I could answer simply that that’s the way the universe works. But a more specific answer is that the universe exhibits translational symmetry—meaning that the laws of physics are the same from your current vantage point as they would be if you shifted a meter to your left. Shifting your vantage point along some linear path—a process called translation—doesn’t affect the laws of physics. The laws of physics are said to be symmetric with respect to translations and, because translations of any size are possible, this symmetry is considered to be continuous in character (as opposed to mirror reflection, which is a discrete symmetry). Whenever the laws of physics exhibit a continuous symmetry of this sort, there is a related conserved quantity. The conserved quantity that accompanies translational symmetry is known as momentum. An isolated object’s momentum can’t change because momentum is a conserved quantity—it can’t be created or destroyed. Since momentum is related to motion, an isolated object that’s at rest and has no momentum must remain at rest with no momentum. And an isolated object that’s moving and has a certain momentum must remain in motion with that same momentum.

Incidentally, the laws of physics also exhibit rotational symmetry—meaning that turning your head doesn’t change the laws of physics—and this symmetry leads to the existence of a conserved quantity known as angular momentum. The laws of physics also don’t change with the passage of time, a temporal symmetry that leads to the existence of a conserved quantity known as energy.

Why does food become soggy after heating in the microwave oven, particularly pas…

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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.

How did the sniperscopes used in World War II work? They seem to have used an in…

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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.

How does a steam whistle work?

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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.

How is infrared light produced?

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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.

How does wing shape affect flight?

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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.

How does a toilet work?

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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.

How can you make a hologram?

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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.