Author: Lou Bloomfield

What causes things to glow in the dark? Why does phosphorus glow? Why does the g…

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What causes things to glow in the dark? Why does phosphorus glow? Why does the glow die? — DB & EB

Most glow-in-the-dark materials store energy when they are exposed to visible light and then glow dimly as this stored energy is gradually converted back into light. In such a material, exposure to light promotes some of the electrons in the atoms or molecules to excited states and these electrons become trapped in lower-energy excited states from which they have trouble escaping. It takes a very long time for each of these trapped electrons to return to their original states by emitting light. Since that return is a random process, a glow-in-the-dark object glows with an ever diminishing light as the excited electrons return at random moments to their original states. Eventually almost all the electrons have returned and the glow weakens to essentially nothing.

White phosphorus also glows in the dark, but not for the same reason. You don’t need to expose white phosphorus to light to make it glow; you need to expose it to air. The chemical reaction between phosphorus and oxygen causes the phosphorus to emit light. This reaction can also cause the white phosphorus to burst into flames. Because of its dangerous flammability and its toxicity, white phosphorus isn’t something you want to have around.

I would like to know a little more about the ac slip ring motor and its uses, pa…

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I would like to know a little more about the ac slip ring motor and its uses, particularly in elevators. – M

A normal induction motor uses a set of stationary electromagnets to produce a magnetic field that seems to rotate rapidly around the motor’s rotating central component—its “rotor.” The rotor consists of a cylindrical aluminum metal cage and the rotating magnetic field causes currents to flow in the cage so that it becomes magnetic. The nature of the magnetism in the rotor causes it to be dragged along with the rotating magnetic fields around it and it begins to turn with those fields. When you first turn on the induction motor, the stationary rotor leaps into rotation as it tries to follow the spinning magnetic fields. That sudden start is acceptable for many applications, but you wouldn’t want it in an elevator—the sudden starting of the elevator car that would accompany the sudden starting of its motor would throw the occupants to the floor. Instead, the aluminum cage in the rotor is replaced by a group of wires that are connected by way of metal ring (the “slip rings”) and some stationary conductive brushes to some components outside the rotor. During the starting process, the currents that are induced in the rotor’s wires are limited by the components outside the rotor. The rotor starts spinning gradually and gracefully. When the rotor has reached full speed, the brushes are retracted from the slip rings and the slip rings are shorted together so that the rotor behaves like the aluminum cage of a normal induction motor.

How do thermals affect the atmosphere and air currents?

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How do thermals affect the atmosphere and air currents? — RM, Praire Farm Schools, Wisconsin

Thermals are air currents in the atmosphere. When sunlight and exposure to warm ground raises the temperature of surface air, that air expands—its molecules travel faster and bounce against one another more vigorously, so they push themselves farther apart. This expanded air weighs less per cubic foot or meter than cooler air, so the cooler air around it lifts it upward in a rising current of warm air—a “thermal.” The air can’t simply accumulate way up overhead forever, so cooler air descends to take its place. The overall result is rising warm air and descending cool air. These air currents are part of giant circulation loops or “convection cells” that also include surface winds and high altitude winds.

Would it be possible for a spacecraft to use electrically powered propulsion? Co…

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Would it be possible for a spacecraft to use electrically powered propulsion? Could it gather atoms and molecules from space and then use an electromagnetic field to push them through a nozzle? — JC, Burnaby, British Columbia

Not only is it possible to use electrically powered propulsion, such systems are already in use on several spacecraft. While they don’t scavenge atoms and molecules from space, these ion propulsion engines uses electric forces to accelerate ionized atoms to enormous speeds. As the engine pushes on the ions it accelerates, those ions push back on the engine. The ions rush out into space in one direction and the engine experiences a modest thrust in the opposite direction. While the overall thrust from an ion engine is small, it uses its stored-atom “fuel” very efficiently and can be sustained for a very long time in a solar- or nuclear-powered satellite. Ion engines are used in spacecraft that need small but steady thrust for a long time. Scavenging atoms from space would allow these engines to run for an even longer time, but it’s probably not realistic. The atoms in space are typically so rare and so fast-moving that they would be more trouble than they’re worth.

Can you explain gyroscopic precession?

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Can you explain gyroscopic precession? — BW, Newport, RI

When a gyroscope is spinning rapidly, it has a large amount of a conserved physical quantity called angular momentum. Angular momentum is a special measure of rotational motion that can’t be created or destroyed—it can only be transferred between objects. As long as nothing tries to transfer angular momentum to or from the spinning gyroscope, it will continue to spin at a steady pace about a fixed axis in space. But when an external torque (a twist) is exerted on the gyroscope, a transfer of angular momentum takes place. The gyroscope’s rate of rotation or its axis of rotation begins to change so that its angular momentum changes. If you apply a twist to the gyroscope around its axis of rotation, it will either spin faster or slower, depending on which way you twist it. But if you twist the gyroscope about a different axis, its axis of rotation will shift—the gyroscope will undergo precession. The direction of this precession depends on how you apply the twist and tends to be very non-intuitive.

How can people lay on a bed of nails and still survive?

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How can people lay on a bed of nails and still survive? — LW, Marion, OH

If you push gently on the tip of one nail, it won’t pierce your finger. When you push on the nail, it pushes back on you, but the force pushing the nail against your finger isn’t strong enough to break your skin. If you push twice as hard on two nails at once, using two different fingers, then the force you exert on each nail will be the same as before and each nail will push back against one of your fingers with the same force as before. Once again, the nails won’t break your skin. If you now push 100 times as hard against 100 nails, each nail won’t push hard enough against you to break your skin. In fact, a few hundred nails will be able to push on you with an overall force equal to your weight without piercing you. That’s the idea behind a bed of nails—by lying on many nails at once, you allow so many nails to push upward on you that, while the overall force they exert on you is enough to balance your weight, the force exerted by each individual nail isn’t enough to draw blood. These nails have to be spread out around your body so that no individual nails bear more than their fair share of your weight. If one of the nails took too much of your weight, you’d be hurt by it.

What happens when salt is added to water? If I mix 1 cup of salt with 1 cup of w…

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What happens when salt is added to water? If I mix 1 cup of salt with 1 cup of water, will I end up with 2 cups of solution? – RT

As a crystalline solid, salt consists of a beautiful cubic lattice of sodium atoms that have lost one electron to become sodium positive ions and chlorine atoms that have gained one electron to become chlorine negative ions. The crystal is held together by the attractive forces between these oppositely charged atomic ions. When a salt crystal dissolves in water, it decomposes into individual sodium positive ions and chlorine negative ions that are then carried about by shells of water molecules. Water molecules are electrically polar, meaning that they have positively charged ends and negatively charged ends. The water molecules line up around a positively charged sodium ion with their negatively charged ends inward and carry that ion about. Similarly, water molecules line up around a negatively charged chlorine ion with their positively charged ends inward and carry that ion about. Whether you will end up with 2 cups of solution after mixing 1 cup of salt and 1 cup of water depends on how tightly the atoms and molecules pack together in each case. Remember that your 1-cup of salt contains a fair amount of air between the salt grains. You’ll have to try it to find out the answer—I’m not sure what the answer will be.

Why does popcorn pop? – AB

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Why does popcorn pop? – AB

Inside the hard, dry hull of a popcorn kernel is a portion of moist starch. When you heat the kernel well above the boiling temperature of water, the water in the starch converts to hot, high-pressure steam. The hotter this steam gets, the higher its pressure rises and the stronger the outward forces it exerts on the hull. Eventually, the hull rips open under the stress and exposes the starch to the low-pressure air around it. The pressurized steam then pushes the starch outward, expanding it to many times its original size. The kernel “pops.”

Is there an inexpensive device for detecting leaks from a microwave oven?

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Is there an inexpensive device for detecting leaks from a microwave oven?

Yes. You can get one from a hardware or appliance store for about $5 to $30. ComfortHouse.com sells one on-line at www.comforthouse.com. While I have tended to downplay the leakage issue in the past, I bought a tester and found that the microwave oven in my laboratory actually leaked significantly. I had used it in many class demonstrations, so it had been abused and the door wasn’t properly aligned any more. I retired it. Incidentally, the tester contains only two components: a fast diode and a current meter. It detects microwave in the same way that a crystal radio detects an AM radio broadcast. However, I should note that both the International Microwave Power Institute (IMPI) and the FDA caution against trusting those simple and not particularly accurate meters, and recommend that you take your microwave oven to a service shop for inspection with an FDA certified meter.