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Page 92 of 160 (1595 Questions and Answers)

911. How long will a magnetic tape stay magnetized? Won't it lose its magnetization very fast, like we saw with the iron nails?
At room temperature, a magnetic tape will remain magnetized for years and years. It is made of much harder magnetic materials than the nails are made of and it is much harder to demagnetize than the nails. In effect, it is covered with tiny permanent magnets and you have seen permanent magnets that remain magnetic for decades or centuries.

912. When you talk about the magnetic tape and recording, is it the pressure or frequency that is being recorded? Are pressure and frequency interrelated?
Sound consists of pressure fluctuations. The stronger those pressure fluctuations, the louder the sound. The rapidity with which the air goes between a pressure increase and a pressure decreases determines the frequency of the sound and the pitch that we hear. So the extent of the pressure fluctuations, their amplitude, determines the sound volume while the number of pressure fluctuations each second, their frequency, determines the sound pitch. The tape recorder detects both and records both. The louder the sound, the deeper the recorder magnetizes the tape. The higher the frequency of the sound, the more often the tape recorder reverses the magnetization of the tape's surface.

913. How is sunlight both harmful and beneficial? - CP
Sunlight provides virtually all the energy in our world. Without it, plants wouldn't grow and we wouldn't have food or daylight. We wouldn't even have fossil fuels such as coal and petroleum because those were formed from vegetation that itself derived energy from the sun. However, sunlight also contains ultraviolet light, which can damage chemicals in biological tissue. Long exposure to ultraviolet light can age your skin or cause cancer.

914. How can one tell the difference between a gravitational red shift of light and a red shift caused by motion? Could the red shift of quasars be from gravity and not speed, therefore making the quasars closer than we think they are? - FG
At astronomical distances, there is no way to tell the difference between the two red shifts. An object that is deep in the gravitational potential well of a very massive object experiences time slowly and its light appears shifted toward the red (low frequency and long wavelength) when it reaches us. The light from an object that is moving away from us rapidly also appears red shifted (low frequency and long wavelength), but this time it's due to the Doppler effect.

Quasars exhibit enormous red shifts and one explanation for those red shifts is that the quasars are located near the other side of the universe. If so, they would be moving away from us rapidly, along with their surroundings in the expanding universe, and their light would appear highly red shifted. Moreover, their light would have been traveling almost since the beginning of the universe so that we would be observing very ancient objects. However, it's also possible that quasars are much near to us and that their red shifts are caused by gravitational effects rather than relative motion. As far as I know, this possibility can't be ruled out and remains a concern amount the astronomical community.

915. We know that spinning objects on earth can lose their spin (angular momentum) due to friction (fluid or sliding) with the air or ground. However, if an object is set spinning in space, will it lose its initial angular momentum eventually or will it spin forever assuming no outside forces (e.g., gravity) act upon it? If it does come to rest, how does the earth maintain its spinning motion? — RD, Kingwood, TX
If a spinning object is truly free of outside torques—the influences that affect rotation—then it will spin forever. Angular momentum is a conserved quantity in our universe, meaning that it can't be created or destroyed and can only be transferred between objects. Thus if you set an object spinning (by exerting a torque on it) and then leave it entirely alone, it will not be able to change its angular momentum. The earth is a good example of this situation—it's almost free of torques and so it spins steadily about a fixed axis in space. Its angular momentum is essentially unchanging.

Since gravity acts at the center of rotation of a freely falling object (which is that object's center of mass), gravity exerts no torque on freely falling objects. Because of that fact, even objects in orbit around the earth are essentially free of torques and satellites that are set spinning when they're launched continue to spin steadily for centuries. The space shuttle astronauts encounter this result each time they release or catch a satellite. If they set it spinning when they let go of it, it will still be spinning when they retrieve it years later.

916. I have read that sometimes two very slick things rubbing together have more friction than two rough things. Is that true? Why? — A
Friction is caused by contact and collisions between the tiny projections that exist on all surfaces. When you put one block on top of another, the tiny projections on the bottom of the upper block touch the tiny projections on the top of the lower block. If you then try to slide one block across the other, these projections begin to collide with one another and they oppose the sliding motion.

If the two blocks have rough surfaces, then the projections that are colliding are obvious to your eyes. But if the two blocks have very smooth surfaces, you can't see their surface projections. However, the invisibility of these projections doesn't make them insignificant. Even the smoothest surfaces are rough at the atomic scale. When you press two smooth surfaces against one another, their microscopic projections still touch one another and those projections still collide when you try to slide the surfaces across one another. In short, smooth surfaces still experience friction.

But it's also possible for attachments to form between portions of the two smooth surfaces when they touch. This molecular adhesion makes it even harder to slide the two surfaces across one another. You can feel this adhesion when you press two pieces of very clean glass against one another—they form bonds that partially stick them together. Actually, this sort of sticking would be quite common if it weren't for water. Almost all surfaces are coated with a layer or two of water molecules. These water molecules lubricate the interface between any two surfaces and make it hard for those surfaces to stick to one another. But if you get rid of the water molecules, the sticking becomes quite severe. This effect causes trouble in my laboratory, where sliding mechanisms that move easily in air stop working properly when we put them in a vacuum chamber and remove the water on their surfaces.

917. Can you tell me the difference in lifting power of helium versus hydrogen? — FL, Napa, CA
A balloon experiences an upward buoyant force that's equal in amount to the weight of the air it displaces. If that balloon is filled with helium or hydrogen, both of which have very low densities, then this upward buoyant force may be more than the balloon's weight and the balloon may accelerate upward. Helium weighs a little more per cubic foot or cubic meter than hydrogen does, so replacing the helium with hydrogen will make it easier to float the balloon. A cubic foot of hydrogen weighs 0.0056 pounds less than a cubic foot of helium and a cubic meter of hydrogen weighs 89 grams less than a cubic meter of helium. Any weight saving made by replacing helium with hydrogen in your balloon can be viewed as extra lifting power. As you can see, the effect is small and hydrogen is a whole lot more dangerous than helium.

918. How does a halogen cooktop unit heat up food? — BS, Logan, UT
A halogen cooktop unit uses thermal radiation to transfer heat to a pot or pan. All objects emit thermal radiation, but that radiation isn't visible until an object's temperature is at least 500° C. At higher temperatures, a significant fraction of an object's thermal radiation is visible light. In a halogen cooktop unit, an electrically heated tungsten filament is heated to the point where it emits a large amount of thermal radiation. Since the filament is small, it takes only a second or two for the filament to reach full temperature and begin emitting its intense thermal radiation. Any dark object above the unit will absorb this thermal radiation and experience a rise in temperature. When you turn off the unit, the filament cools rapidly and stops emitting its thermal radiation. The filament itself is protected from oxygen in the air by a heat-resistant glass envelope that's filled with halogen gas. This gas helps to keep the filament intact and prevents it from depositing tungsten atoms on the insides of the glass envelope.

919. Why is an incandescent light bulb hotter than a fluorescent light? — TJ, Woodbridge, VA
An incandescent light bulb produces light by heating a small filament of tungsten to about 2500° C. At that temperature, the thermal radiation that the filament emits includes a substantial amount of visible light. But the filament also emits a great deal of infrared light (heat light) and it also transfers heat via conduction and convection to the glass bulb around it. When you put your hand near the bulb, you feel both the infrared light and the heat that has worked its way to the surface of the bulb. The bulb feels hot.

In contrast, a fluorescent lamp tries to produce light without heat. It collides electrons with mercury atoms to produce an atomic emission of ultraviolet light. This ultraviolet light is then converted to visible light by the layer of white phosphor powders on the inside of the lamp's glass envelope. In principle, this whole activity can be performed without creating any thermal energy. However, many unavoidable imperfections cause the lamp to convert some of the electric energy it consumes into thermal energy. Nonetheless, the lamp only becomes warm rather than hot.

920. How does ammonia refrigeration work?
There are actually two answers to this question. First, like the more modern chlorofluorocarbon (Freon) and hydrofluorocarbon refrigerants, ammonia (NH3 converts easily from a gas to a liquid near room temperature. If you squeeze ammonia to high density, it will release heat and convert to a liquid. If you let it expand to low density, it will absorb heat and convert to a gas. A compressor-based ammonia refrigeration unit makes use of that easy convertibility. First, it uses a compressor to squeeze the ammonia gas outside the refrigerator. The hot dense ammonia gas that leaves the compressor enters a condenser, where it releases heat to its surroundings and condenses to a cool ammonia liquid. This liquid enters the refrigerator and passes into an evaporator, where it's allowed to expand into a gas and it absorbs heat from its surroundings. The gas then returns outside the refrigerator to repeat this cycle again and again.

But there is a second type of ammonia refrigerator that makes use of an absorption cycle—ammonia dissolves extremely well in cool water but not so well in hot water. In an absorption cycle refrigerator, a concentrated solution of ammonia in water is heated in a boiler until most of the ammonia is driven out of the water as a high-pressure gas. This hot, dense ammonia gas then enters a condenser, where it gives up heat to its surroundings and becomes a cooler liquid. The liquid ammonia then enters a low-pressure evaporator, where it evaporates into a cold gas. This evaporation process draws heat out the evaporator and refrigerates everything nearby. Finally, the ammonia gas must be returned to the boiler to begin the process again. That return step makes use of the absorption process, in which the ammonia gas is allowed to dissolve in relatively pure, cool water. The gas dissolves easily in this water and thus maintains the low pressure needed for evaporation to continue in the evaporator. The now concentrated ammonia solution flows to the boiler where the ammonia is driven back out of the water and everything repeats.
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