. How does radar absorbent materials work. How effective is stealth technology? — DP, Scottsbluff, NE
I believe that most radar absorbing materials are partially conducting plastic composites. As a microwave from the radar transmitter penetrates these composites, the electric field in that wave drives charges back and forth through the composites. Since the composites don't conductor electricity well, they turn the wave's energy into thermal energy and thereby absorb it. A similar effect occurs for light waves when you shine them on a pile of powdered charcoal. (According to David Ingham, some radar absorbing materials include lossy magnetic materials—materials such as ferrite and carbonyl iron that respond to the magnetic field in a microwave.) Because there is always some reflection whenever an electromagnetic wave enters a material that slows the wave down, stealth aircraft are also careful to deflect the reflected wave away from the radar transmitter so that its receiver won't detect the return wave. In fact, these materials can be corrugated so that any microwaves hitting them reflect into the corrugations and have many opportunities to be absorbed. As I understand it, the microwaves that return to the radar receiver from a stealth plane are remarkably weak. I wouldn't be surprised if a whole stealth plane reflected less microwaves back at the radar unit than would reflect from a foil chewing gum wrapper.
. Does water drain in the opposite direction in the southern hemisphere? - TL
In principle, yes, but in practice, no. To explain why, I'll begin with the origins of directional circulations on earth. Because the earth is turning, motions along its surface are complicated. The ground at the equator is actually heading eastward at more than 1000 miles per hour. The ground north or south of the equator is also heading eastward, but not as quickly. The ground's eastward speed gradually diminishes until, at the north and south poles, there is no eastward motion at all. As a result of this non-uniform eastward motion of the ground, objects that travel in straight lines because of their inertia end up drifting eastward or westward relative to the ground. For example, if you took an object at the equator and threw it directly northward, it would drift eastward relative to the more slowly moving ground. If someone else threw an object southward from the north pole, that object would drift westward relative to the more rapidly moving ground. In the northern hemisphere, objects approaching a center tend to deflect away from that center to form a counter-clockwise circle around it. This process is reversed in the southern hemisphere so that objects approaching a center there tend to form a clockwise circle around it. Thus hurricanes are counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere.
When water drains from a basin in the northern hemisphere, it flows toward a center and should have a tendency to deflect into a counter-clockwise swirl. However, the effect is very weak in a small washbasin. The direction in which the water swirls as it drains is determined by other effects such as how the water was sloshing before you opened the drain or how symmetric the basin is. For this earth's rotation-driven swirling effect (the Coriolis effect) to dictate the direction of a circulation the objects involved must move long distances over the earth's surface. Even tornadoes don't always rotate in the expected direction; they're just not big enough to be spun consistently by the Coriolis effect.
. How does a gravity powered water pump work? — JA, Hiawassee, GA
I believe that the pump you're interested in is one that uses the energy released when water flows downhill to lift a small fraction of that water upward. While there are many possible designs for such a pump, the classic version used a phenomenon called "water hammer" to lift water upward. In this technique, a column of water is allowed to accelerate downhill through a pipe until it's flowing at a good speed through the pipe. The pump then closes a valve at the lower end of the pipe, so that the water has to stop abruptly. Since water accelerates in response to imbalances in pressure, the stopping process involves an enormous pressure surge at the lower end of the moving water column. A one-way valve at the lower end of the pipe opens during this pressure surge and allows a small fraction of the water to escape from the pipe. The escaping water rises upward through a second pipe for delivery to a home or business. According to a reader, the escaping water actually enters a head tank that is normally filled with air and thus compresses that air. The compressed air is then used to push water through the pump's outlet and provide the pumping action. This pumping scheme is apparently called a "hydraulic ram."
The only trick to operating such a pump is opening and closing the valve at the lower end of the first pipe. This valve must open long enough that the water in the pipe reaches a good speed and then it must close very suddenly to provide the pressure surge that lifts the small amount of water upward for delivery.
. How does heat conduct through different materials? - B
In electric insulators, heat is carried by motions of the atoms themselves. You can think of this heat transfer as a bucket-brigade process—one atom jiggles its neighbor, which in turn jiggles its neighbor, and so on. If one end of an insulator is hotter than the other, this jiggling effect will gradually transfer thermal energy from the hotter end (more vigorous jiggling) to the colder end (less vigorous jiggling). Imperfections and weaknesses in most electric insulators make them relatively poor conductors of heat, although there are a few exceptional materials such as diamond that use the bucket-brigade mechanism very effectively and are excellent thermal conductors. In electric conductors, mobile electrons help out by carrying thermal energy from one atom to another over long distances. Even in a material that doesn't make good use of the bucket-brigade mechanism, the mobile electrons provide substantial thermal conductivity. Thus good electric conductors, such as copper, silver, and aluminum, are also good thermal conductors.
. Why does water boil at lower or higher temperatures under varying atmospheric pressures? Do changing vapor pressures above a liquid play a role in changing boiling points of liquids? — KC, East Greenwich, RI
A liquid boils when its vapor pressure reaches atmospheric pressure. While a liquid will evaporate at temperatures below the boiling temperature, that evaporation only occurs from the surface of the liquid. That's because atmospheric pressure crushes any bubbles that try to form within the body of the liquid. Every once in a while, a few molecules of the liquid break free inside the liquid and form a bubble of gas. The pressure inside such a bubble is the vapor pressure of the liquid at its present temperature. If the liquid's temperature is below its boiling temperature, atmospheric pressure is greater than the pressure inside one of these spontaneous vapor bubbles and it crushes the bubble. But once the temperature of the liquid reaches the boiling temperature, the bubbles will have enough pressure to remain stable against atmospheric pressure. Each bubble that forms begins to float upward toward the top of the liquid and more molecules evaporate into it as it rises, so that it grows larger and larger.
If you lower atmospheric pressure, the liquid will boil at a lower temperature because the vapor pressure reaches atmospheric pressure more easily. If you raise atmospheric pressure, the liquid will boil at a higher temperature because the vapor pressure must rise higher before it reaches atmospheric pressure.
. What are the relative efficiencies and reasons for power losses in sprocket and chain drives, rubber cogged belt drives, pulley drives, and gear drives? — RA, Montreal, Quebec
The only power loss mechanisms I can think of in each case are sliding friction and vibration. The drive system most likely to experience substantial sliding friction is a pulley (or smooth belt) drive. If the belt slips as the pulleys turn, the belt will do work against the force of sliding friction and that work will be converted into thermal energy. But, as one of my readers points out, if the belt is properly tightened, has an adequate coefficient of friction to prevent slipping, and has a high tensile strength so that it doesn't creep across the pulley surface, then it can operate with very little power loss.
In the other drive systems, there is no possibility of slippage so that any power loss that occurs must be due to internal sliding friction within the components, or from vibrations. Flexing a chain involves some internal sliding friction and wastes some power. I suppose this could be minimized with careful chain construction and I wouldn't be surprised if large change drive systems placed bearings in the chain links to eliminate sliding friction altogether. Flexing a rubber-cogged belt also involves some molecular friction within the belt material so it wastes some power. I'm not sure which system is more efficient, the chain drive or the cogged belt drive. Finally, the gear drive is the least likely to waste significant energy. The only sliding friction that occurs is between the gear teeth. If the teeth are designed well and cut carefully, they should slide very little. In that case, the only significant power loss would be through vibrations. If everything is carefully mounted to prevent vibrations, there should be very little power loss in a gear drive.
. I have a gas steam heating system and the second floor radiators don't heat well. How does this system work and how can I balance the system so that the upstairs radiators warm at the same rate as the first floor radiators?
In a steam heating system, steam rises upward from a boiler in the basement and condenses in the radiators. As the steam transforms into water, it releases an enormous amount of heat and this heat is transferred to the air in the rooms. The condensed water than descends back to the boiler to be reheated. The beauty of this system is that the rising steam and the descending water can both pass through the same pipes, propelled by gravity alone. The low-density steam is lifted upward by the high-density water.
However, there are a few potential problems with this system. If there is air trapped in the pipes, the steam will have trouble reaching the radiators. Even though steam is lighter than air, it will diffuse slowly through the trapped air. That's why each steam radiator has a small bleeder valve. When the steam pressure exceeds atmospheric pressure, it should push the air in the pipes out the bleeder valves of the radiators. You ought to be able to hear the air leaving and the valves may continue to sputter a bit even when the pipes and radiators are essentially full of steam. I suspect that the bleeder valves on your upstairs radiators aren't functioning well so that steam isn't reaching them.
. Do whales drink salted water? — GR, Montreal, Quebec
No. Marine mammals rely on water obtained from their food. Because they don't sweat, they only lose water through their urine, which they concentrate to minimize the loss of water. What little water these animals do need comes from eating foods that are already relatively low in salt. Most of the lower sea animals, including fish, have active systems—ones that consume ordered energy—for eliminating salt so that when a sea mammal eats one of the lower animals, it inherits that animal's relatively salt-free water. Moreover, metabolizing fats and carbohydrates produces water as a byproduct.
. I recently visited an audio store where I saw electrostatic speakers. These speakers have no moving parts like conventional speakers and are more expensive. How do they produce sound? — BC, Ottawa, Canada
Electrostatic speakers uses the forces between electric charges (so called "electrostatic forces") to move a thin metal diaphragm back and forth rapidly. The motions of this diaphragm compress and rarefy the air in front of it, producing sound. On each side of the diaphragm is a rigid metallic grill that can hold electric charges. When the speaker is silent, the diaphragm has a large positive electric charge on it and both the metal grills have large negative charges on them (it could be the other way around, depending the speaker's exact design). The diaphragm is then attracted equally toward both grills and the electrostatic forces cancel perfectly. The diaphragm doesn't undergo any acceleration. To make the speaker produce sound, the electric charges on the two grills are changed so that the electrostatic forces on the diaphragm don't cancel. Instead, the diaphragm is pulled strongly toward whichever grill has more negative charge on it (or less positive charge). The charges on the grills fluctuate as the music plays and the diaphragm accelerates back and forth between the grills. It pushes on the air as it does and produces sound. You'll notice that the diaphragm is a moving part, so the claim that the speaker has "no moving parts" is misleading. The speaker cone of a conventional speaker only moves back and forth, too, so it has an equal claim to having "no moving parts." The relative expense of an electrostatic speaker comes from the requirement of careful construction and the need for a high voltage adapter to match an amplifier to the speaker.
. Don't microwaves change the molecular structure and composition of food, by ejecting some electrons from atoms and forming cancer-causing free radicals? If I should stand away from a microwave to avoid possible leakage, why would I eat microwaved food?
Microwaves don't affect the molecular structure of the food, except through the thermal effects we associate with normal cooking (e.g., denaturing of proteins with heat and caramelizing of sugars). That's because, like all electromagnetic waves, microwaves are emitted and absorbed as particles called "photons." The energy in a microwave photon is so tiny that it can't cause any chemical rearrangement in a molecule. Instead, it can only add a tiny amount of heat to a water molecule. During the microwave cooking process, microwave photons stream into the food and heat it up. But millions of them would have to work together in order to cause non-thermal chemical changes in the food molecules and they don't normally do that. The photons can only work together if there is a conducting material, such as a metal wire, inside the oven. In that case, the photons can accelerate mobile electric charges along the conducting paths and create sparks. Such sparks can cause chemical damage, but nothing worse than the chemical damage caused by scorching food with a flame or broiler. Even if your microwave is full of sparks for some reason, I doubt that the food will be any worse for you than it would be if you cooked it over an open flame or barbecue.