. How does a radio receive transmissions from one station and not another, and how does it turn them into audible waves? — T, Chester, VT
A radio wave contains an electric field that pushes on any electric charge it encounters. That's why, when a radio wave passes the antenna of your radio, it causes electric charges in that antenna to accelerate up and down. There is also a resonant circuit connected to the antenna—a circuit that oscillates strongly only when charge is pushed up and down the antenna at exactly the circuit's resonant frequency. If the circuit's resonant frequency is the same as that of the radio wave, the small pushes exerted on charges in the antenna add up so that charge moves more and more vigorously through the resonant circuit. But if your radio isn't tuned to the frequency of the radio wave, the overall motion of charge on the antenna and this resonant circuit is small. That's why your radio only responds to the radio transmission of one station and not others. To understand this effect, imagine pushing a child on a swing. If you push rhythmically at just the right frequency, the child will swing higher and higher. But if you push rhythmically at the wrong frequency, the child will just jitter about a bit.
Once charge is moving strongly through the resonant circuit in your radio, the radio can monitor various features of that moving charge. If the station is using the AM or amplitude modulation technique to represent sound, your radio studies the amount of charge moving back and forth through the resonant circuit. When that flow of charge—that current—is strong, it moves the speaker cone toward you and produces a compression of the air. When that current is weak, it moves the speaker cone away from you and produces a rarefaction of the air. These changes in air density and pressure reproduce the sound that the station is transmitting.
If the station is using the FM or frequency modulation technique to represent sound, your radio studies the frequency at which charge moves back and forth in the resonant circuit. Very small changes in this frequency, caused by frequency changes in the radio wave itself, are used to control the speaker cone in your radio. When the frequency is raised slightly above normal, your radio moves the speaker cone toward you and produces a compression of the air. When the frequency is lowered slightly below normal, your radio moves the speaker cone away from you and produces a rarefaction of the air. Again, these changes in air density and pressure produce sound.
. How does a strobe light work? — JM, Kettering, OH
A strobe light passes a brief, intense pulse of electric current through a gas, which then emits a brilliant burst of light. The gas is usually one of two inert gases, xenon or krypton, that emit relatively white light when they're struck by the fast moving electrons in the electric current. When it hits a xenon or krypton atom, an electron may give up some of its kinetic energy—its energy of motion—to the electrons in the atom. Those atomic electrons shift from their usual orbitals (quantum mechanically allowed orbits) to higher-energy orbitals that they usually don't travel in. The atomic electrons remain only briefly in these higher-energy orbitals before dropping back to their original orbitals. As they drop back down, these electrons give up their extra energy as light. Because krypton and xenon atoms have a great many electrons and their electronic structures are very complicated, they emit light over a broad range of wavelengths. Moreover, the gases are at relatively high pressures and collisions between the atoms while they are emitting light further smooth out the spectrum of light they produce. Thus the strobe emits a rich, white light during the moments while current is passing through the gas.
Supplying the enormous current needed to maintain the brief arc in the strobe's gas is done with the help of a capacitor, a device that stores separated electric charge. A high voltage power supply pumps positive charge from the capacitor's negative plate to its positive plate, until there is a huge charge imbalance between those two plates. You can often hear a whistling sound as this power supply does its work. The capacitor plates are connected to one another through the gas-filled flashlamp that will eventually produce the light. However, current can't pass through the gas in the flashlamp until some electric charges are injected into the gas. These initial charges are usually produced by a high voltage pulse applied to a wire that wraps around the middle of the flashlamp. When a few charges are inserted into the gas, they accelerate rapidly toward the positive or negative wires that extend from the charged capacitor. As these charges pick up speed, they begin to collide with the gas atoms and they deposit energy in those atoms. Electrons are occasionally knocked out of atoms or out of the wires at the end of the flashlamp and these new charges that enter the gas also begin to accelerate toward the wires. A cascade of collisions quickly leads to a violent arc of charged particles flowing through the flashlamp and colliding with the gas atoms. The flashlamp emits its brilliant burst of light that terminates only when the capacitor's separated electric charges and stored energy are exhausted.
. What is an H-Bomb made of?
A hydrogen bomb or thermonuclear bomb is a nuclear weapon that obtains most of its energy from the fusion of hydrogen nuclei into helium nuclei. This fusion typically involves deuterium and tritium nuclei, the heavy isotopes of hydrogen. Deuterium is a stable, naturally occurring isotope with one proton and one neutron in its nucleus, and can be extracted from normal water. Tritium is an artificial, radioactive isotope with one proton and two neutrons in its nucleus, and can be formed in nuclear reactors or, during a nuclear explosion, by the exposure of lithium nuclei to the neutrons formed in that explosion.
Since hydrogen nuclei are positively charged, they repel one another. To get these heavy hydrogen nuclei close enough together to fuse into helium nuclei, the hydrogen nuclei must be heated to fantastic temperatures. This heating is done with a fission bomb—a uranium or plutonium bomb. When the fission bomb explodes, its heat is enough to trigger the hydrogen bomb.
. What is a vortex? — M
A vortex is a region of fluid that's circulating in one direction around a line passing through that region. If you imagine yourself looking along that line, you would see the fluid flowing either clockwise or counter-clockwise around the line itself. Tornadoes and whirlpools are both vortices since they involve fluids circulating in one direction around a central line.
. What is convection? — DB, Corona, CA
Convection is the transfer of heat by a circulating fluid, such as air or water. This heat is carried from a hotter object to a colder object. The fluid first passes near the hotter object and receives heat. The fluid becomes warmer and more buoyant, and it's lifted upward by the colder fluid around it—just as a hot air balloon is lifted upward by the colder air around it. The rising fluid carries the heat with it. Eventually the rising fluid spreads outward and it pass near colder objects, giving up its heat. The fluid becomes cooler and less buoyant, and soon it begins to descend back toward the ground. Eventually it's drawn back past the hotter object and this cycle begins again.
. How does a snow making machine work? — IB, Blue Ash, OH
A snow-making machine simply sprays a fine mist of water high into the cold air overhead, so that that mist can freeze into tiny particles of ice before falling back to the ground. If the air is cold enough, the mist will solidify before it hits the ground and before it has time to evaporate into water vapor. This freezing process isn't as simple as it sounds because water can't turn into an ice crystal without a seed on which that crystal can grow. Forming a seed crystal is a random process in which a couple of water molecules accidentally arrange themselves in a crystalline lattice. In snow making, each water droplet has only a few seconds in which to freeze and it can easily take that long for a seed crystal to form. However, people have found that adding certain chemicals or other materials to the water before spraying it into the air can speed the formation of seed crystals and dramatically increase the fraction of water that becomes artificial snow.
. How do sound waves travel in space? — PS
When sound travels in air, it takes the form of compressions and rarefactions of that air. Similar compressions and rarefactions occur when sound travels in a liquid or in a solid. But sound can't travel through space because space is entirely empty. Sound requires a medium in which to travel and space doesn't contain any such medium. Astronauts talk to each other by radio during space walks. With nothing at all between them, they simply can't hear one another directly.
. If you stand near a microwave oven, looking at your food, is it dangerous—tissue damage or make you blind?
Properly built and undamaged microwave ovens leak so few microwaves that they aren't dangerous at all. Even if they did leak enough to be in violation of the safety limits, those safety limits are very conservative. While there is no reason to court disaster by holding your face right up to the microwave for hours and hours, it shouldn't hurt you at all.
. Why can you put a can of frozen concentrate juice in the microwave? The metal doesn't spark or burn.
The microwaves in a microwave oven consist of electric and magnetic fields. Since electric fields push on electric charges, microwaves cause electric currents to flow through any metal objects they encounter. These movements of current don't necessarily cause any problems in a microwave oven. In fact, metal objects only cause trouble in the microwave oven when they are so thin or narrow that they can't tolerate the electric currents that flow through them or when they have such sharp ends that electric charges leap off them as sparks. A thin object like a twist-tie can't tolerate the currents and becomes very hot. Its sharp ends also allow charges to leap out into the air as sparks. But the thick, rounded end of a juice concentrate can easily tolerates the currents sent through it by the microwaves and doesn't have the sharp ends needed to send charges into the air as sparks. It doesn't present any problem for the microwave oven.
. How does a "touch lamp" work? — LAM, Enosburg Falls, VT
A touch lamp detects your touch by looking for changes in the electric properties of the lamp's surfaces. It monitors these properties by putting a fluctuating electric charge on them. As electric current flows toward the bulb through the lamp's wires, it passes through an electronic device that places a high frequency (about 60 kHz) alternating current onto those wires. This added current causes the lamp's surfaces to take on a small fluctuating electric charge—first positive, then negative, then positive, over and over again. This surface charging involves electrostatic forces, which extend long distances between charged objects, and occurs even though the lamp's surfaces aren't directly connected to the lamp's wires. The more surface the lamp has, the more easily it can hold that electric charge—the greater the lamp's electric capacitance.
When you plug the lamp in, the electronic device uses its fluctuating charge to determine how easy it is to add or subtract charge from the lamp's surfaces. In other words, it measures the lamp's capacitance. It then begins to look for changes in that capacitance. When you touch the lamp, or even come close to its surfaces, your body effectively adds to the lamp's surface and its capacitance increases significantly. The electronic device detects this increase in capacitance and switches the lamp's state from on to off or from off to on. The fact that you don't have to touch the lamp to affect its capacitance means that a touch lamp can have insulating paint on its metal surfaces yet still detect your touch. You can also buy touch lamp modules that plug into the wall and turn the lamp that's connected to them into a touch lamp. These modules are so sensitive to capacitance changes in the lamp that you can trigger them just by touching the lamp cord.