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How Everything Works  
Page 82 of 160 (1595 Questions and Answers)

811. I have to do an experiment for school on the electromagnetic properties of iron, steel, and aluminum. The only problem is that I am not too sure what I should be testing. Any ideas? — CP, Nassau, Bahamas
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Iron and steel (not stainless) are ferromagnetic metals, meaning that they are intrinsically magnetic. While this magnetism is normally hidden by the formation of millions of tiny, randomly oriented magnetic domains, it becomes apparent when you hold a magnet near the iron or steel: they are attracted! Aluminum has no intrinsic magnetism and is not attracted to a magnet. There are far more non-magnetic metals than magnetic ones. Why don't you try to see which metals will stick to a magnet. Only the ferromagnetic ones will. Even common stainless steel is non-ferromagnetic.

812. What is analog? I hear about digital audio being better than analog, but nobody defines what analog is. — DG, Houston, TX
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In analog audio, the air pressure fluctuations of sound at the microphone are represented by a continuously variable physical quantity such as an electric current, a voltage, or a magnetization. Thus as the air pressure at a tape recorder's microphone rises during one moment of a song, an electric current in the recorder will rise and a region of a magnetic tape surface will become particularly strongly magnetized in a particular direction. Overall, each value of air pressure is converted to a particular value of the physical quantity.

The problem with analog recording is that when the sound is recreated, any defect in the physical quantity representing air pressure will lead to an imperfection in the reproduced sound. For example, if the magnetization of the recording tape has changed slightly due to how it was stored, the sound that the tape recorder produces won't be exactly the same as the sound that the microphone heard. Digital recording avoids this problem by recording the information as bits. The physical quantity such as magnetization is representing bits (which take only two possible values) rather than the air pressure itself (which can take a broad range of values). Minor changes in the physical quantity representing these bits won't change the bits. Thus imperfections in the recording or playback process won't affect the sound quality.


813. If I want to create a radio controlled device, how do I make sure it does not create interference with other devices or receive interference. How does digital RF work and does it stop interference problems? — KG, New York, NY
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Radio interference occurs whenever two nearby radio transmitters are simultaneously emitting radio waves that overlap in space and frequency. The receivers for these two waves can't tell them apart and end up receiving both at once. This interference is familiar with AM radio, where you can sometime hear two broadcasts at the same time. With FM radio, the receivers are clever enough to distinguish one radio wave from another, but they can't determine which broadcast they're supposed to follow. Instead, they lock onto whichever wave is strongest and will often flip back and forth from one station to the other as their signal strengths fluctuate.

The only way to avoid interference completely is to choose a radio frequency that no one else nearby is using. That way your transmission is certain to be stronger than any other at the same frequency and your receiver will follow only your broadcast. If you have no choice but to share a particular frequency, then you must use some encoding scheme such as digital transmission so that your receiver can tell when it's receiving a broadcast from your transmitter and not from some other transmitter. Your receiver looks for your personal encoding scheme and won't respond to that of some other transmitter. However, if that other transmitter is strong enough, it will probably prevent your receiver from detecting your transmission. That trick of overwhelming a receiver with a second transmission is the principle behind jamming of a radio transmission.


814. How can I check the magnetron in a home microwave oven? I have checked the HV (high voltage) transformer, the rectifier, and capacitor and all are OK. Does the magnetron output decrease with age? The oven has a hum that is much louder than normal. — AA, Ontario, CA
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While I have only a little experience repairing microwave ovens, I can make reasonable guesses. The loud hum you hear is probably an indication that something is overloading the power transformer. That suggests that the diode, capacitor, or magnetron are bad. If you have checked the first two carefully, at full operating voltage, and found no problems, then I would suspect the magnetron. I have been told by a reader that magnetrons usually fail by shorting out, the result of electromigration of the filament material. The tube would then draw excessive currents from the high voltage transformer. That has probably happened in your case. Still, free advice like mine is only worth what you've paid for it. I'd suggest you consult a local repairperson, who has test equipment that can pinpoint the problem in seconds.

815. How does a dishwasher machine work? — WW, Bochum, Germany
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A dishwasher is really a number of simple machines that work together to clean dishes. These machines are controlled by a mechanical or electronic timer and include an electrically operated water valve, a water level sensor, one or two water pumps, a thermostat, an electric heating element, one or more rotating spray nozzles, and a fan.

The cycle begins when the timer sends electric current through a coil of wire in the water valve, making that coil magnetic and pulling the water valve into its open position. Water flows then flows from the high pressure in the water line to the atmospheric pressure in the cleaning chamber. When the water sensor detects that the dishwasher is adequately filled, it shuts off current to the valve and the valve closes.

The thermostat measures the water temperature and may delay the start of the cycle if the water is too cool. If so, it directs electric current through the heating element, where that current's energy is converted into thermal energy and transferred to the water. When the water is hot enough, the cycle continues.

During the cleaning cycle, one or more pumps operate. They add energy to the water and increase its pressure. This high-pressure water flows slowly to the rotating nozzles and then accelerates to high speeds as it enters the narrow openings and sprays out into the low-pressure cleaning chamber. As the high-speed water collides with the dishes and slows down, its pressure rises again and begins to exert substantial forces on the food particles. The food particles are pushed off the dishes and fall into the bottom of the dishwasher. Soap added to the cleaning water forms tiny spherical objects called micelles that trap and carry away fats that would otherwise not mix with water. At the end of the cycle, the water, food particles, and fat-filled soap micelles are pumped down the drain.

The cleaning cycle may repeat with fresh water and is then followed by a rinse. A soap-like surfactant may be added to the rinse water to lower its surface tension and prevent it from beading up on the dishes. When the pumps have removed the last of the rinse water, a fan begins to blow air over the dishes. The heating element may heat this air to assist evaporation. The water molecules leave the surfaces of the dishes and become gaseous water vapor. The dishes are left clean and dry.


816. How does a rotary phone switching system distinguish between the off-hook signal and the dialing signals, one through ten? - B
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It doesn't. When you dial a rotary phone, it briefly hangs itself up one time for every number on the dial. Thus if you dial a "5", it hangs itself up briefly 5 times. In fact, you can dial the phone by tapping the switchhook briefly one time for every number. For example, if you want to dial a "5", tap the switchhook (hang up the phone) briefly 5 times very quickly. It takes some skill, but you can "dial" just fine without ever touching the dial. It used to be that people installed key locks on the rotary dial to prevent unauthorized use of the telephone. Unfortunately, this action didn't prevent someone with a nimble hand from dialing with the switchhook.

817. When I read of scientists discovering galaxies "on the edge of the universe," perhaps 15 billion light years away, I wonder if they are including the distance the objects must have traveled in the time it took for the light to reach their telescopes. Very distant objects are said to be receding from any other point in space at a higher rate than closer objects. If a galaxy is discovered 15 billion light years away today, the light left that galaxy 15 billion years ago while receding at a high rate. Where is it today, really? Twice as far away? — DK, Missouri City, TX
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This seemingly simple question has a surprisingly complicated answer. You might expect that if the earth and one of these distant galaxies had been very near one another at the creation of the universe and had both been moving away from one another at almost the speed of light, that after 15 billion years each would have moved almost 15 billion light years in opposite directions and would thus be separated by almost 30 billion light years. That's not the case. That simple view ignores the important effects of special relativity on rapidly moving objects.

To understand these effects, suppose that there was an observer who was stationary at the creation and watched the earth and galaxy head off in opposite directions at almost the speed of light. From that observer's perspective, the two objects are heading away from one another at almost twice the speed of light. After 15 billion years, this observer sees the galaxy as almost 30 billion light years away from the earth.

Now suppose that there was another observer who was on the earth at the creation. From this person's perspective, the galaxy recedes from the earth at almost the speed of light, but no more. Nothing can move faster than speed of light! After 15 billion years, this observer sees galaxy as almost 15 billion light years away from the earth.

These two observations don't seem to agree. The problem lies in how the two observers perceive time and space. According to special relativity, observers who are moving relative to one another don't perceive time and space in the same way. Their perceptions will be so different that they will not even agree about just when 15 billion years has passed.

With this long introduction, here is the answer to your question: no distant galaxy in the observable universe can ever be farther from us than the distance light has traveled since the creation of the universe. Since that creation was about 15 billion years ago, the most distant possible galaxy is almost 15 billion light years away.


818. Why doesn't a helium balloon pop when it reaches the ceiling?
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The buoyant force lifts the helium balloon upward—the denser air flows downward to fill the space vacated as the balloon is squeezed upward. When the balloon finally reaches the ceiling, the ceiling exerts a downward force on the balloon and prevents it from rising further. But the force the ceiling exerts on the balloon's skin is gentle enough and spread out enough that it doesn't injure the rubber. The balloon simply comes to a stop and remains suspended until enough helium diffuses out of the balloon to cause it to descend.

819. How does air pressure affect the distance a soccer ball can be kicked? — SR, Pittsburgh, PA
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In general, the greater the air pressure, the greater the air resistance. As the soccer ball moves through the air, the air in front of it experiences a rise in air pressure and pushes the ball in the direction opposite its motion. While there are various other changes in air pressure around the ball's surface, this rising pressure in front of the ball remains largely unbalanced and it slows the ball down. The higher the air pressure was to start with, the greater its rise in front of the ball and the stronger the backward push of air resistance. Thus if you were to play soccer in the Rocky Mountains, where the air pressure is much less, you'd be able to kick the ball significantly farther.

820. How does a heat lamp work and could it be harmful to the eyes of pets from extended exposure? — DM, Osceola, IA
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A heat lamp is much like a normal incandescent lamp, except that the heat lamp's large filament operates at a much lower temperature. Because of this lower temperature, the filament emits relatively little visible light. Instead, it emits mostly invisible infrared light. While you can't see infrared light, you can feel it as heat. Looking at a heat lamp is no more dangerous than looking at the glowing coals in a fireplace. Their thermal radiation heats your skin and the surfaces of your eyes, and is likely to make you uncomfortable enough to turn away before it causes real damage. In contrast, ultraviolet light from a sunlamp can injure your skin and eyes without causing any immediate pain—it's only much later that you feel the sunburn on your skin and corneas. That's why a heat lamp is relatively safe while a sunlamp is not.

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