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Page 98 of 160 (1595 Questions and Answers)

971. What types of gas are used in light bulbs and how do their effects differ? — SF, Westfield, NJ
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The glass envelope of an incandescent bulb can't contain air because tungsten is flammable when hot and would burn up if there were oxygen present around it. One of Thomas Edison's main contributions to the development of such bulbs was learning how to extract all the air from the bulb. But a bulb that contains no gas won't work well because tungsten sublimes at high temperatures—its atoms evaporate directly from solid to gas. If there were no gas in the bulb, every tungsten atom that left the filament would fly unimpeded all the way to the glass wall of the bulb and then stick there forever. While there are some incandescent bulbs that operate with a vacuum inside, most common incandescent lamps contain a small amount of argon and nitrogen gases.

Argon and nitrogen are chemically inert, so that the tungsten filament can't burn in the argon and nitrogen, and each argon atom or nitrogen molecule is massive enough that when a tungsten atom that's trying to leave the filament hits it, that tungsten atom may rebound back onto the filament. The argon and nitrogen gases thus prolong the life of the filament. Unfortunately, these gases also convey heat away from the filament via convection. You can see evidence of this convection as a dark spot of tungsten atoms that accumulate at the top of the bulb. That black smudge consists of tungsten atoms that didn't return to the filament and were swept upward as the hot argon and nitrogen gases rose.

However, some premium light bulbs contain krypton gas rather than argon gas. Like argon, krypton is chemically inert. But a krypton atom is more massive than an argon atom, making it more effective at bouncing tungsten atoms back toward the filament after they sublime. Krypton gas is also a poorer conductor of heat than argon gas, so that it allows the filament to convert its power more efficiently into visible light. Unfortunately, krypton is a rare constituent of our atmosphere and very expensive. That's why it's only used in premium light bulbs, together with some nitrogen gas.

Incidentally, the filament in many incandescent bulbs is treated with a small amount of a phosphorus-based "getter" that reacts with any residual oxygen that may be in the bulb the first time the filament becomes hot. That's how the manufacturer ensures that there will be no oxygen in the bulb for the tungsten filament to react with.


972. I know that adding salt to water will raise its boiling point, which would seem to imply that it would take longer to come to a boil. But does it take longer? As a cook I've always been told to add a little salt to the water to bring it to a boil faster. It seems to work or is that just the power of suggestion? If it does boil faster, why does it? — ND, Ashland, OR
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I think that power of suggestion is at work here. Salt water boils at a higher temperature than pure water. Thus if you set two identical pots of water, one salty and one pure, on burners and heat them at equal rates, the pure water will reach its boiling temperature first.

However, water boils more vigorously when it contains impurities that can nucleate bubbles of water vapor. Just before the water in a pot reaches a full boil, its temperature is often nonuniform and there are some regions that are boiling while others aren't. The edges and corners of crystals are particularly good at nucleating bubbles, so that tossing salt grains into such nearly boiling water will encourage its hot regions to boil more vigorously, at least until those salt grains dissolve away. The appearance of bubbles makes you think the water is at a full boil when it really isn't.


973. Are some frequencies of sound more directional than others and, if so, why? — BKZ, Dayton, OH
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In open air, sound waves travel in straight lines regardless of frequency or wavelength. But low frequency (long wavelength) sounds don't fit well in confined spaces and have less directional character to them. That's why you only need one subwoofer for a sound system—you can't hear where the lowest frequency sounds are coming from any way. Higher frequency sounds remain relatively directional, even in confined spaces. The same effects apply to electromagnetic waves—in confined spaces, long wavelength radio waves are effectively less directional than short wavelength light waves.

974. How does a stereo convert 110 volt electric current into the positive and negative current that is sent to power the speakers? — JF
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A stereo contains a power supply that converts 110-volt alternating current into lower-voltage direct current. This direct current is ultimately when powers the speakers. The stereo's power supply first lowers the voltage with the help of a transformer. Alternating current from the power line flows back and forth through a coil of wire in this transformer, the primary coil, and causes that coil to become magnetic. Since the coil's magnetism reverses 120 times a second (60 full cycles of reversal each second), along with the alternating current, it produces an electric field—changing magnetic fields always produce electric fields. This electric field pushes current through a second coil of wire in the transformer, the secondary coil, and transfers power to that current. There are fewer turns of wire in the secondary coil than in the primary coil, so charges flowing in the secondary coil never reach the full 120 volts of the primary coil. Instead, more current flows in the secondary coil than in the primary coil, but that secondary current involves less energy per charge—less voltage. In this manner, power is transferred from a modest current of high voltage charges in the primary coil to a large current of low voltage charges in the secondary coil.

Having used the transformer to produce lower voltage alternating current, the power supply than converts this alternating current into direct current with the help of four diodes and some capacitors. Diodes are one-way devices for electric current and, with four of them, it's possible to arrange it so that the alternating current leaving the transformer always flows in the same direction through the circuit beyond the diodes. The diodes act as switches, always directing the current in the same direction around the rest of the circuit. The capacitors are added to this circuit to store separated electric charge for the times while the alternating current is reversing and the diodes receive no current from the transformer. The capacitors store separated charge while there is plenty of it coming from the transformer and provide current while the alternating current is reversing. Overall, the stereo's power supply is a steady source of direct current.


975. If a microscope's resolution is limited by the wavelength of the light it uses, why don't we use very short wavelength light instead of electrons or other particles for studying very small objects? — SJ, Philadelphia, PA
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Ultraviolet light isused in microscopy to achieve higher resolution than can be obtained with visible microscopes. But beyond ultraviolet light comes X-rays and it's difficult to build imaging optics for X-rays. There are some X-ray microscopes, but they aren't nearly as common and practical as electron microscopes. The electrons in electron microscopes have very short wavelengths (atomic and subatomic length scales) and yet electron optics are easy to build. So while very short wavelength electromagnetic waves can be made, they're just not practical for microscopy.

976. Why can't light resolve details smaller than about half its wavelength? — SJ, Philadelphia, PA
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Suppose that you have a white card with what appears to be a black line on it. That line might actually be two very closely spaced lines; you're not sure. To find out, you focus a beam of light to the smallest possible spot and then move this tiny spot of light across the line. You realize that if there are two separate lines on the card, then the spot of light should cross first one line and then the other, and you should see two changes in the reflected light rather than just one.

It turns out that, however, that no matter how hard you try you can't focus the light to a spot much smaller than the wavelength of the light. An equivalent problem would occur if you tried to use water waves to create a narrow spike of water above the surface—no matter how you worked with the water waves, you would be unable to make them to merge together into a spike that's much narrower than the wavelength of the water waves. Because of his limitation, your spot of light can't be much smaller than the wavelength of light and you can't distinguish between one line or two if those lines are much closer than a wavelength of the light you're using. Since visible light has a wavelength of 400 nanometers or more, you can't use it to resolve details much smaller than 400 nanometers wide.

Actually, there is an exception to this general rule—near-field scanning optical microscopy or NSOM uses light emerging from the tiny tip of a glass fiber to resolve details far smaller than the light's wavelength. In NSOM, the resolution is determined by the tip size and not the light's wavelength.


977. Since spent fuel rods from propulsion reactors are still quite hot would it be possible to harness the heat produced for energy needs? It seems like a possible source, and a waste not to harness what we can. — SS, Lakewood, CO
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While the radioactive decays from spent nuclear fuel rods continue to produce thermal energy, the amount of energy released each second isn't enough to make it cost effective to use that energy. Since the power output from a spent fuel rod would only be in the watt range, it wouldn't justify the hazardous job of trying to extract that power without encountering the radiation. Furthermore, the laws of thermodynamics make it much harder to use heat from a warm object than heat from a hot object and spent fuel rods would at best be warm objects.

978. How do different airplane wings help or hurt the airplane?
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An airplane wing's main job is to generate a large upward lift force while experiencing as little backward drag force as possible. To obtain the lift force, a wing must make the air flowing over its top to speed up while the air flowing under its bottom slows down. The wing must also avoid introducing turbulence into the main airstream because that will result in severe pressure drag. There are many cross sectional shapes for wings that achieve both large lift forces and small drag forces, but some are better suited to each style of airplane than others. For example, private propeller-driven planes travel relatively slowly and need broad, highly curved wings to obtain enough lift to support them. In contrast, commercial jets have much narrower, less curved wings because they travel faster and produce lift more easily. But during takeoff and landing, even jets need to increase the curvatures of their wings. That's why many jets have slats and flaps that extend from the leading and trailing edges of their wings to increase the wings' breadths and curvatures for low-speed flight.

979. What is the frequency, amplitude, wavelength, etc. of a sound wave at the sound barrier? — KT, Ocean Springs, MS
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The sound barrier is something of a myth that dates to the early days of transonic flight. As early airplanes approached the speed of sound, they suffered various flight instabilities—a significant rise in air drag and a tendency for supersonic shock waves to interfere with the operations of control surfaces. Exceeding the speed of sound appeared problematic at the time and the expression "the sound barrier" came into common use. However, there is no real sound barrier. Once Yeager had exceed the speed of sound in an experimental plane, it became clear that the speed of sound was not a firm barrier.

However, there is one peculiar thing that does happen once a plane has exceeded the speed of sound. You can no longer hear the plane coming because it is outrunning its own sound waves. Instead of having its sound spread out in front of it, the plane has its sound swept back in a cone behind it. The edges of this cone are a shock wave and you experience a sudden pressure rise as this cone passes across you—you hear a sonic boom. A supersonic plane carries this conical shock wave with it at all times and everyone hears a sonic boom as this shock wave sweeps across them. What you should remember is that the sonic boom doesn't occur when the plane "breaks the sound barrier"; the sonic boom is a continuous feature of a supersonic plane that you hear as its shockwave passes you by.


980. What is a digital display and how does it work?
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The term "digital display" usually refers to a system that reports the value of a physical quantity in numerical form. A digital watch display is a good example. The physical quantity it reports is time and it makes its report in the form of hours, minutes, and second—all in numerical form. In a digital watch, the display makes use of liquid crystals that are sensitive to electric fields. When you look at the display, you are actually looking through a layer of polarizing filter, some transparent electric wires, and a layer of liquid crystals. Liquid crystals are liquids that contain molecules that naturally orient themselves relative to one another. In the display, these liquid crystals adopt different orientations when they are exposed to electric fields than when they're not exposed to such fields. This electrically altered orientation affects their optical properties and causes them to appear dark when viewed through the polarizing filter. The watch can control the appearance of each segment of its digital display by the pattern of electric charge on its transparent wires. Since it takes very little energy to change the orientation of the liquid crystals, the watch uses almost no power for its display and can operate for years on a button battery.

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