Month: December 1996

Why does water boil at lower or higher temperatures under varying atmospheric pr…

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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.

How does heat conduct through different materials? – B

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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.

How does a gravity powered water pump work?

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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.

Does water drain in the opposite direction in the southern hemisphere? – TL

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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 radar absorbent materials work. How effective is stealth technology?

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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.

What is the “memory effect” of a NiCad (nickel-cadmium) battery? Is it reversi…

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What is the “memory effect” of a NiCad (nickel-cadmium) battery? Is it reversible or minimizable? – MF

NiCad batteries are more rechargeable than most batteries because the chemicals that power NiCad batteries remain solid throughout the discharge cycle. The chemicals in most other batteries, including alkaline batteries, go into solution or otherwise change shape during the discharge cycle so that it difficult to reconstruct the original battery electrodes during recharging.

Unfortunately, the two solid electrodes in a NiCad battery are damaged by repeated charging and discharging. These electrodes work best when they are both fine powders (the positive electrode is nickel hydroxide powder and the negative electrode is cadmium metal powder). With repeated use, the powder particles grow larger and larger and they stop contributing to the battery’s power. “Memory” appears during the discharge cycle when all the useful small particles have been used up and only the undesirable large particles remain. Repeated charging and partial discharging tends to convert many of the small particles into large particles. You can improve the battery by fully discharging it before recharging it, presumably because this deep discharge breaks up the larger particles so that the battery contains mostly small particles once again.

What is the most explosive and energy releasing combination of chemicals?

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What is the most explosive and energy releasing combination of chemicals? — RC, Chapman, Australia

A mixture of 1 part hydrogen and 19 parts fluorine by weight is the most energetic possible mixture of chemicals, releasing approximately 13,600 joules of energy per gram. The next most potent mixture is 8 parts oxygen and 1 part hydrogen by weight, releasing approximately 13,400 joules of energy per gram. Because fluorine is such a vigorous oxidizer that tends to cause fires, it isn’t practical for rocket propulsion. The hydrogen/oxygen mixture is the basis for the Single Stage to Orbit rockets that are currently being developed. — Thanks to Gary V. Lorenz at NASA for help on this question.

What are the risks of occupational exposure to “black” fluorescent lamps? – MB

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What are the risks of occupational exposure to “black” fluorescent lamps? – MB

By “black” lamps, you mean ultraviolet lamps. Since ultraviolet light is able to cause chemical damage to biological tissue, long-term exposure to this light isn’t so good. How much risk there is depends on how much ultraviolet light they produce and how near you are to them. Sunlight contains a considerable amount of ultraviolet, so long exposure to sunlight burns and ages skin. The photons of ultraviolet light contain enough energy to cause changes in molecules and thus upset the cellular machinery that keeps us healthy. Ultraviolet lamps will do the same thing, given enough intensity and time.

I have heard that diode lasers won’t work in ring laser gyroscopes because these…

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I have heard that diode lasers won’t work in ring laser gyroscopes because these lasers are not single frequency. If this is true, will a prism or a diffraction grating isolate one of the frequencies? – M

While most diode lasers operate at several frequencies simultaneously, it’s possible to make lasers that function at only one frequency. In fact, such “single mode” diode lasers are extremely stable light sources and the basis for much current research in optical science. For example, the recent observations of Bose condensation in vapors of alkali metal atoms were made with the help of single mode diode lasers.

The phrase “single mode” refers to a single longitudinal wave that travels back and forth through the device while it is operating. This single wave has one frequency and one wavelength. It is selected from other possible waves through the use of interference effects. For the wave to be stable inside the laser cavity (the laser is bounded at each end by a mirror, thus forming an optical cavity), the cavity’s length must be an integer or half integer multiple of the light’s wavelength. While that criterion alone will allow several possible waves to form, coupling a second cavity to this laser cavity further restricts the wave so that only a single wave can operate inside the laser. The diode laser will then have only a single mode of operation and will emit a single frequency of light.

How is the xerographic copying process related to laser and led printers? – BG

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How is the xerographic copying process related to laser and led printers? – BG

The same basic printing process is used in both xerographic copiers and laser or led printers. In all cases, a charge image is formed on the surface of a photoconductor and this pattern of electric charge attracts a pattern of colored plastic powder. The powder is then transferred to paper and melted or pressed into the paper’s surface to form a permanent print.

The main difference between a copier and a printer is in the source of light used to produce the charge image. In a copier, lenses and mirrors are used to form a real image of the original document on the surface of the photoconductor. Wherever light from the white portions of the document strikes the photoconductor, the photoconductor becomes an electric conductor and charge is able to move. The pattern of light then becomes a pattern of charge—a charge image.

In a printer, a laser or an array of light emitting diodes is used to form the pattern of light on the surface of the photoconductor. Wherever the light strikes the photoconductor, charge is again able to move about. Dot by dot or row by row, the charge image takes shape. The pattern of charge that’s written on the surface of the photoconductor eventually becomes the printing itself.