. What happens to water in space? — DZ, Illinois
That depends on the water's temperature. At extremely low temperatures, ice remains stable indefinitely. That's why comets that are as old as the solar system have been able to hold on to their water despite having almost no gravity. But at more moderate temperatures, ice and water both slowly lose water molecules. These water molecules evaporate (or sublime, in the case of ice) and drift off into space. Because there's no air pressure in space to prevent evaporation from occurring inside the body of water, water will actually boil at any temperature. That's what boiling is: evaporation into steam bubbles located inside the water. Atmospheric pressure normally smashes these bubbles as long as the water temperature is below 212° F (100° C), but in empty space the bubbles form without opposition at any temperature.
. When ice placed in water melts, does the overall volume of water increase, stay the same, or decrease? — AB, Riverside, CA
The volume decreases. That's because ice at 32° F (0° C) is less dense than water at that same temperature. As the ice melts to form water, the density of its molecules increases and the overall volume of material decreases. This situation, in which the solid form of a material is less dense than the liquid form of that material, is virtually unique in nature and explains why ice floats on water.
. What is an electron and what keeps its mass and charge together so that when the mass moves, the charge moves with it? — WG, Calgary, Canada
An electron is a fundamental particle that has as two of its attributes, a mass and an electric charge. Because the electron appears to be structureless, it has no size and it wouldn't make sense for its mass to be located at a distance from its charge. With a less fundamental particle such as a proton, the charge and mass can be somewhat spread out and displaced so that the charge and mass can move slightly independently. Still, even in the case of a proton there are effects that keep the mass from getting far away from the charge.
. How does electricity travel through wires?
When the atoms that make up a metal assemble together, some of their electrons become delocalized—they stop associating with specific atoms and can move throughout the overall metal. Most importantly, these mobile electrons can respond to the presence of electric fields and electric forces by accelerating and traveling through the metal. When you turn on a flashlight, you are creating a system in which positive charges on one terminal of the battery and negative charges on the other terminal can begin to push electrons through the flashlight's wires. The mobile electrons in those wires are negatively charged and they accelerate toward the positive terminal of the battery. New electrons from the negative terminal of the battery replace the departing electrons and soon a steady flow of electrons through the flashlight is established.
. How do scientists measure the speed of light? — DZ, Illinois
There are many possible methods for measuring the speed of light, but the classic technique is easiest to describe. In this method, a rapidly spinning mirror is used to direct a beam of light down a long pipe toward a stationary mirror at the end of that pipe. The first mirror is spinning in such a way that the beam it reflects sweeps across the pipe and can only strike the second mirror during that brief moment when the first mirror is perfectly aligned to direct the light down the pipe. A scientist then looks into the spinning mirror to observe the flash of light that returns from the second mirror. Because it takes a small but finite amount of time for the light to travel back and forth through the pipe, the spinning mirror will have turned a little between the moment when it sent the beam of light toward the far mirror and the moment when that beam of light returns to the spinning mirror. By studying the angle at which the reflected beam leaves the spinning mirror and by knowing how quickly the mirror is spinning, the scientist can determine the speed of light.
However, something has changed since those sorts of measurements were done: the speed of light is now a defined constant. It isn't measured any more—it's simply defined to be 299,792,458 meters per second. The second is defined in a similar manner—as 9,192,631,770 periods of a particular microwave emission from the cesium-133 atom. Because of these two definitions, an experiment that "measures the speed of light" is now used to determine the length of the meter.
. I have seen some new 48" fluorescent tubes rated at 25W compared to the standard 40W. I was told I could use these in my existing fixtures without doing anything to the ballast. What effect will replacing a 40W bulb with these 25W bulbs have on my fixtures and ballasts? - ST
I would guess that the lower wattage tubes will work fine in your existing fixtures, but I am not expert enough to be certain. The 25W tube itself is evidently built so that a smaller current flows through it than through a normal 40W tubes when the two are exposed to similar voltages. The ballast's job is to prevent a catastrophic rise in that current by adjusting the voltage across the tube dynamically during each half cycle of the power line and to keep the tube operating even as a half cycle is coming to an end. Although the 25W tube will draw less current than the ballast expects, the ballast should behave pretty well. I would expect that the tube designers have anticipated this situation and have built the tube to operate with the standard ballast. If a reader knows better, please let me know.
. Why is CD audio better than that of a cassette? — MK, Baltimore, MD
CD audio is recorded in a digital form—as a series of numerical pressure measurements. This digital recording is a very accurate representation of the air pressure fluctuations associated with the original sounds that arrived at the microphones. During playback, these air pressure measurements are read from the CD and the original air pressure fluctuations are recreated by the speakers. While there are imperfections in the whole process of measuring air pressure fluctuations and recreating those fluctuations, the CD itself doesn't introduce any imperfections—the information read from the CD during playback is absolutely identical to the information that was recorded on the CD at the manufacturer's plant.
The same isn't true of analog recording on a cassette tape. Cassette audio is recorded in an analog form—as magnetizations of the tape surface that are proportional to the air pressure fluctuations associated with the original sounds. During playback, these magnetizations of the tape are analyzed and used to recreate the sounds. But the tape itself introduces imperfections in the reproduced sound. The information read from the tape during playback isn't quite the same as the information that was recorded on the tape at the manufacturer's plant. The tape isn't perfect and the sound that's reproduced by a tape player isn't quite the sound that was originally recorded.
. How does a violin create sound? Is the sound only made by the strings or is it made with the help of the violin's shape and structure? — RH, North York, Ontario
While the vibration of the strings is ultimately responsible for the sounds a violin emits, it is the body of the violin that emits most of that sound. Strings are very poor emitters of sound because they aren't able to push on the air effectively. When the string moves back and forth through the air, the air simply flows around it to the other side. So instead of compressing and rarefying the air, as it must do in order to produce sound waves, the string just stirs the air around. But the bridge of the violin rocks back and forth as the strings' vibrate and it conveys this motion to the belly of the violin. The belly moves in and out, compressing and rarefying the air and doing a fine job of producing sound.
. How does a laser printer work? What is the toner made of and how does the printer place it on the paper? — PC, Brussels, Belgium
A laser printer uses the light beam from a laser to control the placement of electric charges on a photoconductor surface. A photoconductor is a material that only conducts electricity when exposed to light, so that charges can move through the photoconductor only when the laser beam hits it. The printer uses a corona discharge to place charges on the darkened photoconductor and then uses the laser beam to remove charges from certain places. The end result is a pattern of electric charges that's an image of the final print. The toner particles, which are made of black plastic, are given an electric charge so that they cling to the charge image on the photoconductor. This pattern of toner particles is then transferred to electrically charged paper and fused to that paper with heat and pressure.
. How dangerous is the radiation from high voltage power lines? - K
Probably not very dangerous. The radiation itself is so weak that it can't cause significant heating in your body (as the microwaves used in diathermy treatment do) and so low frequency that it can't do chemical damage (as the X-rays from a CT scan do). The only possible source of trouble is the small electric and magnetic fields from the power lines and there is still no credible evidence that these affect biological tissue. Moreover, there are sound physical arguments why those fields should not be able to affect biological tissue. Only in rare cases of an organ that is devoted to sensing magnetic fields (e.g., in migratory birds) is there any reasonable interaction between tissue and small magnetic fields.