Author: Lou Bloomfield

Why is CD audio better than that of a cassette?

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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.

I have seen some new 48″ fluorescent tubes rated at 25W compared to the standar…

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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.

How do scientists measure the speed of light?

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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.

How does electricity travel through wires?

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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.

What is an electron and what keeps its mass and charge together so that when the…

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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 is chlorine gas used to disinfect water at treatment plants? – KM

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How is chlorine gas used to disinfect water at treatment plants? – KM

Chlorine molecules (Cl2) dissolve easily in water, where they react with water molecules to form hypochlorous acid (HOCl), chlorine ions (Cl–) and hydrogen ions (H+). Hypochlorous acid is a weak acid that partially dissociates into hydrogen ions (H+) and hypochlorite ions (OCl–). Studies have shown that it’s predominantly the hypochlorous acid molecules and the hypochlorite ions that disable and kill microorganisms. These molecules and ions diffuse onto and into the microorganisms and oxidize important biological components, such as the protein coats of some viruses, key enzymes in many bacteria, and the genetic materials in both bacteria and viruses. — Thanks to J. Symons for pointing out this mechanism to me and providing me with detailed reference materials.

Why is it bad to put metal in a microwave oven? – OR

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Why is it bad to put metal in a microwave oven? – OR

It isn’t necessarily bad to put metal in a microwave oven, but it can cause cooking problems or other trouble. Microwaves cause currents to flow in metals. In a thick piece of metal, these currents won’t cause problems for the metal. However, in thin pieces of metal, the currents may heat the metal hot enough to cause a fire. Metallic decorations on fine porcelain tend to become hot enough to damage the porcelain. But even thick pieces of metal can cause problems because they tend to reflect the microwaves. That may cause cooking problems for the food nearby. For example, a potato wrapped in aluminum foil won’t cook at all in a microwave oven because the foil will reflect the microwaves. The currents flowing in the metal can also produce sparks, particularly at sharp points, and these sparks can cause fires. In general, smooth and thick metallic objects such as spoons aren’t a problem, but sharp or thin metallic objects such as pins or metal twist-ties are.

What is the black holey stuff on the doors of microwave ovens? Is it for looks, …

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What is the black holey stuff on the doors of microwave ovens? Is it for looks, protection, or what? – K

The black holey stuff on the window of a microwave oven is a metal shield that keeps the microwaves inside the cooking chamber. Because the holes in this metal sheeting are so much smaller than the wavelengths of the microwaves (about 12 cm), the microwaves respond to the sheeting as though it were solid metal and they reflect almost perfectly. By keeping the microwaves inside the oven, this sheeting speeds cooking and protects you from the microwaves.

What is the principle of the Trinitron Sony TV system?

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What is the principle of the Trinitron Sony TV system? — JPD, Spiennes, Belgium

To form a color image, a color television illuminates a dense pattern of tiny spots—some red, some green, and some blue. By mixing various amounts of these three primary colors of light, the color television can make us perceive any color. But the television must control the amounts of these three colors at each spot on the screen, a very difficult task. A typical color television does this by shining three separate beams of electrons through a mask with holes in it and onto a screen that’s covered with tiny phosphor spots. Because the three beams approach the mask at different angles, they illuminate different portions of the screen after passing through the holes. Thus the “blue” beam only illuminates spots of blue phosphor, the “red” beam illuminates red spots, and the “green” beam illuminates green spots.

However, the Sony Trinitron system uses a line mask rather than one containing holes and the phosphors are coated onto the screen in stripes rather than spots. Again, three separate electron beams are used but they now illuminate specific stripes of phosphor rather than spots of phosphor. The advantage of the stripe approach is that there is more active phosphor on the screen (fewer dark places between spots) so the image is brighter.

Is it possible to sense when a person touches a car, even if the car is painted?…

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Is it possible to sense when a person touches a car, even if the car is painted? – AW

Yes. I wouldn’t try to detect mechanical contact, because you’d have trouble differentiating between forces exerted on the car by a hand and those exerted on it by sound waves. But you can tell whether a conducting object (such as a person) is near the car by looking at the car’s electric properties. If you were to send electric charge on and off the car rapidly with a source of high-frequency alternating current, you would find that the amount of charge that flowed on or off the car during each cycle would change as the person’s hand approached the car. That’s because the charges on the car would push or pull on charges in the person’s hand and the charges in the person’s hand would move. In effect, the person’s hand would make the car “larger” and it would draw more charge from your current source. Even if the person didn’t touch the car, the nearness of the hand and car would change the way current flowed on and off the car. Such a change would be easy to detect with laboratory equipment and could probably be made by cheap consumer equipment, too. The only complications would be in not detecting everything—passing cars for example—and in not damaging the device with static discharges. Still, I think all of that could be done.