Month: October 1996

Does the volume in the cooking chamber of a microwave oven affect the rate at wh…

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Does the volume in the cooking chamber of a microwave oven affect the rate at which it cooks the food? In other words, which cooks faster, a small microwave oven or a large one? – RP

The size of a microwave oven’s cooking chamber should have little or no effect on how quickly it cooks food. The oven’s magnetron tube delivers a certain amount of microwave power to the cooking chamber and virtually all of that power will eventually be absorbed in the food. It may take a few moments longer for a large cooking chamber to fill with microwaves when you first start the oven, but soon the food inside it will be exposed to the same intensity of microwaves as food cooking inside a smaller microwave oven with a similar magnetron power.

On the other hand, the magnetron’s power does affect cooking speed so that an oven with a more powerful magnetron will cook food faster than one with a less powerful magnetron. The speed of cooking in a microwave oven also depends on how much food it contains because the food shares the microwave power. In general, doubling the amount of food in the microwave doubles the cooking time.

What is the “sound barrier”? – EL

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What is the “sound barrier”? – EL

The “sound barrier” is more a psychological barrier than a real impediment. In the early days of high-speed flight, there was concern that a plane flying at or beyond the speed of sound in air would encounter unanticipated phenomena that would rip it apart. However, when Chuck Yeager finally did exceed the speed of sound for the first time in 1947, he found the transition from subsonic to supersonic uneventful. The only way that he could tell he was traveling faster than the speed of sound was with the help of his instruments.

What is a shockwave and a sonic boom? – EL

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What is a shockwave and a sonic boom? – EL

A plane that is flying faster than the speed of sound is outrunning its own sound. As a result, its sound spreads out behind it as a conical structure, with the plane located at the apex of that cone. This cone moves along with the plane. Since the planes sound is all contained inside the cone, you can’t hear the plane until the cone passes by you. When the edge of the cone does pass you, you hear a great deal of sound all at once. In fact, there is a pressure jump right at the surface of the cone (sound and pressure are closely related) and this cone itself is a shockwave. As the shockwave (or cone surface) passes you, you hear a loud booming sound, a “sonic boom”. Note that the sonic boom occurs when the shockwave passes your ears, not when the plane “breaks the sound barrier”. When you hear the sonic boom depends on where you are relative to the moving plane, so different people hear it at different times.

If airplane cabins are pressurized to provide adequate oxygen for the passengers…

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If airplane cabins are pressurized to provide adequate oxygen for the passengers to breathe, what provides this compressed air? – EL

The air that you breathe inside an airplane is actually pumped into the cabin through the jet engines. The first component of a jet engine is a compressor that takes the low-density air outside and boosts its pressure and density. While most of this air then continues through the engine to the combustion chamber, part of it is diverted to the cabin. But before it can be released into the cabin, the air must be chilled by an air conditioner. That’s because compressing air adds energy to it and raises its temperature. The compressed air leaving the jet engine’s compressor is hot, even though no combustion has taken place yet. So the air is first cooled and then sent into the cabin.

What does the SPF on sun screens mean? – RC

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What does the SPF on sun screens mean? – RC

Sunscreens contain pigments that absorb invisible ultraviolet radiation. While they appear clear and transmit visible light so that you can’t see them when they’re on your skin, sunscreens are almost opaque to ultraviolet light. A sunscreen’s SPF is related to the fraction of ultraviolet light that it absorbs. An SPF of 15 means that a normal layer of this sunscreen on your skin transmits only 1 part in 15 of the ultraviolet light that reaches it from the sun. An SPF of 40 means that a layer of this sunscreen transmits only 1 part in 40 of the ultraviolet light. The true transmission of the sunscreen depends somewhat on how you apply it and how much you apply, so these SPF ratings are only approximate. A sunscreen contains a mixture of dye molecules that transmit visible light but absorb ultraviolet light (and convert its the light’s energy into thermal energy). Most if not all of these dye molecules are artificial organic compounds that have been carefully selected to be non-toxic and non-irritating. The first popular sunscreen contained a compound called PABA that caused skin reactions in many people, but more recent dye choices are less likely to cause skin trouble.

How much natural pressure is around us when we are on the ground? Does this pres…

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How much natural pressure is around us when we are on the ground? Does this pressure decrease in higher places? Why don’t people in aircraft explode because the pressure is lower?

Near sea level, the air around us has a pressure of about 100,000 newtons per square meter or 15 pounds per square inch. That means that each square meter of surface on your body is exposed to an inward force of 100,000 newtons or that each square inch of your body is exposed to an inward force of 15 pounds. Your body is thus exposed to enormous inward forces. However, you don’t notice these forces because your body is composed of solids and liquids that resist compression ferociously. To see that this is so, try to squeeze a sealed bottle of soda or to squash a coin by stepping on it. It’s very hard to shrink the volume of a solid or liquid by squeezing it.

The origin of the large pressure around us is the weight of the atmosphere overhead. The air near you is supporting the weight of several miles or kilometers of air overhead and the weight of this air is squeezing the air down here. When you ascend a mountain, the amount of air overhead decreases and so does the pressure of the air around you. Your body becomes less tightly squeezed by the air around it. However, you don’t explode because releasing the pressure on you doesn’t change your volume very much. Solids and liquids don’t expand very much when the pressure on them is released.

What is a microwave and what does it do?

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What is a microwave and what does it do? — AH, Rochester, MN

A microwave is an electromagnetic wave with a frequency and a wavelength that are intermediate between those of a radio wave and those of light. An electromagnetic wave consists of both an electric field and a magnetic field. These two fields travel together in space and perpetually recreate one another as the wave moves forward at the speed of light. An electric field is a phenomenon that exerts forces on electric charges, while a magnetic field is a phenomenon that exerts forces on magnetic poles. Electric and magnetic fields are intimately connected, so that whenever an electric field changes, it creates a magnetic field and whenever a magnetic field changes, it creates an electric field. By combining a changing electric field and a changing magnetic field, the electromagnetic wave uses their abilities to create one another to form a self-perpetuating entity—the wave’s changing electric field creates its changing magnetic field and its changing magnetic field creates its changing electric field.

If you were to freeze an electromagnetic wave at one instant and look at its structure in space, you would find that its electric and magnetic fields had a periodic spatial structure. Just as a water wave has crests and troughs, an electromagnetic wave has spatial fluctuations in its two fields. The distance between adjacent “crests” in either one of these fields is that wave’s wavelength. Different types of electromagnetic waves have different wavelengths. Radio waves have long wavelengths that range from about 1 meter to hundreds or even thousands of meters and visible light has short wavelengths that range from about 400 billionths of a meter (400 nanometers) to about 750 billionths of a meter (750 nanometers). Microwaves are those electromagnetic waves with wavelengths between 1 millimeter and 1 meter. The microwaves used in microwave cooking have wavelengths of 12.2 centimeters.

Microwaves are often used to carry information in satellite communication and telephone microwave links. Whenever you see a dish antenna (a satellite dish or a communication link dish on a building or tower), you are looking at a microwave system. Astronomers use radio telescopes to look at microwave emissions from celestial objects. Radar bounces microwaves from objects to determine where they are or how fast they’re moving. And microwave ovens use microwaves to add thermal energy to water molecules in order to heat food.

I’ve seen tops that rest with their large parts down but that flip up onto their…

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I’ve seen tops that rest with their large parts down but that flip up onto their handles when you spin them. What is the reason that they have a different equilibrium when they are spinning versus when they are not? — CH, Renton, WA

While I’m not an expert on these “tipple tops,” I believe that I understand how they work. These tops have large round heads and look like wooden mushrooms. When you hold the handle (the mushroom’s stem) and spin it with its head down, it quickly flips over so that it spins on its handle. The flipping is caused by a torque that friction exerts on the top’s round head as the tops surface slides across the table. If the top were perfectly vertical as it spun on its head, friction between the top and the table would exert a torque (a twist) on the top that would simply slow the top’s rotation. But when the top isn’t perfectly vertical, the torque that friction exerts on it does more than slow its rotation. This torque also causes the top to precess (change its axis of rotation) in such a way that the top’s handle gradually becomes lower and the top’s head gradually becomes higher. Eventually, the top’s axis of rotation inverts completely so that it begins to rotate on its handle. Once that happens, the precession stops because the handle is too narrow for anything but the slowing effects. Only when the top stops spinning does it shift from this dynamically stable arrangement (handle down) to its statically stable arrangement (head down).

What’s the difference between fluorescent, phosphorescent, and triboluminescent?…

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What’s the difference between fluorescent, phosphorescent, and triboluminescent? – DS

Fluorescence is the prompt emission of light from an atom, molecule, or solid that has extra energy. For example, when some of the dyes used in modern swimwear and clothing are exposed to ultraviolet light, they absorb the light energy and promptly reemit part of that energy as visible light—typically brilliant greens and oranges. In contrast, phosphorescence is the delayed emission of light by an atom, molecule, or solid that has extra energy. Glow-in-the-dark objects are phosphorescent—they are able to store the extra energy they obtain during exposure to light for remarkably long times before they finally release that stored energy as visible light. Systems that exhibit phosphorescence rather than fluorescent are those that have special high-energy states that have enormous difficulty radiating away energy as light. Finally, triboluminescence is the emission of light from a surface experiencing sliding friction. Since sliding friction introduces energy into the surfaces that are sliding across one another, it’s possible for that energy to be emitted as light.

Why do the earth’s oceans appear blue to an observer on the moon?

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Why do the earth’s oceans appear blue to an observer on the moon?

The earth’s oceans and sky both appear blue to everyone who observes them. They do this because water absorbs blue light less strongly than it absorbs other colors. When ocean water is exposed to sunlight (white light), it absorbs most of the red light quickly and a good fraction of the green light. But the blue light penetrates to considerable depth in the water and there is a reasonable chance that this light will be scattered back upward to an observer on the shore, in the air, or even on the moon.