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Microwave Ovens
Page 4 of 13 (127 Questions and Answers)

569. Were microwaves invented for the microwave oven?
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While microwaves were known long before anyone know how to produce them efficiently, they became important during World War II as the basis for radar. The ability to detect and locate enemy aircraft at long distances and at night was crucial to the defense of Allied cities during the war. The 1945 discovery that microwaves also cooked food was an accidental offshoot of radar development.

583. If you microwaved bean plant seeds over a period of weeks while they were growing, would they grow faster or longer, and if they would, would that be due to the heat or some effect of the microwave radiation? - DS
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Microwaving the bean plant seeds would be no different from heating them, except that the distribution of temperatures in the seeds and soil might be a little different from what you would get if you simply used a space heater. The particles or photons of ultraviolet light, X-rays, or gamma rays have enough energy to cause chemical changes in organic molecules and can induce mutations in living organisms. However, the photons of microwaves have so little energy that all they can do is heat living things. The most likely result of microwaving the bean plant seeds will be that the seeds will overheat and won't grow at all. You'll have bean stew.

584. My mother owns a microwave oven that is about 20 years old. It looks like new and has always been well taken care of. However, I was wondering whether it is still safe to use. Should I have it tested for leakage? — KE, Milwaukee, WI
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As long as it still cooks, it's probably fine. Leakage of microwaves can only occur if the cooking chamber has holes in its metal walls. These walls include the metal grid over the front window and the seals around the door. If the metal grid is intact and the door still appears to close properly, the oven shouldn't leak any more microwaves now than it did 20 years ago. However, to set your mind at ease, you can have it tested or test it yourself. www.comforthouse.com sells a simple microwave leak tester for $30. You can probably find similar devices at local appliance stores or, for a more accurate and reliable test, take your microwave oven to a service shop for inspection with an FDA certified meter. [Note added 1/10/97: I have finally found one microwave oven that leaks enough that a simple tester identifies it as dangerous—it's the microwave oven in my laboratory and I've moved it around frequently and taken it apart several times for my classes. Evidently, I damaged its door hinges during my experiments because the door now sags a bit and doesn't seal properly. The tester worked nicely in finding the leaks.]

601. Is 2.45 gigahertz the best frequency for a microwave oven? Is that frequency at or near a water molecule resonant frequency? Do water molecules have a resonant frequency?
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The frequency of the microwaves used in most microwave ovens, 2.45 gigahertz or 2,450,000,000 cycles per second, isn't related to any resonance of the water molecules themselves. While the isolated water molecules in steam or moist air have clear resonances associated with various vibrational and rotational modes of oscillation, these resonances are smeared out in liquid water. The water molecules in liquid water touch one another and their resonances are disturbed in much the same way that the resonances of a bell are disturbed when you touch it.

Rather than interacting with the water molecules via a resonance, the microwaves in an oven heat the water by twisting its molecules rapidly back and forth so that they rub against one another. The molecules are heated by the molecular equivalent of sliding or dynamic friction. The choice of 2.45 gigahertz gives the water molecules about the right amount of time to twist in each direction. The precise frequency isn't important, but microwave ovens are required to operate at exactly 2.45 gigahertz so that they don't interfere with communication systems using nearby frequencies. I believe that there are 2 other frequencies allocated to microwave ovens, but only a few ovens make use of those frequencies.


607. Why does a microwave oven heat organic material and not inorganic material? — JM, Columbus, OH
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A microwave oven heats anything that contains liquid water. Since many organic materials contain water, they will become hot in a microwave oven. But some organic materials such as pure salad oil don't contain water and won't become hot in a microwave oven. There are also some inorganic materials such as damp unglazed pottery that contain water and that will become hot.

608. Does cooking in a microwave oven destroy the nutritional value of foods? Are microwaves radioactive? Does radiation "leak" from the oven? - DL
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Microwaves are essentially high frequency radio waves. They heat food by twisting its water molecules back and forth so that those water molecules rub against one another. Like all electromagnetic waves, microwaves are absorbed and emitted as particles or "photons," but the photons of microwaves have so little energy that they are unable to cause chemical changes in the molecules they encounter. They simply heat food; they don't "irradiate" it. The only way a microwave oven damages the nutritional value of foods is if it overheats. Microwaves are not radioactive—radioactivity is the spontaneous fragmentation of the nuclei of atoms and is usually associated with the emission of high-energy particles; particles that can induce chemical changes in the molecules they encounter. Finally, if a microwave oven was properly constructed and hasn't been damaged, virtually no microwaves leak from it. A small amount of microwaves won't hurt you anyway—they are present all around us already because of satellite transmissions, cellular telephones, and even the thermal radiation from our surroundings.

609. I've heard from many people that you should not stand directly in front of and no closer than 30 feet while a microwave oven is on? Why? If this is a myth how did it get started?
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This idea is just a myth. There should be virtually no microwaves leaking from the oven so it shouldn't matter where you stand. If you're concerned about microwaves, you can buy a microwave oven tester from a local appliance store or from www.comforthouse.com (or for a more accurate and reliable measurement, take your microwave to a service shop for inspection with an FDA certified meter). I have no idea how such a myth got started, but it's clear that microwave ovens scare people because they don't understand them. Given how easy it is to burn yourself on a conventional oven, I'd guess that there are fewer health risks with microwave cooking than with conventional cooking.

614. Can plastic melt in a microwave oven? How does this process work? Can plastic burn in a microwave oven? - HD
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Most plastics are unaffected by microwaves and do nothing at all in a microwave oven. For them to absorb energy from the microwaves, the plastics must either conduct electricity or their molecules must undergo the twisting motions that water molecules experience in the microwave oven. There are a few conducting plastics and these may melt or burn in a microwave as the microwave electric fields propel electric currents through them. There are also some plastics that trap water molecules and these may also melt or burn as the water molecules gather energy from the microwaves. I suppose that there are also a few plastics that have polar molecules in them that respond to the microwaves the way water does. However, most plastics do none of these and only melt or burn if they accidentally come in contact with very hot food or pieces of metal that happen to be in the microwave oven.

624. I have heard that microwaving can destroy certain nutrient molecules in food, such as vitamins. Is this true? — D, Boulder, CO
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A microwave oven heats the food it cooks; nothing more. If it damages nutrients, then it's by overheating those nutrients. Such overheating could happen in a microwave oven if you don't move the food about during cooking. That's because the microwaves aren't uniformly distributed in the cooking chamber and some parts of the food heat faster than others. Some parts of the food could become hotter than you intend and this overheating could damage sensitive molecules. However, I think that microwave cooking is probably less injurious to the food than conventional cooking. It's pretty hard to burn food in a microwave!

634. How does a magnetron work? — MM, Czech Republic
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A magnetron has a ring of resonant electromagnetic cavities around a hot central filament. Each resonant cavity acts like an electromagnetic "tuning fork"—electric charges and electromagnetic waves swing back and forth inside a resonant cavity at a particular frequency; the cavity's resonant frequency. As electrons are "boiled" off the hot filament, a high voltage attracts them toward the walls of the resonant cavities. The resonant cavities tend to have at least small amounts of electric charge "sloshing" back and forth in them at their resonant frequencies and the electrons from the filament are attracted more strongly to the cavities' positively charged walls than to their negatively charged walls.

However, there is also a magnetic field present in the magnetron and this field deflects the streams of electrons so that they hit the wrong walls of the resonant cavities. Instead of canceling the charge sloshing in the walls of the resonant cavities, the newly arrived electrons add to it. As electrons flow to the resonant cavities, more and more charge sloshes in the resonant cavities and these cavities accumulate huge amounts of energy. Some of this energy is tapped by a small wire loop and a microwave antenna. This antenna radiates some of the energy from the cavities into a metal channel that leads away from the magnetron. In a microwave oven, this channel leads to the cooking chamber so that energy from the resonant cavities is delivered to the food in the oven. Energy is extracted from the magnetron slowly enough that the filament and high voltage power supply can replace it and the operation continues indefinitely.


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