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Microwave Ovens
Page 6 of 13 (127 Questions and Answers)

759. Why do metal objects spark/arc in the microwave? Why don't the metal walls of the microwave spark? - JR
Like all electromagnetic waves, microwaves are composed of electric and magnetic fields. Since an electric field exerts forces on charged particles, a microwave pushes electrons back and forth through any metals it encounters. It is this motion of electrons back and forth through the metal walls of the microwave oven that allow that metal to reflect the microwaves and keep them inside the oven. If you leave a spoon in you cup of coffee as you heat it in the microwave, electrons will move back and forth through the spoon. This motion of charge will cause no problems so long as (1) the spoon can tolerate this flow of charge without overheating and (2) the spoon doesn't allow the charges at its ends to leap into the air as a spark. To keep the spoon from overheating, it must be a good conductor of electricity. Since most spoons are pretty thick, the modest currents flowing through them in the microwave will leave little energy inside them and they won't overheat. But a thin twist-tie or small bit of aluminum foil may well overheat and begin to burn. To keep the spoon from sparking, it should have smooth ends. Electrons are more likely to leave the end of a metal surface at a sharp point, so avoiding points is important. Most spoons are smooth enough that no sparks will occur. But a fork, a sharp piece of foil, or a twist-tie may well begin to emit electrons into the air as those electrons pile up at one end of the wire while the microwave oven is on. Like a spoon, the walls of the oven are good conductors of electricity and they have no sharp points. While electrons move back and forth in these walls, they simply reflect the microwaves without becoming very hot and without emitting any sparks. You'll note that the light bulb for the microwave is always outside the cooking chamber because it contains small bits of metal that would have trouble inside a microwave oven.

789. Is heating milk by microwave advisable? - I
Microwave cooking leaves no permanent mark on the food. It causes virtually no chemical damage and absolutely no radioactivity. The only drawback with heating milk by microwave is that the heating may be uneven and may denature some protein molecules in regions of the milk that become excessively hot. Since most protein molecules are disassembled by your digestion anyway, this treatment probably has no effects worth worrying about. Even with infant formula, my only concern would be the hot spots. If you carefully shake the milk after heating, so that its temperature is uniform, it should be just fine. I suspect that companies warn you not to heat milk in a microwave because they are worried that you will either not shake the milk to distribute its temperature evenly or that you will overcook it until it boils and the bottle explodes.

814. How can I check the magnetron in a home microwave oven? I have checked the HV (high voltage) transformer, the rectifier, and capacitor and all are OK. Does the magnetron output decrease with age? The oven has a hum that is much louder than normal. — AA, Ontario, CA
While I have only a little experience repairing microwave ovens, I can make reasonable guesses. The loud hum you hear is probably an indication that something is overloading the power transformer. That suggests that the diode, capacitor, or magnetron are bad. If you have checked the first two carefully, at full operating voltage, and found no problems, then I would suspect the magnetron. I have been told by a reader that magnetrons usually fail by shorting out, the result of electromigration of the filament material. The tube would then draw excessive currents from the high voltage transformer. That has probably happened in your case. Still, free advice like mine is only worth what you've paid for it. I'd suggest you consult a local repairperson, who has test equipment that can pinpoint the problem in seconds.

825. Don't microwaves penetrate metal at all? — DR, Tampa, FL
If the metal is a good conductor, then the microwaves don't penetrate more than a fraction of a millimeter. That's because the microwave electric fields push on the metal's mobile electrons and those electrons immediately rearrange in such a way that they cancel the microwave fields inside the metal. Only the skin of the metal responds to the fields and it shields the rest of the metal from the microwaves.

826. How can we clean the microwave oven? - PTW
Since the cooking chamber of a microwave oven doesn't get hot, there is no way to make a "self-cleaning" microwave oven. Instead, you have to clean it by hand with a sponge and perhaps a little soapy water. As long as you get the soap or any other cleaning agents out, you can clean the cooking chamber just as you'd clean the top of a stove.

827. If the condenser in a microwave is bad, what is the most likely reaction the microwave generator will exhibit? — IF, Bakersfield, CA
According to a reader, most microwave oven capacitors have fuses in them so that when they fail, they usually become open (they lose all of their ability to store separated charge and behave as a simple open circuit). You'd need a capacitor checker to find this open circuit within the capacitor.

828. Is it possible to eat a microwave while you eat food that was cooked in the microwave oven? - PTW
Not one that came from the microwave oven. Microwaves are all around us and are completely innocuous. Your body emits weak microwaves all the time, as part of its thermal radiation! Like light, microwaves don't remain still in objects so you can't eat one that was put in the food by the oven.

829. Where is the best place to put a microwave oven? Is it dangerous to place it on the refrigerator? - PTW
You can put a microwave oven anywhere that it's stable and where it has adequate ventilation. A microwave oven has a fan and vents through which it gets rid of its excess heat. You mustn't block the vents or the oven will overheat.

849. In cooking, what are some examples of absorbing microwaves, transferring microwaves, and reflecting microwaves? - K
In a microwave oven, water-containing foods absorbs microwaves. The microwaves disappear as they pass through the food and the food becomes hotter. Microwaves are transferred from the small antenna near the magnetron to the cooking chamber by sending those microwaves through a metal pipe. This rectangular pipe is typically a few inches wide and an inch or so tall, and is called a "wave guide." Finally, the walls of the cooking chamber reflect the microwaves. When a microwave encounters a metal surface, it pushes electric charges back and forth in the metal and this moving charge causes the microwave to reflect.

850. In your explanation of why microwaves don't penetrate the oven door, you said it is because the holes in the screen are smaller than the wavelength of a microwave. Wouldn't it be the amplitude of the wave and not its wavelength? - P
When a microwave tries to pass through the holes in the metal screen, electric charges in that screen begin to move. The microwave's electric field fluctuates back and forth rapidly and the charges reverse directions rapidly as a result. If the electric current made up of these charges has enough time to travel all the way around each hole before it reverses directions, it will be as though the screen were made of solid metal and the screen will be able to completely reflect the microwave.

Like any electromagnetic wave, a microwave has a wavelength (the spatial distance between adjacent wave crests) and a period (the temporal spacing between adjacent wave crests). The electric current that a microwave propels through a metal travels about one microwave wavelength during one microwave period. Therefore, the current can work its way around a hole in the metal only if the hole is significantly smaller than the microwave wavelength. The amplitude of the microwave doesn't matter—increasing the amplitude of the microwave just makes more current flow.
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