Tag: microwave ovens

How does the resonant cavity in the magnetron work?

When it’s active, the magnetron’s cavity has electric charge sloshing back and forth along its tines. The charge moves at a frequency determined by the shape and size of the cavity and these are carefully controlled so that the cavity’s natural resonance frequency is 2.45 gigahertz. To keep the charge sloshing, the magnetron adds negative charge from a hot filament wire located in the center of the cavity. Electrons flowing off of this wire are steered toward the negative tines by a magnetic field. As a result, the charges continue to slosh back and forth indefinitely. A small wire connected inside the magnetron extracts some of the energy in the magnetron and converts it into microwaves outside the magnetron. This wire acts as an antenna. The antenna is located in the pipe that carries the microwaves to the cooking chamber.

I have a friend who refuses to stand in front of the microwave oven in his kitchen, because he feels the “nuclear waves” leak and will cause his sperm to deform (and he doesn’t want ugly kids). Is this true? What about car phones? He heard they were bad, too!

Both microwave ovens and car phones emit electromagnetic radiation. But that radiation has relatively long wavelengths (about 12 cm in the case of microwave ovens and about 40 cm in the case of car phones) and is not at all like the electromagnetic waves emitted by nuclear processes. Nuclear electromagnetic radiation, usually called gamma rays, has extremely short wavelengths (less than 0.001 nanometer or about a millionth of the wavelength of visible light). All electromagnetic waves are emitted and absorbed as particles called photons. The energy in a photon is inversely proportional to its wavelength (in vacuum). Gamma rays, with their short wavelengths, have very energetic photons that can do lots of chemical damage to your tissue. But the longer wavelength radiation from microwave ovens and car phones comes as very low energy photons. These photons can’t do chemical damage. The only thing those waves can do is heat things. Microwave ovens are carefully shielded so that they keep most of the microwaves inside. If those waves did emerge, they would simply warm your tissue up. This warming won’t cause genetic damage but it could cook your tissue. There has been recent concern about low frequency electromagnetic fields causing subtle damage to tissue, but these have not be substantiated by scientific research and no physically reasonable scenarios for how such damage could occur have been offered.

I’d heard that if I cook in the microwave oven, there will be a possible formation of free radicals. Is it true? If yes, how? — Angela I.

It’s doubtful that microwave cooking forms free radicals in food. The microwaves in a microwave oven cook by exerting torques on the water molecules and gradually increasing the water molecules’ thermal energies through friction-like effects. There is never enough energy present in a single molecule at one time to shatter that molecule and form a free radical. While ultraviolet light, such as that found in sunlight, carries enough energy per photon (particle of light) to split a molecule and form a free radical, microwave radiation carries very little energy per photon. That’s why microwave photons can’t do chemical damage the way ultraviolet photons can. However, even if microwave radiation could form free radicals in food, that wouldn’t necessarily cause you trouble when you eat that food. So much happens to the food before it enters your blood stream that a free radical probably won’t survive. The more harmful free radicals are ones that are actually created inside your body, where they can immediately attack important molecules in your cells.

If a microwave does not melt ice, how does the “Defrost” setting on the microwave work?

I’ve already noted the issues of warming frozen food. However, the “defrost” setting is an interesting issue. If you’ve ever watched a microwave trying to defrost food, you’ve probably noticed that it heats the food briefly and then waits. It repeats this process many times. What it is doing is depositing energy (via the microwaves) into whatever water molecules are able to absorb microwaves. It then waits for this energy to flow as heat into the nearby food. Once the heat has been distributed rather evenly, the oven adds some more energy by turning the magnetron back on. This cycle of heating and waiting allows the food to defrost fairly evenly. Still, microwaves are likely to create hot and cold regions in the food so that some parts of the food will cook rather than defrost while some parts remain frozen.

If a radio station operated at 2.45 gigahertz, could you pick it up when your microwave was turned on and attached speakers?

If some radio station were to operate at 2.45 gigahertz, the main effect would be very poor reception of that channel on your radio. The oven isn’t a transmitter for microwaves; it just makes them like crazy. Most of the microwaves never leave the cooking chamber and there are strict regulations on any leakage. But it would only take a few thousandths of a watt of leaking microwave power to cause trouble in your reception of the radio station. Your radio wouldn’t be able to distinguish that station’s transmission from microwaves leaking out of your oven. The radio would struggle to pick up the signal and you would probably hear lots of noise in the background.

In microwaves – you heat up food really fast. Is it true that microwaved food will cool down faster than oven heated food? Someone told me “if it heats fast, it will then cool fast.”

No. Microwaves cook the food in a very different manner than normal thermal heating, but microwaved food has the same thermal energy that it would have if it had been warmed by more traditional methods. Microwaves heat food by exerting torques on the individual water molecules in the food. These molecules jiggle back and forth and sliding friction between them heats the food. This peculiar route to energy addition explains why frozen portions of the food don’t heat well: the water molecules are rigidly oriented and can’t jiggle back and forth in order to become hot. But despite the fancy heating scheme, the food retains no memory of how it was heated. Once it is uniformly hot, it cools at a rate that depends only on how heat is transported out of it. Microwaved food cools just as slowly as normally cooked food.

Inside the microwave oven, what is it that heats the food? How does the heat come out; where did it come from?

The food is heated by the microwaves themselves and these microwaves are piped into the cooking chamber from the magnetron. The magnetron has electric charge sloshing back and forth in its tines. A small antenna uses that sloshing charge to emit microwave radiation. The water molecules in the food absorb this microwave radiation and turn its energy into heat. The usual rules of heat transfer don’t apply in the heating process—the energy arrives at the food as microwaves, not heat.

Why does water react in a violent and dangerous way when overheated in a microwave oven? CA

Water doesn’t always boil when it is heated above its normal boiling temperature (100 °C or 212 °F). The only thing that is certain is that above that temperature, a steam bubble that forms inside the body of the liquid will be able to withstand the crushing effects of atmospheric pressure. If no bubbles form, then boiling will simply remain a possibility, not a reality. Something has to trigger the formation of steam bubbles, a process known as “nucleation.” If there is no nucleation of steam bubbles, there will be no boiling and therefore no effective limit to how hot the water can become.

Nucleation usually occurs at hot spots during stovetop cooking or at defects in the surfaces of cooking vessels. Glass containers have few or no such defects. When you cook water in a smooth glass container, using a microwave oven, it is quite possible that there will be no nucleation on the walls of the container and the water will superheat. This situation becomes even worse if the top surface of the water is “sealed” by a thin layer of oil or fat so that evaporation can’t occur, either. Superheated water is extremely dangerous and people have been severely injured by such water. All it takes is some trigger to create the first bubble-a fork or spoon opening up the inner surface of the water or striking the bottom of the container-and an explosion follows. I recently filmed such explosions in my own microwave (low-quality movie (749KB), medium-quality movie (5.5MB)), or high-quality movie (16.2MB)). As you’ll hear in my flustered remarks after “Experiment 13,” I was a bit shaken up by the ferocity of the explosion I had triggered, despite every expectation that it would occur. After that surprise, you’ll notice that I became much more concerned about yanking my hand out of the oven before the fork reached the water. I recommend against trying this dangerous experiment, but if you must, be extremely careful and don’t superheat more than a few ounces of water. You can easily get burned or worse. For a reader’s story about a burn he received from superheated water in a microwave, touch here.

Here is a sequence of images from the movie of my experiment, taken 1/30th of a second apart: