March 20, 1995

One Minute Papers - Questions and Answers

Are microwaves harmful to you? Is eating microwaved food harmful?

Microwaves can heat your body by adding thermal energy to the water molecules in you. This heating can be damaging if it is not controlled. Most of your body is protected from slow heating because your blood carries heat away from any local hot spots so that you warm evenly. However there are a few places that are not cooled by your circulation and can heat up locally enough to denature the protein molecules and cause biological injury. The cornea of your eye is a good example. It can be heated and damaged because it is not cooled well. That is why you must be careful not to look into a strong beam of microwaves. As for microwaved food, the only effect of cooking with microwaves is hot food. There is no "radiation damage" or "radioactivity", as there might be with x-ray or gamma radiation. Some foods should not be cooked in a microwave only because the uneven heating may allow certain parts to become too hot. Those parts may burn you when you eat them or they may suffer thermal damage that diminishes their nutritional value.

How does a microwave oven defrost foods? Doesn't it only work with water, not ice?

In any frozen food, there are some water molecules that are relatively free to turn about. These molecules may be at the surfaces of ice crystals or sitting on the surface of food particles. These water molecules can absorb microwaves and heat. However, the heating is very uneven because as soon as any water crystal absorbs enough heat to melt, the resulting liquid water will begin to absorb microwaves much more strongly. That is why defrosting must be done slowly. Then the microwave deposited heat will have time to flow through the food and melt it uniformly. Otherwise, you can end up with boiling hot spots mixed together with frozen icy spots.

How do the metal tines short out the microwaves?

If you arrange a metal rod so that it is parallel to a microwave's electric field, the microwave will push electric charges up and down that rod. This moving charge will waste some of the microwave's energy by creating heat in the rod. But the main effect will be that the rod will reflect or scatter the microwave. The moving charge will emit its own microwave and this new microwave will interfere with the original one.

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

If some radio station were to operate at 2.45 GHz, the main effect would be very poor reception of that channel on your radio. The oven isn't a receiver of 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 all it would take would be a few thousandths of a watt of microwave power to cause trouble in your reception of the radio station. Your radio would not be able to distinguish that station's transmission from microwaves leaking out of your oven. It would struggle to pick up the signal and you would probably hear lots of noise in the background.

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.

In the interferometer, how do the microwaves go through the mirrors to the mirror opposite it? Shouldn't the waves split?

In the interferometer, there are two different kinds of mirrors. The central mirror or beamsplitter is not a perfect mirror. It is a semi-transparent mirror like the "one-way mirror" used to watch people from a darkened room. The two mirrors at the ends of the interferometer's arms are fully reflective mirrors. They reflect microwaves almost perfectly.

What happens if you start the microwave oven with nothing inside?

The magnetron creates microwaves that travel into the cooking chamber and should be absorbed there. If there is no food (or rather no water-containing food), those microwaves will not be absorbed and will eventually find their way back to the magnetron. Eventually the magnetron will absorb as many microwaves as it emits. This situation is hard on the magnetron, which works best when it has very little radiation returning to it. That is why you should never run a microwave empty.

How does the resonant cavity in the magnetron work?

When it is 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 GHz. 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.

You said an ice cube will not get hot in the microwave because the molecules won't "flip". If this is so, then why do frozen foods cook in the microwave?

As noted previously, the water molecules in frozen foods are not all bound up perfectly inside ice crystals. As long as there are a few relatively mobile water molecules, even frozen food will eventually absorb enough energy to melt. Once that happens, the food can cook easily. Of course, the melting process is frequently very non-uniform so that food comes out with hot and cold regions. In general, frozen food cooked in a microwave is not very satisfying.

Are microwaves distributed unevenly in the oven? Why do manufacturers claim that microwaves with turntables are more effective than microwaves without turntables?

As the microwaves bounce around the inside of the cooking chamber, they tend to interfere with one another. There are usually regions in which the waves that follow various paths almost cancel one another and regions in which the waves reinforce one another. These regions do not cook food equally well. If the microwaves are canceled in one region, cooking will be slow there. If the microwaves reinforce one another in another region, cooking will be fast there. If you simply leave food in one place and try to cook it in the microwaves, the cooking will be uneven. However, if the food is rotated continuously, these good and bad cooking regions will be blurred away so that the food will all cook at about the same speed.

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.

What is the purpose of the wire in a microwave?

I'm not sure what wire you are referring to. Most microwaves contain no wires in their cooking chambers because any wire inside will tend to reflect the microwaves. The exception is temperature probes, which allow the oven to monitor the food's internal temperature. Such a probe does upset the microwave fields around it and will probably have some effect on the cooking taking place around it.

What exactly goes on when you're cooking a potato in the microwave and it explodes?

A microwave oven heats food by depositing energy in its water. If you cook the food long enough, that water can begin to boil. If the food has a hard outer shell (e.g. a potato or a corn kernel), the boiling water can create enough pressure in the food to make it explode. That is what pop's the corn in microwave pop-corn and why the potato explodes if you don't pierce it so that steam can escape.

Why do microwave ovens cook so rapidly?

When you put solid food (a potato, not soup) into a conventional oven, the heat flows slowly into the center of that food. This heat must work its way into the food via thermal conduction, in which adjacent atoms and molecules transfer their motional energies in a long bucket-brigade process. The last part of a potato to become hot is its center. However, in a microwave oven, the microwaves travel well into the solid food and deposit their energy everywhere. The potato cooks throughout at a relatively even rate. The actual amount of heat and energy involved in conventional and microwave cooking is about the same. However, the microwaves can heat the food throughout without having to wait for the slow process of conduction to carry it inward from the food's surface.

Why do some microwave ovens not seem to have a metal surface in the cooking area?

The cooking chamber of a microwave oven is always metallic. Even the glass door has a metal grid across it to keep the microwaves inside. This metal chamber may be coated with paint or plastic but it is there nonetheless. Without it, the microwaves would leak out and the oven would be hazardous and inefficient. It would cook objects throughout the kitchen.

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