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Zap a snack; how microwaves get water molecules moving to heat food in a flash.

How microwaves get water molecules moving to heat food in a flash.

A friend recently told me a horrible story. She said that the grandmother of a friend of another friend had just received a microwave oven as a present. Although the grandmother was the traditional sort, she began to use the new microwave, and noted how quickly it warmed things. So, after bathing her poodle one cold day, she decided to pop him into the microwave to dry him off. She punched up the controls, and ... bloof!--the poodle exploded.

I have no love for poodles, but even I flinched when I heared the story. Lest you be sick, let me add that it's probably fiction: I have now heard similar versions from people all around the country. I just cannot believe that America is swarming with grandmothers who explode their dogs.

But the story does contain an interesting lesson about how microwave ovens work.


Microwaves are a type of electromagntic energy--sort of like light waves, except your eyes can't use them for seeing (see spectrum diagram, p. 16). That's because microwaves have longer wavelengths than light waves; your eyes can't detect them. The different wavelengths also make the waves behave differently.

Light waves, for example, can travel through a glass of water because water does not absorb them well. Microwaves, on the other hand, are absorbed by water, and the water heats up as a result.

To understand how this absorption generates heat, you need to take a "look" at the smallest bits of water--water molecules.


Water molecules are much too tiny to see, even with a microscope. But if you could see a water molecule, it would be shaped like a V (see molecule diagram, p. 16). The two prongs might point up, or in any other direction. In a cup of water, for example, you'd find incredible numbers of water molecules pointing in all different directions--a crazy and confusing scene. When you stick the cup in the microwave and turn the power on, however, the scene changes drastically.

As microwaves travel through the water, they force the water molecules to line up and spin (see diagram, p.16). In other words, the molecules absorb the microwave energy and convert it to kinetic energy (motion).

The spinning molecules are jammed so close together that they rub against each other. The rubbing creates friction, which changes the kinetic energy into thermal energy, or heat. That's how the water gets hot.

Regular ovens work by getting molecules moving, too. But the heat energy that sets them in motion comes from the hot air outside the food. The heat is passed inward as vibrating molecules on the outside of the food set those farther inside in motion. This molecule-to-molecule action takes time.

Microwave energy, on the other hand, can penetrae deeper into the food. So water molecules there heat up all at once. Result: The food cooks faster.


Most foods contain water, so they can be heated in microwaves. Consider a roast. The meat fibers are partially filled with water. As the microwaves pass through, the water gets hot, heating and cooking the meat fibers around it.

But don't try cooking a whole egg in your microwave. The problem is that an egg has a lot of water. When the water heats up, it changes from liquid to steam, which is a gas. The steam expands.

When an egg is cooked sloely on a regular stove, the expanding steam has time to escape through pores in the shell. But when the egg is cooked rapidly by microwaves, the steam forms so quickly that it has no time to escape. Its outward push becomes so great that the egg explodes.

Presumably, the poddle in my friend's story exploded for the same reason: The steam formed inside its body by the microwaves expanded very rapidly. So when it comes to microwaves, keep the eggs and dogs out.


If you're like most people, you probably think metals should be included on the list of items that are "unsuitable-for-microwaving." But, contrary to popular belief, metal can be used in a microwave. In fact, many new models come with metal racks. And many microwave cookbooks call for using aluminum foil to keep certain parts of food cool.

That technique works because metals reflect microwaves, just like a mirror reflects light waves. (Covering your food with foil or using metal pans might bounce so many microwaves away from your food that it doesn't get hot.)

Whatever you do, make sure that the metal never touches the walls, door, floor, or ceiling of the microwave chamber. These structures, which are also metal, carry an electric charge when the oven is on. Touching them with another metal can cause the oven to short-circuit and burn out. (Older models may have other restrictions, so check your microwave owner's manual before using metal in your microwave.)

Interestingly, the metal walls of a microwave serve an important safety function: They keep the microwaves bouncing around inside the oven instead of letting them leak out.

Older models may leak some microwaves around the door edges--especially if the door has worn loose from overuse. And if you stand near such a leaky oven, the water in your body will absorb the microwaves.

Usually the absorption won't hurt you and you won't even realise it is happening. But there are scattered reports of such ill effects as brain damage and sterility in people exposed to microwaves at close range. If you want to avoid the danger, just walk away from the oven while it is on. There is no danger when the oven is off--there are no leftover microwaves lurking about, just as there is no leftover light when you turn off a lamp.
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Title Annotation:includes related article on science experiments for microwave ovens
Author:Walker, Jearl
Publication:Science World
Date:Jan 14, 1994
Previous Article:One photogenic planet.
Next Article:To eat or not to eat; the dieter's dilemma.

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