Dear Bill,

How does a microwave oven work? Why does the food get hot, but not the air around it?

-- Microwavin' Maven

Dear Maven,

Microwaves affect molecules that have slight differences in their electric properties from one side of the molecule to the other. Water is just such a molecule. Microwaves cause liquid water molecules to rub against one another and heat up. So your microwave oven is able to heat food by causing the water molecules in the food to rub against each other. When the water molecules are a gas (when they're water vapor) they're too far apart to rub against each other to heat up much--that's why the air stays relatively cool.

How microwaves workMicrowave ovens use two invisible force fields working across one another to convert the flow of electrons in wires--electricity--into the flow of electromagnetic waves through air--microwaves. A circuit in the oven stores electricity in such a way that its voltage gets much higher than it is in your house's wiring, while the number of

electrons flowing at any moment is somewhat smaller than in your house wiring.

These flowing electrons are directed into a small metal wire that acts very much like a tiny antenna. Around the wire is a metal ring. The wire and the ring are connected to the same electric circuit, so there's an electric field between them. It's like a force field in a science-fiction movie, only real. Another example is lightning, which jumps to Earth under the influence of an "electric force field."

Electrons would be forced to zip out straight from the wire to the ring, following paths like the spokes of a wagon wheel. But they don't, because the whole assembly is packed between two strong magnets. The magnets set up another type of field--a magnetic field. Now, a fundamental feature of the universe makes electrons create magnetic fields of their own, when they're in motion.

You may know from experience that one magnet can push or pull on another magnet--the same goes for magnetic fields. Under the influence of the electric and magnetic fields, instead of going straight, the electrons take curved paths. When the wire, the ring, the voltage, and magnetic fields are configured just right, the whole assembly becomes a miniature broadcasting antenna, and it radiates microwaves.

These assemblies, which can fit in the palm of your hand, are called magnetrons. They are often called cross-field devices, because they use an electrical field that crosses a

magnetic field. One very good way to think of electromagnetic energy is as a wave or set of waves zooming through empty space or the low-density spaces between and within molecules.

How heat worksMicrowaves are electromagnetic energy. So is light, so is heat, and so are computer signals. The difference between these seemingly very different kinds of energy is not their speed (they're all going the speed of light); it's their respective wavelengths. Light waves are a few hundred billionths of a meter long. Green light's wavelength is about 550 nanometers (0.000000550 meters) long. Heat's wavelength is about twice that, around 1,000 nanometers. We have, of course, another name for heat--infrared, which means below red. It's designated "below" because as the wavelength goes up, the frequency goes down. Infrared energy makes molecules move faster. The kinetic or "moving" energy of molecules is actually a definition of temperature. More kinetic energy means more molecular motion and a hotter temperature.

Microwaves are much, much longer than heat waves. Microwaves are about 12 centimeters long. That would be 12 million nanometers. When these waves pass through some materials they set these substances tumbling--on the molecular level. That

motion becomes heat. Liquid water is just such a substance. It's H2O, and its molecules are almost lined up: H-O-H. But, although they're symmetrical, the molecules are bent in the middle.

Chemists say the water molecule is polar. It has electromagnetic poles, just like Earth's north and south. But of course, water molecules are somewhat diminutive in comparison--about a billion, billion times smaller. The top of the molecule (as pictured) has a slightly positive charge compared with the bottom, which carries a relatively negative charge. As microwaves move through liquid water, the electromagnetic waves slightly attract then slightly repel the H2O molecules, and things heat up.

Why popcorn popsThis polarity is what makes popcorn pop. Popcorn kernels carry just the right amount of water inside--around 14 percent of a properly dried popcorn kernel is liquid water. So when microwaves pass through a kernel, the water turns to steam and blows the kernel inside out.

In the air of a microwave oven, we don't have liquid water. We have water vapor. With the molecules much, much farther apart, the gentle tumbling brought on by the microwave energy doesn't cause them to rub on each other the way they do when they're a liquid. So the air in a microwave oven hardly heats up at all. But foods and materials with polar molecules do. When the microwave magnetron is on, it's a molecular tumble-thon.