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Microwave ovens cook food by agitating water molecules
"Nuking" food in the microwave is a convenient, fast, efficient, and generally safe method of cooking. It almost seems like magic, but it's just physics. MicrowavesMicrowaves are a form of light. They are photons. There is more to light than just what we see in the visible spectrum. The spectrum of light extends from X-rays and gamma rays that are extremely energetic light waves that have high frequencies and short wavelengths, to radio waves that have very low frequencies and long wavelengths. Visible light is near the middle of the spectrum, though it is still quite energetic. Microwaves have longer wavelengths than visible light, but much shorter wavelengths than radio waves. Dielectric HeatingMicrowaves interact with water, and to a lesser degree, fat, sugars, and some other materials in food that are dipoles. A dipole is a molecule that has an area that is negatively charged and an area that is positively charged. Microwaves force dipoles to rapidly rotate back and forth. This produces friction, and the friction produces heat. This is known as dielectric heating. Water is a very strong, well-defined dipole. However, frozen foods are more difficult to heat with microwaves since the water molecules are stuck together and aren't as free to rotate back and forth. The MagnetronA magnetron generates microwaves, and each microwave oven has one. A magnetron is a type of vacuum tube (or thermionic valve), which is a device that frees electrons from a metal filament (the vacuum tube is a descendant of the lowly light bulb). A regular vacuum tube simply attracts the electrons to an anode, a positively charged plate. In a magnetron, a permanent magnet produces a magnetic field that forces the electrons to spiral around to the anode. The anode is the case of the magnetron, and the cavity has a funny spoke and lobe shape. Each spoke on the anode has one side that is more positively charged than the other. The electrons spiraling by bump electrons in the spoke, creating a little circuit around the circumference of the lobe. This induces an electromagnetic field that generates microwaves, a frequency of light. It is the spacing and size of the spokes and lobes that determine the frequency generated.
The copyright of the article Cooking with Light in Atomic/Molecular/Optical Physics is owned by Katharine M. J. Osborne. Permission to republish Cooking with Light in print or online must be granted by the author in writing.
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