KATIE BURNS

Staff Writer

Q: How do microwave ovens cook food?

A: “As the name implies, they broadcast microwaves into thefood,” said Steve Drew, who teaches chemistry and physics at SanMarcos High School. “The microwaves are absorbed by the watermolecules in the food, and it causes the water molecules tovibrate. And as everyone knows, the vibration of molecules isheat.”


The water molecules obtain energy from the microwaves, which areelectromagnetic waves made of oscillating electric and magneticfields. An H2O molecule moves in an electric field because itsoxygen atom attracts its electrons more than its two hydrogenatoms, giving one side a negative charge and the other a positivecharge.

“The more motion you have, the more heat you have -- becausethat’s what heat is, the motion of the atoms,” said JulieBridgewater, a physics teacher at Ramona High School, as she put apotato in a microwave oven.

Microwaves reach far into food, even building up enough steampressure to blow up a baking potato into mashed potatoes. Theelectromagnetic waves don’t heat most dishes -- ceramic, glass orplastic containers only warm up because of the food inside. Butmicrowaves and metals don’t mix well.

“If you broadcast microwaves against a piece of metal, that’sgoing to set up a current in the metal,” Drew said.


The electrons in metal move easily, so an electromagnetic wavecan push them around. Microwaves also can’t penetrate metal, whichis why microwave ovens have a metal mesh in the door. But a metalobject inside the oven causes sparks and arcs -- and sometimesfire. More often, the currents damage one of the parts of theappliance.

“The microwaves are emitted by something called a magnetrontube,” Bridgewater said.

The magnetron operates on high voltage to make the microwaves.The microwaves flow from the magnetron along a metal channel, orwaveguide. They hit a rotating metal stirrer blade, which bouncesthem into the oven. Most machines last a long time, but they canbreak.

“They have components inside that deteriorate for a variety ofreasons,” said Paul Page, owner of Mart’s Appliance Service inEscondido. “All of those components go bad, in addition to thecircuit board that controls the microwave.”


Most people will replace a malfunctioning microwave oven with anew one because the appliances are relatively inexpensive. But somepeople will still bring in a microwave for repairs, generally whenit for some reason stops making microwaves -- and a potato justnever warms up.

“Typically, what happens is it doesn’t heat any longer,” Pagesaid. “It still goes through all the gyrations.”

Please direct your science and technology questions, along withyour name and hometown, to Katie Burns at (760) 740-5442,kburns@nctimes.com or 207 E. Pennsylvania Ave., Escondido, CA92025.

11/11/01