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Microwave oven

Microwave ovens

Last updated: March 27, 2008.

How our ancestors would have loved microwave ovens! Instead of sitting around smoky wood fires for hours on end, boiling up buffalo stew for their Stone-Age friends, they could have just tossed everything in the microwave, pressed a few buttons, and had a meal ready in a minute or two. Of course, they had no electricity, which might have been something of a problem…

When microwave ovens became popular in the 1970s, they lifted household convenience to a new level. A conventional oven heats food very slowly from the outside in, but a microwave oven uses high-powered radio waves to cook food from the inside out. This is why a microwave can cook a joint of meat roughly six times faster than a conventional oven. Microwave ovens also save energy, because you can cook immediately without waiting for the oven to heat up to a high temperature first. Let's take a closer look at how they work!

What is heat?

Cooking cavity in a microwave oven

Microwave ovens are so quick and efficient because they channel heat energy directly to the molecules (tiny particles) inside food. Microwaves heat food like the sun heats your face—by radiation.

A microwave is much like the radio waves that zap through the air from TV and radio transmitters. It's an invisible up-and-down pattern of electricity and magnetism that races through the air at the speed of light (300,000 km or 186,000 miles per second). While radio waves can be very long, microwaves have much shorter wavelengths and frequencies. The microwaves that cook food in your oven are just 12 cm (roughly 5 inches) long.

Despite their small size, they carry a huge amount of energy. One drawback of microwaves is that they can damage living cells and tissue. This is why microwaves can be harmful to people—and why microwave ovens are surrounded by strong metal boxes that do not allow the waves to escape. Microwaves can be very dangerous, so never fool around with a microwave oven. Microwaves are also used in cellphones (mobile phones), where they carry your voice back and forth through the air, and radar.

Photo (above): The "cooking cavity" of a Panasonic microwave oven. This strong metal box stops harmful microwaves from escaping. The microwaves are generated by a device called a magnetron, which is behind the perforated metal grid on the right hand side (just behind the lamp that illuminates the oven inside). If you peer through the grid, you might just be able to see the horizontal cooling fins on the magnetron (which look like a stack of parallel, horizontal metal plates). Note also the turntable, which rotates the food so the microwaves cook it evenly. The back of the door is covered with a protective metal gauze to stop microwaves escaping. You can see into the oven when the door's shut because light can get through the holes in the gauze. Microwaves, however, are much bigger than light waves, so they're too big to get through the holes and remain safely "locked" inside.

How do microwaves cook food?

Simple artwork showing how a microwave oven works

A microwave oven has several main parts. Inside the strong metal box, there is a microwave generator called a magnetron. When you start cooking, the magnetron takes electricity from the power outlet and converts it into high-powered, 12-cm radio waves. It blasts these waves into the food compartment through a channel called a wave guide. The food sits on a turntable, spinning slowly round so the microwaves cook it evenly.

When the microwaves reach the food, they don't simply bounce off. Just as radio waves can pass straight through the walls of your house, so microwaves penetrate inside the food. As they travel through it, they make the molecules inside it vibrate more quickly. Vibrating molecules have heat so, the faster the molecules vibrate, the hotter the food becomes. Thus the microwaves pass their energy onto the molecules in the food, rapidly heating it up.

Who invented the microwave oven?

Like many great inventions, microwave ovens were an accidental discovery. Back in the 1950s, American electrical engineer Percy Spencer (1894—1970) was carrying out some experiments with a magnetron. At that time, the main use for magnetrons was in radar: a way of using radio waves to help airplanes and ships find their way around in poor weather or darkness. One day, Percy Spencer had a candy (chocolate) bar in his pocket when he switched on the magnetron. To his surprise, the bar quickly melted because of the heat the magnetron generated. This gave him the idea that a magnetron might be used to cook food. After successfully cooking some popcorn, he realized he could develop a microwave oven for cooking all types of food. He patented the idea in 1953 and his first oven was around 1.5 meters (5 ft) high! Since then, microwaves have become much more compact and millions of them have been sold throughout the world.

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