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.
What is heat?
Microwave ovens are so quick and efficient because they channel heat
directly to the
molecules (tiny particles) inside food. Heat is a type of energy. When
things are hot, they have more heat energy than when they're
cold. If you heat a metal bar by placing it in a fire, the heat from
the fire passes into the bar by a process called conduction.
Although the atoms and molecules inside the bar are fixed in position,
they vibrate (shake around) randomly. The hotter the bar becomes, the
faster the molecules inside it move about.
A similar thing happens when you put food inside an oven. Heat from
hot
electric bars in the oven walls (or a gas flame in the base of the
oven) travels toward the food by radiation.
Radiation is a way
in which heat travels invisibly between two things that are not in
direct contact. Sunlight feels hot because of the invisible radiation
traveling from the Sun's surface onto your body. Once the heat
touches the outside layer of the food, it passes to the deeper layers
by conduction. Like a metal bar placed in a fire, the food slowly
heats up from the outside in. As it does so, the molecules inside it
move around more quickly.
There's a third way of heating up food with a conventional oven.
Imagine
you're heating soup in a pan on the top of a stove. The soup is
hottest at the base of the pan, where it is closest to the heat, and
coldest at the top of the pan, where it's closest to the air.
Hot soup is less dense than colder soup so it starts to rise upward.
As the hot soup rises through the colder soup above it, it cools and
begins to fall. This effect, which is called convection,
sets
up a kind of fountain of moving soup in the pan, with warm soup
continually rising and cold soup falling. Gradually, convection heats
up all the soup in the pan.
If you've ever tried cooking a chicken in an oven or heating soup on
a stove,
you'll know that conduction and convection take time to heat up
food. Radiation is much quicker. It takes just over 8 minutes for the
Sun's heat to travel 93 million miles to your face. And you
feel the warmth instantly!
What are microwaves?
Microwaves heat food like the sun heats your face.
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.
What happens inside the oven?
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 penetate 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.
How were microwave ovens invented?
Like many great inventions, microwave ovens were an accidental
discovery. Back
in the 1950s, American electrical engineer Percy Spencer (1894-1970)
was carrying on 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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