Pressure cookers
Last updated: October 12, 2008.
Want to cook your dinner in a fraction of the time? You could use
a microwave to zap it with energetic waves. But another popular
solution is to seal it in a pressure cooker: a kind of saucepan that
cooks things quicker by boiling them at a higher temperature than
usual. Although considered old-fashioned by some,
pressure cookers are still a convenient and economical way to prepare
food. Let's take a closer look at how they work!
Photo: A pressure cooker in action. Notice the small valve on the top (just to the right of the central handle) through which steam escapes. Photo copyright © and courtesy of Simon Law 2005, published on Flickr
under a Creative Commons License.
How pressure and temperature affect boiling
Suppose you have a saucepan full of potatoes that you want to
cook. You fill the pan with water, put it on a hot stove, and wait
for the water to boil. Now you probably think the water will boil
"when it's hot enough"—and that's true, but only half true.
The water will actually boil when the molecules it contains have
enough energy to escape from the liquid and form water vapor (steam) above it.
The hotter the water is, the more energetic the molecules are and the more easily they can escape.
So temperature plays an important part in making things boil.
But pressure is important too. The higher the pressure of the air
above the water, the harder it is for the molecules to break free;
the lower the pressure, the easier it is.
If you've ever tried making a cup of tea on a mountain with a
portable camping stove, you'll know the water boils at a lower
temperature at high altitudes. That's because air pressure falls the
higher up you go. At the top of Mount Everest, air pressure is about
a third of what it would be at sea level, so water boils at roughly
70°C or 158°F (see why on this MadSci forum posting). Mountain-top tea tastes pretty disgusting because the water boils
at too low a temperature: even though it's boiling,
the water is too cold to "cook" the tea leaves properly.
Photo: Civilisation is a cup of tea—but only when you make it at sea-level. That's when water boils at what we consider its "proper" temperature (100°C or 212°F).
Read more about pressure, temperature, and how molecules behave when liquids boil.
How pressure cookers work
Let's forget the tea and return to our potatoes.
Suppose we could somehow arrange things so that the air above our
saucepan was actually at a much higher pressure than usual. That
would make the water boil at a significantly hotter temperature,
which would make the potatoes cook more quickly.
This is the basic idea behind pressure cookers. A pressure cooker
is a big steel saucepan with a tight-fitting lid. The outer edge of
the lid has a thick circle of rubber called a gasket that fits
between the bottom of the lid and the top of the pan to make a really
tight seal.
When you fill the pan with water and place it on the
stove, the water heats up and some of its molecules escape to form
steam up above it. With a normal pan, the steam would just drift off
into your kitchen and disappear. But with a pressure cooker, the
gasket and lid stop the steam escaping so the pressure soon builds up.
Although the water inside the pan boils, the higher pressure means it boils at a higher
temperature than normal that cooks your food more quickly. A special
valve on the top of the lid allows a small amount of steam to escape,
keeping the pressure higher than normal but not so high that the
cooker explodes. If the pressure inside the pan builds up too much, the valve pops right out, rapidly lowering the pressure to a safe level again.
What is pressure?
Pressure is the way a force acts over a surface. If you pump air
into a bicycle tire, the energetic molecules of gas
rush about inside, colliding with the tire walls and pressing outward. The tire stays
springy and inflated because the air molecules push its inner walls with as much
(or greater) force than the air molecules outside are pushing the outer walls. If you heat up a
tire, you give the air molecules more energy. They rush about faster,
collide with the rubber walls of the tire more often and exert even
more force. The tire feels more pumped up or, if you're unlucky,
bursts!
In physics, we say the pressure on a surface is the force
pressing on it divided by the area over which the force acts:
Pressure = Force/area
This simple equation tells you that if you apply a given force to half the area, you double the pressure. Apply the force to twice the area and you halve the pressure.
It's very helpful to know about pressure in everyday life. Suppose
you want to put a poster up on your bedroom wall. Assuming you don't
have a hammer, you'll find it much easier to use thumbtacks (drawing pins) than
nails. A thumbtack has a huge flat head connected to a very thin
pin with a sharp tip. When you push on the flat head, you apply a
certain amount of force to a fairly big area. The force is
transmitted right through the pin to the tip, where it now acts on an
area of metal that's maybe 100 times smaller. So the pressure on the
tip is effectively 100 times greater—and that's why the pin enters
your wall so easily. Snowshoes and tractor tires use exactly the same
principle only in reverse. They spread weight (the force of gravity) over a bigger area to
stop your body (or a machine) from sinking into soft ground.
Photo: Drawing pins use the science of pressure.
