Julie-Ann -
There are two easy ways that you can change the
air pressure in a container. One is to actually change how much air
there is in a certain amount of space. This is what you're doing when
you pump air into a bike or car tire. The air pressure in the tire
increases because the amount of air in the tire increases. (You can do
the same thing by changing the size of the container without changing
how much air is in it, but this is probably the harder way.) A cool
activity that you can do using this is building
bottle rockets.
The
other way that you can change the air pressure in something is to
change its temperature. The hotter air gets, the more energy it has and
the more it tries to spread out. But if you keep it from spreading out,
its pressure will increase instead. This is because the air particles
are moving faster and hit the container harder when they bump into it.
The harder they hit the container, the more they push on it, and the
higher the pressure is. One way of demonstrating this is to use a
sealable container (like a metal drum or a plastic soda bottle). First
you heat the container up with the lid open. This makes the air inside
very warm. Instead of increasing the pressure, the air spreads out
through the open lid. Then you close the lid and let the container cool
down. As the air cools down, its pressure decreases, and the container
gets pulled inward and collapses. The experiment you're talking about
doesn't actually happen because of air pressure. This is because the
air is never actually sealed in. When you take an open metal pop can
and heat it up, the metal of the can gets very hot. If you then throw
it in a bowl of cold water, the metal is forced to cool down very
quickly. It's difficult for the metal to change its temperature this
quickly, so some of the metal gets cold while some of it is still hot.
This difference in temperature is very hard on the metal, and sometimes
the metal will actually shatter in response.
A similar thing
to this is something I saw on a cooking show a number of years ago. The
cook had placed a cold slice of lemon on a glass plate, making that
spot on the plate very cold. He happened to set the plate down on top
of his stove burner, which had accidentally been left on. When the cold
glass suddenly had its temperature changed by the hot burner, it put to
much stress on the glass itself, making the plate explode.
-Tamara
(published on 10/22/2007)
