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Q & A: Generating Air Pressure

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Most recent answer: 10/22/2007
Q:
My question is how can U make air pressure in science? Like give me an example how you do an very good air pressure experiment fo a science fair. I am in grade 7. Please help me! My sicence fair project is a pop cane that is emtey. You put on top of a fire than you throw it in to a bowl of cold water. Than it explodes up. Bye now!
- Julie-Ann Lee (age 12)
Coquitlam,British Columbia
A:
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 .

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)

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