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MORE FUN WITH AIR PRESSURE

Here is more fun with air pressure: Cover the top of a funnel with a piece of rubber from a balloon. Be sure the rubber is stretched tightly and held in place with a rubber band.

Place your finger over the small open end of the funnel and turn it in different directions to see if the rubber membrane remains flat.

What would happen if the outside pressure on the funnel were greater than the inside pressure? We'll soon find out. Suck some air from the small end of the funnel and quickly cover

Why is the membrane is depressed? (The air pressure inside the funnel is less than the air pressure outside.)

Turn the funnel in different directions to see if there is any change in the rubber membrane.

You will realize that if the outside pressure were less at any position, the membrane would be less depressed and would even bulge if the outside pressure were small enough.

It is easy to change the air pressure on the inside of the funnel. We can suck out some air, or we can blow in some air. How on earth would we be able to change the air
pressure on the outside of the funnel? Any ideas?

Why don't we take our funnel up in an elevator? Would that make a difference? Maybe this is another kind of altimeter? Let's try it.

---o0o---

Here's another air pressure game: Fill a glass to the brim with water and press a small plate or plastic cover over it. (A thin piece of cardboard can also be used but is not impervious to water.)

[Does anybody know what "impervious" means? (Say "im-PER- vee-us".) It means "not able to be passed through." Glass is impervious to water, because water can't go right through
it. Cardboard is not impervious to water, because water soaks right through it and makes it all soggy.]

Carefully turn the glass upside down over a sink or large pan and slowly remove your hand from the plate. (The plate or plastic cover will remain in place, because the outside air pressure presses upward against it.)

Now hold the glass sideways and turn it in various directions. Does the water come out?

Why not? What does this tell you about air pressure? What would happen if the pressure inside the glass were greater than the pressure outside?

Would less water in the glass make any difference in the test? If there is less water in the glass, there is probably more air pressure inside.

We'll find out, by re-testing with the glass only partially filled with different amounts of water.

You will discover that no matter how much or how little the water is in the glass, the cover will not fall off.

The reason is this: With a partial amount of water, the cover sags slightly from the weight of the water inside, the weight of the cover, and the pressure of the air inside.

The sagging slightly enlarges the space the air inside occupies, the almost imperceptible enlargement being enough to reduce the pressure inside the glass (pressure decreases as volume increases), and the greater outside air pressure is enough to stabilize the situation and hold the cover in place.

What does this tell us about a jar of fruit that Mom "canned?" The lid, which is just a disk with rubbery stuff around the inside edge, is stuck to the rim of the jar, and it bends a little toward the inside of the jar. When we use a hook opener to remove the disk, it sort of pops and hisses a little bit. Sounds like air coming out of the jar, but it is really air going into the jar. That is because boiling the jars for 20 minutes or so increases the molecules of the jar's contents, and pushes out any air that may be inside.

What does this tell us about opening a can of coffee? When we make a hole in the lid, we hear air hissing as it goes into the can. When food packers put dry stuff into containers, they somehow suck out all the air that is inside the container, then they seal it. It's called "vacuum pack."

Isn't air pressure fun?

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