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Air Pressure demonstration

This demo has two goals: to talk about air in static situations, and in dynamic ones.

The first goal is to get kids thinking about air pressure in static situations. For this we use the vacuum pump. You can start off by asking: Did you know that the air is always pushing on you? We call this air pressure. It may not feel like the air is pushing on you, because we are used to it. What if we were able to take all of the air out of the room, would it feel different? You then show them the vaccum pump and give a simple explanation of how it works (It is a container with a good seal, and I'm going to suck all the air out). Also point out that inside this container you have a marshmallow and a balloon. Before you turn on the pump, ask: What is going to happen to the marshmallow and the balloon when we suck the air out of the container? The answer, of course, is that the marshmallow will get bigger but then smaller, and the balloon will only get bigger. Don't give away the answer just yet, though. Instead, proceed to evacuate the container and wait for the wow! Now explain: The marshmallow got bigger first because it has a lot of air pockets inside. When we took air out of the container, there was less air pressure on the outside of the marshmallow than inside, so the inside air could push its way out very easily, making the marshmallow grow. But once all the air escaped, the marshmallow started to get smaller again. The balloon also has some air inside, but this air can't escape, so it never gets smaller, only bigger You can then repeat the experiment but this time open the valve right after you've evacuated it. The change is very sudden, and you can say: When the valve is open, the air from outside can now push its way in, so that everything goes back to normal. The marshmallow is still a little wrinkled, because it lost some of its air

Now move on to the other bell jar. This one is used to talk about sound, but before that there's one more thing related to air pressure. Before turning the bell on, cover the jar with lid and show the kids how easy it is to open it. Then ask for a volunteer from the audience. Before letting them try, evacuate the jar, then ask them to try and open it. Of course, they can't! Ask: what happened? is (volunteer) much weaker than I am? Or did I play a trick? After some of their guesses, explain: I sucked out the air from inside, so that there's very little pressure inside the jar. That means that the air outside can push much much harder than the air inside. So hard, that no matter how much I pull, I can't open this. If we let the air back in -remove the hose to let the air in- it's now easy to open it. Let the volunteer open it now, and confirm that it's very easy.

This part of the demo deals with the second goal. We'll talk about air pressure in dynamic situations, when air is moving. The way you probably learned about these explanations was using Bernoulli's Law, but we can't use equations to explain things! Instead, go with general ideas: Bernoulli's law says that air that's moving fast doesn't have as much pressure as air that moves slowly. The faster it goes, the less it can push. You can start by mentioning this, and then showing them that it's true. There are three experiments we can do related to this law. The first one involves the leaf blower, a funnel, and a small ball that fits in the funnel. Before you start, make sure to point out what you have is a leaf blower and not a vacuum cleaner (for example, by blowing someone's hair!) Put the funnel at the mouth of the leaf blower and the ball inside it. Ask: What do you think will happen to the ball when I turn the leaf blower on? After their answers, turn it on to test their ideas. The ball, of course, doesn't hover above it, like most people guess, but rather gets stuck to the leaf blower. Turn it upside down, to show that even in that situation the ball won't fall. To dispel any doubt that the machine is blowing and not sucking, remove the funnel before turning it off and blow someone's hair again. Then turn it off and ask for explanations. After some ideas, explain: The air all around the ball, on the sides of the funnel, is moving very fast. We learned that fast air can't push very hard. But the air in front of the ball is not moving fast, so that one can push much harder. So much harder, that it forces the ball to stay put, even when we put the whole thing upside down.