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.
There are two more experiments you can do to talk about Bernoulli's law. The next is similar to the one you just did, only that this one doesn't use the funnel. Take a racket ball ask ask the audience what will happen if you turn on the leaf blower and put the ball above it. After some answers, test their answers by turning it on. The ball will float about a foot or so from the end of the leaf blower. Hold the ball until you feel it's staying up in the air. It's rather unstable, so give it some time before you let go of it. This doesn't usually cause too much of a wow! because it's what they were expecting. Turn it off and ask why this happened. The answers will probably be something about how air is pushing the ball up. Ask: So if I were to blow air sideways at it, would the ball stay up, or would it shoot sideways and then fall? After their answers, start the leaf blower again. Start by having the ball above it, like before, but now start tilting it, very slowly. You should move it very gradually, and pause to wait for the ball to stabilize before you continue moving. The whole set up is very unstable, so don't worry if you can't go past a feww degrees without the ball falling. Any amount off the vertical is enough to prove your point, since air is not blowing below the ball anymore. After turning it off, explain: Bernoulli's principle says that fast air can't push as hard, so all the air around the ball that is moving very fast because of the leaf blower has very lowe pressure. The air around that, however, is not moving fast, so it has a much higher pressure. So the ball stays up in the column of fast air because all the air around that is pushing it to stay there. You can even explain that the reason it did eventually fall is because it started moving too much, so it managed to get out of the column of fast moving air for an instant. (Parents are usually the ones who ask that!)
The final demostration about Bernoulli's Law is with the very long blue bag, called a Bernoulli bag. Ask for a volunteer from the audience and ask the rest of the audience how many breaths they think it will take the volunteer to blow up the bag. After their suggestions, ask: Do you think s/he can do it in just one breath? Let's try Stretch the bag out, and ask your volunteer to blow just once. After they do, grab the bag so as to trap the air they blew in and show the audience. Thank your volunteer and say: You think I can do any better than s/he did? Then hold the bad at arms length and blow into it (you should have someone hold it straight for you). Again, grab the bag to as to trap the air you blew into it. The time, it should be a lot more than what your volunteer did. Put the bag next to you and point out the amount of air in there is much more than can fit in your lungs. Ask: What did I do differently from (volunteer)? After some answers, say: I held the bag far away from me, so that instead of just the air from my lungs, I blew in all this air that's in between me and the bag as well. When I blow, air is moving fast, so it can't push as much. This means that all the air around it pushes much harder. In fact, it puhes its way into the bag At this point you can ask you previous volunteer if they would like to try again using your technique.
You can end this demo by pointing out that Bernoulli's law is very important. For example, is what allows airplanes to fly, because they are moving very fast, so the air right next to their wings can't push too hard, just like in the case of the racket ball.