Air Pressure demonstration
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, but we’ve grown used to it. What if we were able to take all of the air out of the room, would it feel different? I think it would, but we wouldn’t be able to breathe, so maybe we should try a safer experiment. I have a container with a good seal, which means air can’t get through the cracks very easily. I’m going to put this marshmallow man inside the container, and then we’re going to take the air out. Let’s give him a ball to play with too (put marshmallow man and ball inside the container). This machine is called a vacuum pump, and its job is to suck air out of the container. It’s a lot like a vacuum cleaner. What is going to happen to the marshmallow man and the ball when we suck the air out of the container? (the marshmallows will get bigger, the ball will not change). (evacuate the container). Wow! The marshmallow man got bigger. Marshmallows are mostly sugar with a lot of air pockets trapped inside. When we took air out of the container, there was less air pressure on the outside of the marshmallow man. When there’s more pressure on one side of a surface than the other, the surface can move if its not very strong. In this case the low air pressure outside the marshmallow man allows the marshmallows to grow. The ball doesn’t have any air pockets, so even though there was less air pressure outside, there was nothing pushing from the inside to make the ball bigger. What’s going to happen if I open up this valve so air can get in and out of the container on its own? (the marshmallows will shrink back to normal size). The air inside the container is at a lower pressure than the air outside, so when I let them mix, air is going to want to go inside. When the air pressure is back to normal, the marshmallows will be back to normal size.
I want to go back to the question of whether or not we can feel air pushing. (remove the marshmallows and the ball from the vacuum chamber) Notice that the two halves of the vacuum chamber don’t stick together at all when they have air in them (lift the top half up without holding the bottom half). Is it any different after I use the vacuum pump to empty the air out of the container? (yes) I’m going to need a volunteer to help me find out. (evacuate the chamber while the volunteer is coming up to the stage. Close off the evacuated chamber and disconnect the hose. Then, have the volunteer pull apart the two halves of the chamber). That looked a lot more difficult than before! (let them put the two halves back together so they can see that there was nothing to stick them together, since the air has now been let back in). Why was it harder to pull the two halves of the container apart when there was no air inside? (air pressure). There was less air pressure inside than outside, so the air pressure outside was pushing the two halves together. So you actually felt the air pressure trying to hold the chamber together while you were trying to pull them apart.
[The plastic bottle crush is the same idea as the shrinking balloon in the LN2 demo, so feel free to skip it if you’re doing the other demo] We can reduce air pressure in a container by taking out some of the air, but there are other ways to reduce air pressure too. We can cool the air to reduce air pressure. We are going to put this closed plastic soda bottle in this very cold liquid (LN2, for those in the know, but if it hasn’t been introduced yet just say that it’s very cold). The liquid is going to cool the bottle and the air inside, but then what’s going to happen? (the bottle will be crushed). (put the bottle in the LN2, and let it sit for a few seconds) Wow! By cooling the air inside the bottle we reduced the air pressure inside the bottle. Since the air outside the bottle was at a higher pressure than the air inside, it pushed in on the walls of the bottle until it crushed them.
We’ve seen how to reduce air pressure in a container, but there is one other very important way to change air pressure, and you don’t need a container to do it. I have a strip of paper that I’m going to put next to my bottom lip and blow straight out over the paper. Is anything going to happen? (the strip of paper lifts up). Wow! When I blow over the top of the paper, it lifts up. We know that if the air pressure was lower on top of the paper than the air pressure under the paper, that difference in air pressure could push it up. In fact, that’s what is happening. Fast-moving air has less pressure than slow-moving air. This is known as Bernoulli’s principle. Just blowing air here, I can make the strip of paper lift up against the force of gravity.
Let’s try a few more experiments with Bernoulli’s principle. Here we have a vacuum that blows out air instead of sucking it in. The first thing I want to try is to make this ping pong ball float in the air. (hold the hose straight up, and hold the ball about 2 feet from the top of the hose when you start the vacuum). This is pretty neat, so why does it work? (Bernoulli’s principle). You might think that it’s staying up because the vacuum is pushing air up at it to keep it up. On the other hand, Bernoulli’s principle says the fast moving air in this column has lower pressure, so the outside air is pushing the ball back in toward the column. We’re going to need another test to find out why the ball is staying up. I’m going to tilt the vacuum slightly. Do you think the ball will stay up then? (Tilt the vacuum very slowly and only a few degrees, just enough to convince your audience. The floating ball is only barely stable, so unless you have very steady hands, you’ll probably drop the ball during the demo. Just act like it’s part of the show). The ball stays up even when we tilt the vacuum a little! If the ball had been staying up just because the vacuum was pushing it up, tilting the vacuum should push the ball off to the side and let it fall down. So we saw that Bernoulli’s principle must be keeping the ball within the low-pressure column of air.
We have one more demonstration to show the power of Bernoulli’s principle. I attached a funnel to the vacuum hose and I’m going to face the vacuum hose towards the ground. The air is still blowing out of the vacuum. I have a foam ball that I am going to hold near the opening of the funnel. What’s going to happen to the ball? (it gets sucked into the funnel). (Try it out. Blow some air at the front row to convince them that the air is still blowing out of the vacuum, and do it again.) Because there was fast moving air in the funnel, Bernoulli’s principle says the air pressure was lower, so the ball was attracted to the mouth of the hose, fighting against both the stream of air and gravity. Bernoulli’s principle is really powerful!
Remember at the beginning I said that Bernoulli’s principle is really important. So far we’ve shown that we can use fast-moving air to lift things against gravity, so what important use could this have? (airplanes). Airplanes use Bernoulli’s principle to fly. When a plane starts moving along the runway, the wings are curved so that air has to move faster to get over the top of the wings than to go below the wings. Bernoulli’s principle says that since the air is moving faster on top of the wings, there is less pressure on top of the wings than below the wings. When the plane gets going fast enough, there is enough of a pressure difference to lift the plane off the ground.