Air Pressure


See how Magdeburg hemispheres demonstrate the properties of air pressure.


Crystal: Hi my name is Crystal, and I'm an Explainer here in the National Air and Space Museum's "How Things Fly" gallery. Behind us, we actually have one of our landmarks - it's the lunar module. Now today we're going to learn a little bit about air pressure. Now, if you did or didn't know, there's actually air pressure all around us, and it has a weight and exerts a force on all surfaces and all people. Now, to give you an explanation of what exactly that means, here I have this little cube, and for every one side of it, it's about one square inch. Now, for every one square inch of surface, there's approximately 14.7 pounds of air pushing down on it. So, for a small child, if they have 2,000 inches of surface on their body, that's approximately 30,000 pounds of air pushing down on them, if you can imagine that. So, why isn't that child crushed if there's 30,000 pounds of pressure pushing down on them at all times? Well, that's because there's air on the inside of their body. So, for all of that 30,000 pounds of air pushing down on them on the outside, there's 30,000 pounds of pressure pushing back from the inside. So, there's equal air pressure on the inside and outside. Now, to get a further understanding of that, we're gonna do an experiment, today, to test that. Out here I have two hemispheres. They're actually called Magdeburg hemispheres, and we'll learn a little bit more about that in a minute, but we're gonna use these two hemispheres to test out the air pressure inside of the museum. So, I'm going to use this rubber o-ring and place it on the inside of one of my spheres, and then put the spheres together just like this. Now, because there's equal air pressure on the inside and equal air pressure on the outside, I'm able to easily take them apart just like that. Now, what would happen if I removed all the air from the inside of the sphere but there was still all the air in this museum pushing them together? Well, let's test it out and see what happens. I'm gonna go ahead and attach a vacuum pump to my sphere. Can I have some air please?

[vacuum starts]

Crystal: Thank you very much. So, right now this vacuum pump is removing all of the air from the inside of the sphere. Now, to give you a little bit of background about where this experiment came from, it was first done in Magdeburg Germany, in 1650, by the mayor himself, who is actually testing out a vacuum pump. Now when they did this experiment, they had a 20 inch sphere, and they used 30 horses to try and pull apart the two spheres, but they still were not able to pull this spheres apart. Now, obviously our sphere is only about four inches and we don't have 20 horses here, but we'll try it out anyway and see if we can do it. I'm gonna invite my strong friend, Ashley, to help me out today, to see if we can pull these two spheres apart. Hey Ashley! —

Ashley: Hey.

Crystal: Alright, so we're gonna grab on either side of the sphere, and give it a good pull, as hard as you can. So, as you can see, no matter how hard we pull on the spheres, we just can't get them to come apart. Thanks Ashley.

Ashley: Bye.

Crystal: Bye. [laughs] And that's because there's no air on the inside, since we removed it all. But there's all the air in this building pushing these two spheres together. That's approximately 750 pounds of air pushing on these spheres to keep them together. Now obviously, we don't have that much strength, so to get them to come apart, I need to put air back on the inside of the spheres.

[air releases]

Crystal: And now that there's equal air pressure again, I should be able to remove the top. Now, what would happen if it were the exact opposite? If there was only air inside the object but no air on the outside? So, let's look at another example. Here, I have a bell jar with a balloon taped inside. As you can see, there's equal air pressure on the inside and the outside of the bell jar. There's also a balloon inside that's full of air, and tied off at the top, so there's also equal air pressure on the inside of the balloon and the outside of the balloon, right? Now what would happen if we change the air pressure on the outside of the balloon? Air please.

[vacuum starts]

Crystal: Thank you. So as you can see, the balloon is getting larger and larger and that's not because we're adding air to the inside of the balloon, because it's tied off at the top. But that's because there's no longer air pressure on the outside of the balloon pushing in, and there's only air on the inside of the balloon pushing out. So, eventually the balloon's gonna keep getting larger and larger until it would maybe even pop. Alright, that's enough.

[vacuum stops]

Crystal: Thank you. This is the same situation as when astronauts are traveling into the vacuum of space, and the only air pressure that they're feeling, is the air from with inside the vehicle that they're traveling in. So, when they were building the lunar module, not only did they have to make sure that it could withstand the extreme conditions of outer space, but it also had to be able to withstand the air pressure from within. So, now you know a little bit more about air pressure and how it affects objects here on earth and in space.

Categories: Gravity & Air