The following is a script for a Science Theatre stage show about pressure. The script is used as a guide for the two presenters. They are encouraged to change the words as they see fit as long as the stucture remains constant. Details about individual demonstrations contained in this show, as well as additional demonstrations and shows, can be found in the Science Theatre demo guide: Recipes for Science
Last Revision 25-July-93
Perishable items: Other: 1 cement brick pressure posters/graphics 20 paper cups vacuum pump + attachments 1 empty soda can vacuum grease + rubber stopper 1 empty rubber balloon Magdeburg Sphere 1 jar of baby food bed of nails sledge hammer + mask gloves plastic case green plastic tarp shield
P1: Hi. My name's P1 and here is P2, and we are both members of Science Theatre. Science Theatre is a group of graduate and undergraduate students from the College of Natural Science at Michigan State University. Among other things, we take our shows and demonstrations to local schools and events like this. We also write a weekly column for the Lansing State Journal every Wednesday in which we answer questions people send us. The column is called, 'Ask Science Theatre' and can be found on the Flip Side of the Today section every Wednesday.
Before we start, I'd like to tell you that a lot of time, energy and thought have gone into the demonstrations we're going to be showing you today. We have designed things very carefully and practiced so that everything we do is safe. By no means should you try any of these things at home, even if they look like they're very simple.
P2: Having said that, I want to tell you a little about pressure. We'll start by thinking about what it means for something to be empty.
P2 takes a balloon and blows in it and holds it.
For example, I have here an empty balloon, right?
P1: Well, the balloon's not actually empty...
P2: What do you mean? There isn't anything in it - no water, or sand..
P1 takes the balloon from P2 and let it go.
P1: If it's empty, then what just came out of it?
P2: But that's just air!
P1: What do you mean just air? Air is important.
P2: Well, yeah, for breathing.
P1: I think you're missing the point here. Why does a balloon stay inflated?
P2: Air is made up of gas molecules. The molecules in a gas are constantly moving. When we put the molecules in a container, the air molecules bounce off the sides of the container. If the container is made out of something which gives -- like the rubber of the balloon -- the motion of the molecules actually keeps the balloon inflated.
P2 takes a graphic with a balloon and arrows for the air molecules.
P1: In scientific language, we say that the air molecules exert a force on the balloon walls. A force is just a push or a pull - the air molecules push on the walls, forcing the balloon out.
P1 points to the balloon graphic.
On this picture of an inflated balloon, we indicate the direction of the force by arrows. If there is no air inside the balloon, then there are no molecules pushing on the balloon walls and the balloon deflates.
P2: If air is made up of molecules, then those molecules must constantly be bouncing off the things -- like you and me -- that they encounter.
P2 jumps around, as if being hit from all sides.
But the forces due to these molecules can't be very strong since we don't feel them.
P1: The air molecules are hitting us from all sides, but the same force is being exerted on all sides.
While P1 is talking, P2 begins to push on the table which starts moving. At that moment, P1 pushes on the other side and so the table stop moving.
But like with the balloon and this table, the molecules of our body are also pushing as strong as the molecules of the air so we don't notice the forces of the air molecules, but they're actually quite strong. For example, here's an empty soda can. (To person in audience) Would you take a look at this can and verify that it is indeed empty?
P2: But the can is not actually empty -- it's filled with air molecules, remember ?
P1: That's right. (To audience) Can anyone guess what might happen if I remove some of the air molecules from the inside of the can? (Pause to take responses) Let's go ahead and see what happens. I've got a rubber stopper here, with a hole punched in the middle. I'm going to put it in the opening of the can to make a good seal.
P2: In order to suck the air out of the soda can, we're going to use a machine called a vacuum pump. A vacuum pump acts very much like a vacuum cleaner. It sucks air. It doesn't feel much different than putting your hand up to a vacuum cleaner.
P1: I'm going to attach the vacuum pump to the can and turn the pump on. You'll notice that I'm holding the can by the bottom so there's no possibility that I'm affecting what's happening.
P1 turns the vacuum pump on and puts the can on the attachment and then shows the can to the audience. Meanwhile, P2 grabs the poster with the can and arrows and holds it besides P2.
P2: Before we turned the pump on, there were air molecules on the inside and the outside of the can exerting equal forces against each other in opposite directions. When the pump was turned on, the molecules were removed from the inside of the can, and there was less force pushing from the inside than from the outside of the can, so the can crumpled.
P1: If you were listening carefully, you probably heard some hissing at the end, and the pump gurgling. As the can crumpled, the walls broke, and air molecules were able to get inside the can. If the can didn't split like this, we could collapse it all the way to a flat pancake. I'll pass the can around, but be careful not to touch the sharp parts.
P1 passes the can around and picks up the Magdeburg sphere.
P2: This is a very impressive demonstration, but the idea is far from new. Over 300 years ago, Otto Von Guericke invented the first vacuum pump in the city of Magdeburg, Germany. However, he needed some nice demonstration to show what his new invention could do for his Emperor, Ferdinand the third. So Von Guericke used the pump to perform the very first pressure demonstration. This is a copy of the equipment he used. In honor of Von Guericke's city, it is called a Magdeburg sphere. The apparatus is just two steel hemispheres which fit together and a rubber ring which serves to make a better seal between the two pieces. If I put the two hemispheres together, I can easily pull them apart. What will happen if I remove some of the air molecules from the inside of these two hemispheres?
P1: The soda can crumpled, but these hemispheres are made of a much stronger metal, so I don't see how they could collapse.
P2: Well, let's give it a try and find out.
P2 and P1 connect the sphere to the vacuum pump and remove the air inside.
I need two volunteers from the audience, two strong volunteers.
P2 asks the name of the volunteers and repeats their name to the public.
The idea is to see if the two of you can pull these hemispheres apart.
The two volunteers try and fail. P1 and P2 thank and applaud them.
P1: When von Geriecke originally did this demonstration, he hooked one end of the spheres up to a team of horses, and the other end to another team of horses. Even pulling their hardest, the two teams of horses couldn't pull the sphere apart.
P2 picks up the poster representing the Magdeburg sphere.
P2: This is basically the same as we showed with the soda can. When there's no vacuum, the molecules on the inside exert the same amount of force as those on the outside. When we remove the air from the inside, the forces due to all of the molecules on the outside press the two halves of the sphere together. If I turn the valve and let air back into the inside of the sphere, I can pull the sphere apart easily.
P1: So when you're trying to pull the two halves apart, you're really trying to pull against the force due to the air around us. These guys looked pretty strong, but they're not stronger than the air!
P1 picks up a jar of baby food.
P2: That's right. There are actually a lot of examples of this in our everyday lives.
P1: For example, a lot of food is packaged in glass jars with metal lids, like this jar of baby food. The lid has a metal button which is pushed in. The writing on the top of the lids says "Reject if button is out" When food is canned, the manufacturer pulls air out of the jar to keep the food from becoming contaminated. The button is pushed in because a small number of air molecules push from inside, but a large number of air molecules push down on the outside of the jar. When I open the jar, air enters, so that the forces are equal and the button pops up.
P2: OK, Let's see what we have learned today.
P1: Air molecules exert a force on everything. And of course, that science is fun!
P2: We'd be happy to take any questions. Thank You.