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The information below is intended to provide a description of the demonstration, an explanation for elementary students, and further explanation for high school students.

Please keep in mind that not all demonstrations are presented at each show.


Figure I. Figure II. Figure III.


Equipment: 2 rubber suction cups
2 metal Magdeburg Hemispheres
Vacuum pump

Step 1: The properties of suction cups are discussed and volunteers are chosen from the audience. The two rubber suction cups are placed firmly together. The two volunteers pull with a lot of force, until finally the suction breaks and the suction cups come apart.

Step 2: The Magdeburg Hemispheres (already together) are shown to the audience and their properties discussed. Two new volunteers are chosen from the audience and they pull with all their force, but are unable to separate the 2 halves.

Step 3: Two new volunteers are chosen and this time they easily pull the two halves apart!


Basic Ideas: A gas exerts pressure on all sides of the container which holds the gas. The amount of pressure is related to the energy of the gas and the amount of gas. The higher the energy, the more pressure is exerted, and the more gas is contained, the more pressure is exerted.

Step 1: A suction cup is designed such that when it is placed firmly against a surface, it will force nearly all the air out between it and the surface. In addition, it is very resistent to allowing air to get in between the suction cup and the surface.

When the two suction cups are placed firmly against each other, nearly all the air between the two of them is forced out. In between the two suction cups, there is nearly no air, and therefore little air pressure. Surrounding the suction cups, however, the air is pressing inwards with a certain amount of pressure. The air pressure surrounding the suction cups forces the two cups together. When the volunteers try to pull the two cups apart, they must pull until a small bit of air is able to sneak in between the two suction cups. When a little air is able to get in between the two cups, the suction is broken and the cups come apart.

Step 2: Before the experiment, the demonstrator pressed the two Magdeburg Hemispheres together and removed all of the air using a vacuum pump. Exactly as before, the volunteers must pull until air is allowed to get inside of the two halves. This time, however, the metal hemispheres are much more resistent than the rubber suction cups -- the volunteers are unable to do it. As in Step 1, the air surrounding the suction cups forces the two cups together.

Step 3: Without allowing the audience to see (hopefully), the demonstrator turns the switch on the hemispheres, allowing air to rush in between them. When the new set of volunteers are in place, the hemispheres are very easy to pull apart! The switch is then revealed to the audience.


Related Topics

The following physics topics are discussed during this demonstration:

Sponsored by the Physics Department and the Center for Science, Mathematics, and Engineering Education -- University of Virginia