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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.


COLLAPSING DRUM

Figure I. Figure II. Figure III.

Description

Equipment: Ordinary 16 oz. Coke can, 1 gallon metal can, or 55 gallon metal drum
Hot plate

Step 1: Note that this experiment can be performed on any metal can. Depending on the size of the show, Phun Physics demonstrators may use a coke can, a one gallon can, or a 55 gallon drum. The pictures above were taken of the 1 gallon can. There are no other differences in the experiment other than the amount of water added and the time it takes for the experiment to work.

A small amount of water (relative to the can) is added to the can. The cap is left off of the can and the can is placed on the hot plate. (See Figure I.)

Step 2: The can is left on the hot plate until the water begins to boil. After the water has started boiling, the can is left on the hot plate for some time. Finally, the can is quickly sealed using the original cap. It is very important that the cap is securely sealed. (See Figure II.) The can is then removed from the hot plate (in order to protect the table, it may be necessary to place the hot can on a peice of wood).

Step 3: After some time, the can begins to slowly crush in on itself, often very loudly! (See Figure III.) (To speed the process along, ice is sometimes placed on top of the can.)

Explanation

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.

When two objects of different temperatures are put in contact with one another, there is an exchange of thermal energy. This exchange, known as heat conduction, causes the warmer object to cool and the cooler object to warm.

Cooling a solid only lowers the temperature of the solid, the solid will not change states. The solid will, however, become more rigid.

Cooling a liquid to its freezing point changes the material to a solid state. This process is known as freezing.

Cooling a gas to its condensation point changes the material to a liquid state. This process is known as condensation.

Heating a solid to its melting point changes the material to a liquid state. This process is known as melting.

Heating a liquid to its boiling point changes the material to a gas state. This process is known as boiling.

Heating a gas only raises the temperature of the gas, the gas will not change states.

Step 1: This step is used simply to set up. Nothing exciting happens here.

Step 2: During this step, the water is allowed to boil. Some of the water's particles are changing from liquid state to gas state. These warm gas particles rise above the liquid. This steam slowly forces the air particles out of the can. Eventually, the can will contain almost entirely the liquid water at the bottom and wator vapor in the rest of the can. Almost all of the air will have been forced out of the can.

The air outside the can is trying to press the can inwards with a certain amount of pressure. The water vapor inside the can, however, is trying to expand the can outwards with the same amount of pressure. Since these two pressures cancel, the can is unchanged. In order to maintain this equilibrium, the water vapor is escaping out the top of the can.

Step 3: Immediately after the cap has been placed securely on the can and the can has been removed from the hot plate, the can contains almost entirely water and water vapor. As discussed above, the water vapor is trying to expand the can with the same amount of pressure that the air is trying to crush the can. As the can sits in room temperature, however, there is an exchange of thermal energy between the warm water vapor and the cool air (the can acts as an intermediary). This causes the water to cool. Eventually, the water cools so much that it begins to condense. Some of the water vapor particles are changing from gas state to liquid state. Since the pressure exerted by a liquid on the sides of its container is much less than the pressure exerted by a gas, the pressure inside the can begins to drop. This pressure drop causes the can to be crushed by the air surrounding the can.

More Specifically


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