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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.


BELL JAR

Figure I. Figure II. Figure III.


Figure IV. Figure V. Figure VI.

Description

Equipment: 10-12 35mm plastic film canisters
250ml Erlenmeyer flask
Foam shaving cream
Food coloring
Airtight glass bell jar
Vacuum pump

Step 1: The film canisters are placed inside of the glass bell jar. Their tops have been securely sealed. (See Figure I.) The demonstrators ask the audience what they think will happen when the air is pumped out of the bell jar.

Step 2: The vacuum pump is turned on and soon (usually within 30 seconds) the tops of the film canisters begin to fly off very violently. After about one minute, all of the canister tops have popped off. (See Figure II.)

Step 3: The film canisters are taken out of the bell jar. The Erlenmeyer flask is partially filled with shaving cream and food coloring is added to several spots of the shaving cream. The flask containing the shaving cream and food coloring is then put in the glass bell jar. (See Figure III.) The demonstrators ask the audience what they think will happen when the air is pumped out of the bell jar.

Step 4: The vacuum pump is turned on and very slowly at first, the shaving cream starts to rise in a snake-like shape. The food coloring creates streaks on the shaving cream. The "snake" slowly grows, until about a minute and a half later, when the snake has reached its maximum size. (See Figure IV., Figure V., Figure VI.)

Explanation

Basic Ideas: A 35mm film canister, when securely sealed, does not allow air to move in or out of the canister.

Shaving cream is made of many chemicals mixed with air. The chemicals are arranged such that there are many pockets of air trapped inside the shaving cream.

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: Since the film canisters are under normal circumstances (the glass bell jar is full of air), the film canisters are merely sitting there. The air outside the canisters is exerting a certain amount of pressure on the canisters, trying to press them inwards. At the same time, however, the air inside the canisters is exerting the same amount of pressure on the canisters, trying to expand them outwards. These two pressures exactly cancel out.

Step 2: The vacuum pump has been turned on and is slowly pumping the air out of the glass bell jar. This causes there to be less air surrounding the canisters, and therefore less air pressure exerted on the canisters from the surrounding air. The amount of air inside the canisters is the same, however, and so the pressure exerted by the air inside the canisters is the same as in Step 1. As the pressure surrounding the canisters continues to drop, eventually the pressure difference is so great that the seal is broken and the top pops off of the canister. Eventually, all of the tops will pop off of the canisters. (Occasionally, one of the canisters will not be securely sealed. When this happens, air is leaking out of the canister, and the top will not pop off.)

Step 3: Since the shaving cream is under normal circumstances (the glass bell jar is full of air), the shaving cream is of normal proportions. The air outside the shaving cream is exerting a certain amount of pressure on the shaving cream, trying to press it inwards. At the same time, however, the air inside the shaving cream is exerting the same amount of pressure on the shaving cream, trying to expand it outwards. These two pressures exactly cancel out.

Step 4: The vacuum pump has been turned on and is slowly pumping the air out of the glass bell jar. This causes there to be less air surrounding the shaving cream, and therefore less air pressure exerted on the shaving cream from the surrounding air. The amount of air inside the shaving cream is the same, however, and so the pressure exerted by the air inside the shaving cream is the same as in Step 1. The two pressures do not cancel out, and the shaving cream expands. This slow expansion causes the shaving cream to move up out of the flask in a snake-like shape. The food coloring makes the "snake" have stripes.

More Specifically

Basic Ideas: A 35mm film canister, when securely sealed, does not allow air to move in or out of the canister.

The chemical structure of shaving cream is such that there are many pockets of air trapped by molecules. These pockets of air are constantly applying pressure to the molecules in the amount of approximately 15 pounds per square inch.

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.

The air pressure pressing on all things at the surface of the Earth is approximately 15 pounds per square inch.

Step 1: The film canisters are under normal circumstances. The air pressure inside the canisters exactly equals the air pressure around the canisters.

Step 2: The vacuum pump is turned on and the air inside of the bell jar is slowly pumped out. This causes a decrease in air pressure around the canisters. Since the air pressure inside the canisters has not changed, there is now a difference in pressure. The force on the canister tops from the air inside is now greater than the force on the canister tops from the outside. When this difference in force becomes too great, the tops pop off the canisters.

Step 3: The shaving cream is under normal circumstances. The air pressure inside the shaving cream exactly equals the air pressure around the shaving cream.

Step 4: The vacuum pump is turned on and the air inside of the bell jar is slowly pumped out. This causes a decrease in air pressure around the shaving cream. Since the air pressure inside the shaving cream has not changed, there is now a difference in pressure. The force on the molecules from the air inside is now greater than the force on the molecules from the outside. The shaving cream expands. Since the shaving cream is placed in a flask with drops of food coloring, the expansion of the shaving cream causes it to rise out of the flask in a striped snake-like shape.


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