Pressure

(or the Amazing Can-Crushing Demo)

Purpose:

To examine the influence of pressure differences

Equipment:

clean and empty aluminum cans (soda cans with ridges at the ends work best), hot plate, shallow dish or pan, safety equipment (heat resistent gloves; tongs, goggles also recommended)

Procedure:

Fill the dish or pan about half full with water.

Put ~15mL of water into a can (just enough to cover the bottom). Place the can on the hot plate and turn on the hot plate. Heat the can and water until the water is boiling. This will likely take just a couple of minutes. Steam will come out of the hole in the top of the can when the water is boiling. Let the can steam for at least another minute to try to maximize the ratio of water vapor to dry air inside the can.

As rapidly and carefully as possible remove the can from the hot plate, turn it upside down, and submerge the top of the can in the water. The faster this is done, the better. It is recommended to have the pan of water near the hot plate so you may grab the can about in the middle with the hand turned over (thumb pointing down) and in one motion turn it over and into the water.

With a little luck and skill the can will be crushed and water will be drawn up into the can.

Explanation:

When it is first put on the hot plate the can is filled with air (except for the very small volume of liquid water at the bottom), and there is no difference between the air pressure on the inside and the outside of the can. When the water begins to boil the gaseous water vapor begins to take up some of the volume of the can and force out dry air that had been occupying the can. As the can top is put into the water, the can and thus the hot water vapor are cooled. The water vapor condenses back into liquid water and thus occupies a much smaller volume. Since the hole in the can is submerged in water, air is not able to rush in and fill that volume. With very little air inside the can the air pressure inside the can is much less than the air pressure outside of it. The can is crushed by the net inward forces exerted by this pressure difference.

Also, since the air pressure pushing down on the water outside of the can is much greater than the air pressure pushing down on the water through the drinking hole of the can, water is drawn up into the can.

To further convince yourself the condensation of the water vapor is what leads to the rapid fall of pressure inside the can, try the same procedure without putting any water into the can. As long as the can is dry, some water will likely be drawn into the can, but it will not be crushed. However, do remember this is an extreme demonstration and that evaporation and condensation have only a small effect on air pressure in the atmosphere.

Conclusions:

1) Atmospheric pressure acts in all directions (right and left as well as up and down).

2) Pressure imbalances exert a net force on objects.