How Powerful is Air Pressure?

The Idea

How powerful is air pressure? We can answer this by observing what happens when we remove air from where it is normally found.

What You Need

• empty soda cans
• or a larger rectangular can with a screw-on cap, if you can get one. (These are used for cooking oil or paint thinner, and they can be purchased to do this experiment.)
• hotplate
• beaker
• water
• oven mitt or beaker tongs
• pail of cold water

Method

1. Rinse the can, so it is reasonably clean.
2. Put a small quantity of water (2 tablespoons) in the can.
3. Place the can on the hotplate and keep it there until the water boils and steam comes out of the can (Figure 35-1).
4. Carefully remove the can from the hotplate using the mitt or tongs.
5. Quickly immerse the soda can, with steam still evolving, top side down in the water. Observe what happens (Figure 35-2).
6. Remove the rectangular can from the hotplate. With steam still coming out of the can, screw on the cap. Wait until it cools in the air or facilitate the cooling with water or ice. (If y

   

   ou are doing this as a demonstration, this may take a while and a sense of drama can be created by pretending that nothing is happening and going on to the next experiment.)

Expected Results

The soda can will be crushed almost instantaneously. The rectangular can may take a few minutes; it gets crushed slowly (as if by a protégé of Darth Vader using the Force).

Why It Works

Air exerts a pressure of 14.7 pounds on every square inch. As the steam in the cans condenses, the air pressure inside the can drops and there is a difference in pressure between the inside and outside of the can. This is primarily the result of the change in state from vapor to liquid and to a lesser degree from the contraction of the gas in the can as it cools. This means a soda can (6.5 inches high and 3 inches in diameter) has a force of over 900 pounds pressing down on its sides!

Other Things to Try

Try this with a 55-gallon drum as shown in Figure 53-3. Use a vacuum pump connected to the drum through a valve to create the pressure difference. This may take some time, but it will be worth the wait. You may want to warn the people you work with that the boom they are about to hear does not require the emergency response team to be sent in.

The Point

Air pressure is substantial, exerting a force of nearly 15 pounds for every square inch that it is in contact with.

This article is brought to you by:

From this Book:

125 Physics Projects for the Evil Genius
Buy this book »