Weight and Pressure of Air

Pressure is defined as the weight of the air on a unit area. At sea level, the weight of the atmosphere above one square inch averages 14.7 pounds. So, the pressure is, on the average, 14.7 pounds per square inch. These activities are designed to show that even though we can't see the air, there is something there.

The most dramatic approach to showing the pressure of air is the old crushed can approach. You need metal can with a screw on lid. Put some water in it and heat it to boiling to replace the air with water vapor. Remove from the heat and carefully screw the lid on the can. As the can cools, the water vapor condenses inside reducing the pressure inside to the vapor pressure of water at the can's temperature, some 50 to 100 millibars at room temperature. The 1000 mb pressure of the atmosphere will generally crush the can. (For some really structurally sound cans, a small dent may be needed to start it.)

It is very difficult to demonstrate the pressure of air without recourse to the standard mercury or water barometer which balances the air with a dense fluid with space at the top. The problem is that air is a fluid and it is difficult to separate a column of air of a unit cross section all the way up to space from its neighboring columns in a convincing way. There are several ways of demonstrating that air is really there.

• Ask the students what happens when they stick their hands out a car window when it is moving. (We know it is unsafe but most people have done it.)
• Immerse an empty glass upside-down into a pan of water and observe that the water doesn't fill the glass.
• Fill the glass to the very top with water, cover it with a sheet of stiff paper, and turn it quickly upside-down while holding the paper. Then stop holding the paper as the pressure of the atmosphere will hold it up.
• Immerse a drinking straw into water, close one end by pressing a finger over the opening, and take out the straw; no water will run out of the straw.