Air Pressure or Splash?


Rationale/Objectives/Purpose:
To demonstrate the reality and power of air pressure.
Time Required:
30-40 minutes of class time
Safety Issues:
Be careful to avoid anyone slipping on spilled water.
Materials:
Procedures:
Teacher:
Explain the concept of air pressure (the pressure of air on the surface of the earth caused by the cumulative weight of air in the atmosphere). Compare air pressure to water pressure, which one can experienced diving to the bottom of a swimming pool.

 
Moisten the edges of the toilet plunger to ensure a good seal, and push it against a solid table or wall. Explain that the plunger sticks because the air has been pushed out from under the cup; with no air, there is no air pressure. The normal air pressure on the outside will now cause the plunger to stick tightly to any surface.

 
Now fill a cup with water and put an index card on top so that it seals the cup. Without air, there is no air pressure inside the cup. When you turn the cup upside down, the air pressure outside the cup is strong enough to counter even the weight of the water in the cup and the card holds. (Unless and until the seal breaks and air rushes in and water out!)

 
Students:
Now let students try it for themselves. You may want to move to an outdoor location, in case of spills.

 
Click here to see setup diagram.
Assessment:
  1. What causes air pressure?

    2. A. The weight of air in the atmosphere.
    B. The magnetic attraction of water molecules.
    C. Suction.
  2. Why is a good seal needed on the plunger?

    3. A. So air won't leak in, causing an increase of air pressure inside the cup.
    B. So air won't leak out, causing a decrease of air pressure inside the cup.
    C. So the magnetic attraction of water molecules will continue.
  3. In your own words, explain why the water stays in the cup when it is turned upside down.
Teacher Background/Extensions:
This can be preceded by activities demonstrating that air is matter (see activity, "Air - something or nothing?") and followed with demonstrations of the principle of flight. Manipulation of air pressure on one side of a wing (as in a bird or an airplane) can be achieved by a design that causes air to move faster on the top. When air is moving fast, it doesn't exert as much pressure - the pressure on the slower-moving underside "pushes" the bird or plane up. Demonstrate this simple concept by holding a narrow strip of notebook paper under your mouth. If you blow down fast, the paper flies up due to the greater pressure under it (the faster moving air on top cannot push as hard).