Office of Air Quality Air Quality Planning & Assessment Division

Address/Phone/Fax aqp@tnrcc.state.tx.us Help Search February 24, 1997

Air Pressure

Purpose:

To show students the affects of air pressure.

Grade level:

Sixth

Essential Elements:

Acquire data through the senses. (3)(B) classify matter and forces, organisms, actions, and events from the environment according to similarities and differences.

Focus:

Show the students a bottle that has a mouth that is too small for an egg to pass through.

Objective:

The student will be able to identify air pressure and how it affects objects.

Materials:

Two hard boiled eggs, matches, paper towel, small jar (opening large enough to let the egg almost pass through), thin wooden board (a strip of thin plywood or paneling that is 2 ft. long by 3 to 4 inches wide.)

Background:

Air pushes on all surfaces that it touches. Air at sea level exerts a pressure of approximately 14 1/4 pounds per square inch. Therefore, on a piece of paper 10 inches square the air exerts a pressure of 1,450 pounds on the top surface. The same pressure is also exerted on the opposite surface, counteracting the pressure on the other side.

Procedure:

A. Begin class with the "Egg in the Bottle."

1. Peel both hard boiled eggs just before doing the demonstration.
2. Set a small piece of paper on fire and drop it into the bottle. Place one hard boiled egg gently on the opening of the bottle, small end first. The egg may "dance" and wobble on top of the opening. Then, the egg will appear to be pulled into the bottle after the heated air from the fire has contracted.
3. Allow the students to explain why the egg went into the bottle.

As the air was heated, it began to expand. Some of the air escaped causing the egg to wobble. When the fire was extinguished, the air began to cool and contract. The egg seals the bottle. There is less air in the bottle causing unequal pressure to occur between the air in the bottle and the air outside the bottle. The greater air pressure on the outside pushes the egg into the bottle equalizing the air pressure inside and outside the bottle.

4. Define air pressure: air pushes on all surfaces that it touches. This push is called air pressure.
5. Allow students to explain how you could get the egg out of the bottle. Keep in mind the first demonstration.
6. Hold the bottle upside down with the small end of the egg in the bottle neck.
7. Tilt the bottle down until there is a small opening between the neck of the bottle and the egg.
8. Blow hard into the bottle making a closed seal with your mouth. Before you remove your mouth, tilt the bottle upside down.

B. Demonstrate the "Weight of Air."

1. Place one end of the thin board on a table with slightly less than half the board hanging off the edge.
2. Lay a sheet of newspaper over the part of the board on the table.
3. Allow students to predict what would happen if the protruding end was struck as hard as possible.
4. Strike the protruding board as hard as you can.

The paper will not move. If you hit hard enough, the stick will break. This is due to the air pressure that is exerted downward. Since the paper is flat against the board and table, no air is beneath the paper to counteract the pressure from above.

5. Calculate the surface area of the newspaper. (length by width)
6. Multiply the surface area by 14.5. The result will be the amount of air pressure exerted on the paper.
7. Have students predict what would happen if you pushed down on the board slowly.
8. Place your hand on the protruding piece of the board and slowly push down.

Checking Understanding:

• What do we call the pressure that pushes on all surfaces it touches.
• Explain how the egg dropped in the bottle.
• Explain why the board did not move the paper.

Acknowledgment:

Martha Suarez, Stephen F. Austin University Nacogdoches TES Course, 1994