Kelly Brechin, student and Charles Ophardt, PhD., editor.

Battle of the Balloons

SCIENCE CONCEPT: Pressure. A polymer is difficult to stretch, but once it is started it is easier to stretch.

MATERIALS:

2 Balloons

2 Twist Ties

Spool

DIRECTIONS:

1.Blow up one of the balloons all the way. Tie the neck of the balloon tightly with a twist tie so the air can't escape.

2. Blow up the second balloon about half way. Tie the neck of the balloon tightly with a twist tie so the air can't escape.

3. Stretch the opening of the balloon that was filled up all the way over the end of the spool.

4. Stretch the opening of the balloon that was filled up way around the other end of the spool.

5. Untie the small balloon.

6. Untie the big balloon.

7. The air should go from the small balloon to the big balloon.

INTRODUCTION:

Well, we've been playing a lot of tricks on you today. In most of our experiments, we've been doing the opposite of what you expect us to do. So since you have all gotten so smart, I want to test you again. Which balloon do you think the air will go into once I untie them? The big one or the small one?

EXPLANATION:

The reason the air traveled from the small balloon to the big balloon was the fact that the air in the small balloon was more crowded (or had more pressure) than the air in the big balloon. The small balloon was only blown up a short way so the rubber was not stretched as far as possible. In the big balloon, the rubber was stretched as far as possible. Since the rubber expanded so much, it had more room for air to enter. So the only place for the air to go is into the big balloon since there is more room. Imagine blowing up a balloon. When you first start to blow it up it is very difficult, but once you get it started, it becomes easier. This is the same process. The balloon is already filled up, so it is easier for the air to enter.

According to Herb Miller (2), the problem can also be defined in terms of the Ideal Gas Law and mathematics. Assume the big balloon is number 1 and the small one is number 2.

PV=nRt
P= nRT/V
P(2) > P(1)
R and T are equivalent in the experiment; thus n(2)/V(2) > n(1)/V(1).

This means there are more molecules of air per unit volume in the small balloon (number 2) than in the larger one and reenforces the ideas in the top paragraph.

This can also be further proven as, per Miller (2), when an empty small balloon is placed in a tube opposite the big balloon, the empty balloon inflates but only to a very small degree. This is about the same size (if not exactly) that a partially filled balloon will deflate to when placed on the opposite end of a tube from the big balloon.

REFERENCE:

1. Mouche, Dinah L. More Magic Science Tricks. Scholastic Books, Inc. New York, NY. 1980.

2. Herbert Miller, Civil Engineer in Jefferson Parish (County), Louisiana, private communication.