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Balloon Pairs

This experiment is an old one that may trick you at first. Once you think about the forces that are involved, it should make perfect sense.

You will need:

  • 2 identical balloons
  • a drinking straw
  • tape
  • a friend to help you

Inflate one of the balloons about half-full. Don't tie it off. Instead, twist the neck of the balloon several times. Then carefully fit the end of the balloon over one end of the drinking straw and use tape to secure it tightly.

Blow up the other balloon, but only inflate it one fourth of the way. Twist the neck of this balloon several times and attach it to the other end of the straw as you did with the first balloon.

Now you should have a straw with a large balloon at one end and a smaller balloon at the other. What would happen if you untwisted the necks of both balloons? Think about it for a minute. Will the two balloons stay the way that they are? Will air move from the large one to the small one? Will air move from the small balloon to the large one?

OK, have you thought about it? Have you decided what you think will happen? Good. Then let's try it. Carefully untwist the neck of one balloon. Then untwist the other. What happens?

Most people are very surprised by the result. Some people think that the pressure will equalize, resulting in two balloons the same size. Others think that higher pressure in the large balloon will inflate the smaller balloon. Instead, the small balloon collapses and all the air winds up in the big balloon. Why?

When you allow the air to flow freely between the two balloons, the pressure equalizes. If you have the same amount of pressure in both balloons, what is the difference? Size. With round, elastic containers, such as balloons and bubbles, size is very important. The larger the balloon gets, the less pressure it takes to make it expand farther. This is known as the Law of LaPlace. At the equalized pressure, the small balloon can resist, but the larger balloon stretches. That lets air move from the small balloon to the larger, making the size difference even greater and continuing the process.

This is part of the reason that it is so hard to inflate a balloon. When you first start, you have a very small balloon, so you have to blow very hard to stretch the rubber. As the balloon inflates, it gets easier and easier as it gets larger and larger.

The same applies to bubbles. Use a straw to blow some bubbles in a glass of milk. Looking from the side, you will notice that as the foam builds up, the smaller bubbles are the roundest. Where a large bubble and a small one meet; the large one will form itself around the smaller one. As with the balloons, it is much harder to stretch the small bubbles out of shape.

It would be a shame to waste the glass of milk, so get some cookies to go with it as you digest this week's science.