Experiment of the Week - #298 Balloon Pairs
This week's experiment is an old one that may trick you at first. Once
you think about it, it should make perfect sense.
You will need:
* 2 identical balloons
* a drinking straw
* 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 ballon 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 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.