Re: How do I tell when a balloon stops expanding?

It is true, a balloon does stop expanding at some point, even though you keep putting air into it. First let's think about what a balloon does. Then we'll examine a few similar systems. Then I'll give you a few ideas on how to figure the volume of the balloon.

What happens when you blow up a balloon? First, you inflate the balloon from empty to the point where the rubber balloon is just full of air. This takes less than one lung-full of air, usually. What is offering resistance when you blow? The balloon's surface is being lifted and gravity is pulling on it. Since the balloon wants to be a flat, empty piece of rubber, gravity pushes against your lungs. Since the balloon is small, gravity doesn't push much, and it is easy to inflate it this far. Next, you blow harder, and the rubber of the balloon stretches. Now, you have to blow harder to inflate the balloon any larger. This is because of surface tension. The balloon's rubber acts like a spring. You push on the rubber with the air from your lungs. The rubber pushes back, wanting to not be stretched. This means that the pressure of the air in the balloon is much greater than air pressure around it.

At some point, the rubber can't stretch any more. Think about a rubber band. You can stretch it to a point, and it stops pulling against you, and it acts like a piece of string. You pull harder, and it eventually snaps. It is just like this in a balloon. You can fill the balloon until it doesn't stretch any more. At this point, the balloon is more like a plastic bag than a balloon; it doesn't stretch out when you put more and more air into it. So what happens? You can still blow into the balloon. Why is that? Your lungs are acting like an air compressor. The pressure of the air in the balloon is pushing out on the walls harder than the rubber will expand. However, the pressure is not pushing so hard that the balloon breaks. Now it is even harder to inflate the balloon because you're just compressing the air in the now stretched balloon. Eventually the air pressure pushes on the wall of the balloon so hard that the balloon breaks.

Another way to think about this is a spring. If you pull a spring, it pulls back. If you pull hard enough, the spring will uncoil into a straight wire. At this point, you can pull harder and harder, but the wire won't stretch. Eventually, if you pull hard enough, it will just break. Just like the balloon.

So- how can you measure the volume of the balloon? Water displacement is ok, but you can't trust the measurments. The water is much more dense than the air, and if you submerge the balloon, the weight of the water above it will press in on the balloon, and compress the air inside. Since the air inside will be compressed, the volume will be smaller. A fun experiment is to take an inflated balloon to a pool. If you swim to the bottom of the deep end of the pool with the balloon, you will see that it will shrink by as much as half. Perhaps the best guess of the volume of the balloon is to use balloons that are more spherical than the typical balloons you find at parties. Even if the balloon isn't perfectly spherical, it is pretty close. If you measure the diameter of the balloon, figure the radius and then use the formula for the volume of a sphere, you could get a good estimate. By the way- be careful blowing up balloons until they explode. The popping balloon can both harm your hearing and your eyes. I would suggest going to a store like Target or Kmart and buying a canister of Helium for about $20. That way, you wouldn't have an exploding balloon right in front of you.

I hope this answers your question.

-Fred