Question

Why do rubber bands contract when heated instead of expanding?

Asked by: Trenton Vogt

Answer

Rubber band entropyRubber bands are unusual because they are made from loosely-packed chains of atoms. These long molecules behave in a special way because of a property in physics that we call Entropy.

Entropy is a way of measuring the amount of 'disorder' in a system. If all the chains are neatly lined up in rows, we say that the entropy is very low. If they are all tangled up in a mess, we say that the entropy is high. We even have ways of calculating actual numbers for the entropy. But all we need to know here is, the messier the system, the higher the entropy.

Now here's an experiment you can do in your head. Imagine you have lots of long pieces of string, screwed up into a ball. They're not rolled up neatly, or coiled... just screwed up like random scribbles on a piece of paper. It's quite a mess, and the entropy is quite high. Now imagine that you grab hold of the two sides of this mess, and start to pull them apart. Something quite neat will happen... Even though they'll still be tangled up, you'll see that in the middle they all line up in straight lines pointing from your left hand to your right hand. You've created order out of a mess, and you've reduced the entropy in the system.

This is exactly what happens inside a rubber band when you stretch it. The messy tangles line up a bit, and the entropy goes down. Now, in physics, when the entropy goes down, the system gives out heat, and when the entropy goes up, the system takes in heat from around it. (There is a complicated way of finding the equations for this, but we don't need to bother with that here.) But you can actually feel this for yourself with a thick rubber band and your upper lip, which is very sensitive to changes in temperature. If you suddenly stretch the rubber band and immediately press it against your upper lip, you'll feel that it's got hot - heat energy has been given out. More impressive is when you let the rubber band back to its normal length - you'll feel the rubber band go amazingly cool against your skin, as it sucks up heat from you. Try it a few times, as the band gives out heat when you stretch it, and sucks it back in again when you let it back again.

The smart thing is that it works both ways... you can push and pull the rubber band, and feel the heat coming and going, or you can push and pull heat into the rubber band, by heating it up and cooling it down, and the rubber band will respond!

You just noticed that when you let the rubber band contract, it sucks heat away from the environment. But what about your question: What happens if the rubber band is already stretched and you heat it up? You already felt that heat disappears into the rubber band when it contracts. So if we actually put some heat into the rubber band, by heating it up, we can force it to contract all by itself! By heating it up, we increase the entropy. The molecules get twisted and tangled tighter together, just the opposite of what happened with that tangle of string that we imagined pulling apart.

So by heating the rubber band up, we increase the entropy - the amount of disorder among its molecules - and we make it pull itself tighter together. That's why a rubber band contracts when you heat it up.

Answered by: Alex Seeley, M.S., Postgraduate physics student, Cambridge, UK.

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