Rubber Fridge - Why elastic bands get cooler as they shrink!

In this extremely simple experiment you can discover a surprising property of a rubber band, and why rubber is so stretchy.

What you need

What to Do

What may Happen

You should find that the rubber band gets slightly warmer when you stretch it, and then slightly cooler as it shrinks again.

What is going on?

Rubber contains millions of randomly coiled long molecules called polymers, which are each made of shorter molecules known as monomers. At certain points the long polymer chains are joined together or crosslinked, but they are otherwise free to move around or vibrate owing to the heat energy in the structure.

When you stretch a piece of rubber the coiled molecules are pulled straight; when you let go again they move around at random through thermal vibrations, which restore them to their more compact coiled configuration.

So why does an elastic band become hotter when you stretch it and colder when it shrinks?

When you stretch the rubber band you are adding energy to the rubber, which causes it to vibrate more strongly and this makes it hotter. Then, when you let go, the rubber molecules must do work to pull the polymer chains back into their compact configuration; since this process uses energy, the eleastic band must have less energy after it has shrunk and therefore it becomes colder.

Below is a simple computer model of a small rubber molecule. You can drag the green end around and see what happens to the average energy and therefore the temperature of the molecule.

Written by Dave Ansell,Chris Smith