Make a Rubber Fridge

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

Hold the rubber band firmly at each end.

Touch it to your lips, and remember its temperature.

Move it away slightly and stretch it as far as it will comfortably go.

Touch it to your lips again.

Hold it stretched for a few seconds, then allow it to shrink back to its normal size again.

Feel the temperature again.

Is there any difference?

What may Happen

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

What is going on?

Rubber is made up of a long molecules called polymers, made up of millions of shorter molecules attached together. These long molecules are occasionally joined together or crosslinked, but they are otherwise free to vibrate due to the heat energy in the structure.

If you stretch your piece of rubber, what you are actually doing is straightening out these molecules, and when you let go they will be moved around at random by these thermal vibrations which will tend to make them more wiggley and so the rubber band gets shorter again.

So why does it get hotter and colder?

Well when you stretch the rubber band you are putting energy into the rubber, which causes it to vibrate more strongly and so it gets hotter. When you let go the rubber molecules are doing work pulling the ends of the rubber in, so they must use up some of their energy and so they get colder again.

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