Step 3

Starting with the rubber band in the stretched position, let it quickly relax and then touch it to your forehead.

Does the rubber band feel warm, cold, or just room temperature?

Note: you may want to repeat these steps a few times to verify your results.

What happened?
Well first, we used our forehead as a heat sensor because it is one of the more heat sensitive places on our bodies (and a lot cheaper than a scientific heat sensor). It is also good to understand that heat is energy, and heat energy always flows from hot to cold.

You should have found that when you rapidly stretched the rubber band it became warmer, or heat energy flowed from the rubber band to your forehead. When you let it quickly relax it should have felt cold, as heat flowed from your forehead to the rubber band. Note, the thicker the rubber band, the more you will notice this change in heat.

How it works.
The first law of thermodynamics deals with the concept that energy can neither be created nor destroyed; but it can be transferred from one object (system) to another. As a simple explanation of the rubber band experiment, energy had to be put into the rubber band (by you) in order to stretch it, so the rubber band's internal energy increased. When you touched it to your forehead, some of this increased internal energy flowed from the rubber band to your forehead in the form of heat. When the rubber band was released, much of it's internal heat energy was also released, resulting in a very cool rubber band.

There is a bit more to this rubber band story than just heat and energy transfer. Rubber bands are unique in how they react to heat. This has to do with their molecular structure, entropy, and the second law of thermodynamics. It would be a bit lengthy for this newsletter, but we will come back to it at a later time.

Explore further:
Experiment with different sizes of rubber bands. Does the length of the rubber band matter (if they are the same thickness)? Do different brands of rubber bands respond differently? Is this because of quality or the materials they are made of?