Rubber Band Experiment

Rubber Band Experiment

BY : Rhiannon Nevinczenko

Let's do an experiment. All you need is a rubber band and a few seconds. Take the rubber band, stretch it out, and touch it to either your cheek or the skin just above your lip (don't break it or release it in such a way that you'll get hurt). Notice the temperature (it should feel warm). Don't leave it in place long enough to absorb your body heat. Now, repeat this, but leave the rubber band unstretched. It should feel noticeably cooler. What's going on here?
This phenomenon can be explained by entropy. The Second Law of Thermodynamics describes that a system will always move from order to disorder - diffusion is a great example of this. This is entropy, the universe's tendency toward disorder, and also what makes rubber work the way it does. Rubber molecules are made of polymers (long molecule chains). The molecule chains tend to be tangled up randomly (having a high degree of entropy). When the rubber is stretched, the molecules are forced to align more in one direction, creating order (decreasing entropy). This tendency toward entropy is why rubber bands snap back into their original shape when you let them go.
What's this got to do with temperature? Well, when you align the polymer chains by stretching the rubber band, they have a chance to pack together into very ordered arrangements. This is called crystallization. Molecules give off heat when they form crystals. This is the heat you were feeling! It dissipates the moment you let go because, once broken out of their crystal formation, the molecules absorb heat instead (ironically, making them feel cold).
Image credit: Bill Ebbesen via Wikimedia Commons, CC BY-SA 3.0.

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