Observations and Results
Did you notice how rubber bands contract when they’re warm and expand when they’re cool? If we’re relying on thermal expansion as our only model for understanding how heated matter behaves, rubber throws us a total curveball! But no law of thermodynamics is being defied here. Rather, knowing how rubber behaves at the molecular level is essential to understanding what you observed.
When atoms are warmer, they vibrate faster. Objects with a simple molecular structure will tend to expand when they’re heated because their vibrating atoms bump into each other harder. What happens to rubber is less straightforward because the atoms in molecules that comprise rubber have a more complex structure: they’re arranged a lot like links in long chains. When an atom in one of these chains gets bumped by another atom, the bumped atom puts a kink in its chain, shortening it. Imagine this process happening to billions of rubber molecules every second. Rubber is constantly contracting because of billions of tiny, vibrating “chains” putting kinks in each other, and it’s all because of heat!
When you added more heat to your system with your blow dryer, you enabled the rubber molecules to pull harder because you caused individual atoms to vibrate faster. When you rubbed the ice cube on your rubber band, you took a lot of thermal energy away, which caused the chains to loosen up and straighten out. Now, think about when you touched the rubber band to your face at the beginning of this activity. When you stretched your rubber band out, you stiffened its chains, causing the molecules to hit one another harder and vibrate faster. This produced more heat. When the band contracted, the collapsing chains became loose and absorbed the vibration of the atoms, causing them to vibrate more slowly. This also caused the rubber band’s temperature to drop.
Your blow dryer essentially turned your rubber band into a heat engine—a machine that turns thermal energy into mechanical work. American physicist Richard Feynman described how to make a more elaborate heat engine that produces steady torque by using rubber bands, a heat lamp and a bicycle wheel . If you replace the spokes on the wheel with rubber bands so that the wheel’s hub remains in the center and mount the wheel on a fixed axle, you can use two heat lamps to cause a cluster of rubber band “spokes” to contract. This displaces the wheel’s center of gravity so that it’s no longer located in the same spot as its axle. As the wheel begins to turn, new rubber band spokes are exposed to the heat lamp and as these bands contract, the wheel’s center of gravity is constantly displaced. The pull of gravity progressively turns the wheel around the axle, generating torque through thermal energy!
More to explore
Richard Feynman: FUN TO IMAGINE 3: Rubber Bands, from YouTube
The Rubber Band Heat Engine, from Adam Micolich on YouTube
How to Make a Rubber Band Heat Engine, from Education.com
Richard Feynman—The Laws of Thermodynamics, from The Feynman Lectures on Physics
Heat Shrink!—Why Rubber Bands Get Shorter When You Heat Them, from The Naked Scientists
This activity brought to you in partnership with Education.com