Key Concepts
Energy
Heat
Physics
Thermal energy
Thermal expansion

Introduction
Ever wonder why rubber bands so easily snap back into place after being stretched? When stretched out, rubber pulls back hard to return to its original shape. It’s pretty resilient stuff! Of course, you’ll find that if you leave a rubber band wrapped around something long enough—say, a year or two—it will eventually stretch out permanently and may even snap. But what is it about rubber that makes it so stubbornly resist being permanently deformed? The answer has a lot to do with its unusual molecular structure and how thermal energy, or heat, constantly acts on its molecules. In this activity you’ll grab a hair dryer and a weight to explore just how rubber molecules go to work.

Background
Matter usually takes up more space when it’s warmer and less space when it’s cooler. Scientists refer to this tendency as thermal expansion, and it’s most obvious in gases. If you’ve ever left a water bottle in a hot car, you’ll be familiar with the hiss of hot air and water vapor that escapes when you remove the cap. When it comes to liquids, thermal expansion helps explain how a thermometer works. Heated by warm temperatures, the expanding red liquid has nowhere to go except up the thermometer’s glass tube. It’s usually toughest to recognize thermal expansion in solid objects, but during a hot summer long sections of train tracks can actually expand and buckle due to the stress of thermal expansion compressing the rails lengthwise.

Materials
• Ice cube
• Tape
• Graph paper
• Hair blow dryer
• Something relatively heavy to act as a weight (a hammer, a stapler or bicycle u-lock all work perfectly well)
• Graph paper

Procedure
• Take a thick, broad rubber band and pinch one end with each hand. Gently pull it taught without stretching it out.
• Carefully put the rubber band to your face. Note the temperature of the rubber band. What do you notice?
• Quickly stretch the rubber band out, and keep it stretched. Put your face to the rubber band again. How did the temperature change? Why do you think this is?
• Don’t let the rubber band return to its original shape just yet—our goal is to let it return to room temperature first! Move the rubber band away from your face, and wait about 15 seconds.
• After 15 seconds have elapsed quickly release the tension on the rubber band without letting go, allowing it to contract and return to its original shape. Put it to your face again. What do you notice about the rubber band’s temperature this time? How do you think we can explain this?
• Tape a piece of graph paper to the wall at about shoulder height.
• Loop your rubber band around your weight so that the weight can hang from the rubber band.
• Pinch the other end of the rubber band between the thumb and forefinger of your nondominant hand (your left hand if you are right-handed, for example). Put the side of this hand against the wall at a point slightly above the graph paper so that the weight hangs freely from the rubber band. Make sure the weight isn’t in contact with the wall.
• With your other hand use a pen to mark the position of the bottom of the weight on the graph paper.
• Take your blow dryer and make sure it’s set to hot. Turn it on and direct the hot air toward the rubber band for about 15 seconds. Angle it downward so that you don’t burn your hand!
• After heating the rubber band, determine if the weight moved up or down. Using your pen, mark the new position of the bottom of the weight. Is what you observe what you expected? How do you think you can explain your observations?
• Wait a few seconds to allow the rubber band to return to room temperature.
• Now, rub the ice cube on the surface of the rubber band. Watch the weight carefully and determine whether it moves up or down. Mark the new position of the bottom of the weight. Is what you observe what you expected? How do you think you can explain your observations? What do you notice about the relative positions of all the marks you made?

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