When a rubber band is stretched, its volume and entropy decrease, causing the temperature to rise. Conversely, when a rubber band is allowed to relax, its volume and entropy increase, leading to a decrease in temperature.
Rubber bands are versatile tools used for various daily tasks, such as securing hair or sealing bags of chips. However, have you ever wondered about the peculiar properties of these rubber rings? When a rubber band is stretched, not only does it appear thinner and change color, but its temperature also changes! On the other hand, when you release a rubber band and it returns to its original size and shape, it feels cool to the touch and returns to its original color.
Rubber band stretching (Photo Credit: Yeti studio/Shutterstock)
If you don’t believe me, try it out for yourself and then come back to learn why rubber bands exhibit such peculiar behavior.
What Are Rubber Bands?
Rubber bands are circular bands made of rubber that come in various sizes, thicknesses, strengths, and colors. They are widely used in different industries worldwide. Typically, rubber bands are made from natural rubber, as it offers superior elasticity compared to synthetic rubber. The rubber used in everyday items often comes from the rubber tree (Hevea brasiliensis), which is primarily grown in Southeast Asian countries like Thailand and Indonesia. Rubber is a unique material because it mainly consists of long-chain molecules called polymers.
Rubber band ball (Photo Credit: FabrikaSimf/Shutterstock)
In its relaxed state, a rubber band’s long-chain molecules are intertwined with each other. However, when a rubber band is stretched, these same polymers untangle and become straight. When a rubber band is stretched tightly, its color often changes to a lighter shade or even white. By stretching the rubber band, you are actually stretching the molecules themselves, not just untangling them. As long as you don’t exceed the “elastic range” and enter the “plastic range,” the rubber band will return to its original form. However, if you stretch the rubber band too far, it will undergo plastic deformation and eventually break!
Why Do Rubber Bands Change Temperature?
After understanding the basic physical nature of rubber bands, we can now explore the unexpected thermodynamic behavior of rubber bands. As mentioned earlier, when rubber bands are stretched, they release heat and feel cool to the touch when they return to their original state. Although many people associate heat release with an energy change, there is no specific difference in energy between a normal rubber band and a stretched one. However, there is a difference in entropy!
When a rubber band is stretched, the polymers align and eliminate all the chaotic tangles. Essentially, the entropy of the molecules decreases, but the universe prefers to move towards entropy. Order requires energy and effort, while entropy is the natural direction of matter. The elastic force that causes the contraction of a rubber band when it is allowed to return to its high-entropy state of tangles and knots is considered an entropic force!
Clearly, the entropic force acting on the polymers resists stretching. Therefore, when we pull on a rubber band, we are doing work on the material. Our fingers or hands contribute energy to the rubber band through this work, but it is not required to untangle the polymers. Some of this energy will cause the polymers to move faster, while the rest will be released as heat, which can be felt (try holding a stretched rubber band against your cheek!).
On the other hand, when a rubber band is released, the polymers must perform some work to overcome the forces that keep them straightened. The energy to do this work comes from heat, which is consumed as the polymers return to their high-entropy position. This utilization of heat from the surrounding air, along with the high vibration of the polymers, causes the rubber band to feel cooler than its surroundings once it returns to its relaxed state.
This is also the reason why rubber bands behave in a peculiar way. While most substances contract when exposed to cold temperatures (such as blood vessels, water, and metals) and expand when warmed up, rubber bands behave in the opposite manner. If you give rubber bands more heat energy, they will continue to shrink and increase their entropy. Conversely, exposing them to cold will cause them to loosen, stretch, and expand!
A Final Word
Next time you put your hair up in a ponytail, you might want to reconsider the rubber band you casually stretch around your locks. While they may seem like simple tools, they represent a unique and fascinating demonstration of the universal laws of thermodynamics. Remember, everything from the rubber band on your wrist to the nuclear heart of the sun is heading towards entropy, even though your life may seem chaotic!