The Effects of Temperature Change on the Entropy of a Rubber Band

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Observations on the effects of hot or cold on a rubber band: a simple Physics experiment and entropy.

Why Does a Rubber Band Expand When Cold and Shrink When Hot?

Summer is nearly over. In our freezer is a nearly empty box of fruit-flavored freezie-pops. These frozen sugar-water confections are a nice treat on a hot day. The depletion of the large box happily coincides with the end of an unusually long and hot summer this year.

To reclaim space in the freezer formerly occupied by this rather large cardboard box, I had gathered the remaining dozen or so freezies and bound them together in a unified bundle using a heavy rubber band. This is so they can stand upright or be more stackable and we may now reclaim more freezer space.

When last I went to retrieve one of these frozen sugary confections I noticed that the rubber band that held them together was now substantially slacker than when the rubber band was first introduced at room temperature. I also know as a fact that from previous experience a rubber band when heated, shrinks. The behavior is contrary to how metals and other solids behave, but it is still serves as evidence of a change in entropy.

Herein lies is a classic observation in Physics, an experiment in entropy. Entropy is orderly or disorderly arrangement of molecules in a substance. A rubber band shrinks when heated and expands when cooled as its entropy state changes. Here are the reasons why this occurs.

The molecular bonds in an elastic band commonly referred to as a 'rubber band' although solid and at rest are very disorganized. There is no 'crystal structure' to rubber bands. The arrangement is very disorganized. It is said to have a high state of entropy.

Explain the Meaning of "Entropy" With an Example

"Entropy" is the state or condition of organization or disorganization in a liquid, gas or solid. The higher the entropy, the greater the disorganization. The more 'ordered' the structure, the lower the entropy is. Think of entropy for a moment as it is in a sugar cube. A sugar cube is a dense, organized crystalline structure with low entropy. It has an orderly state.

But dissolve this sugar cube in a glass of water and the sugar molecules become disrupted, dissociated and mobile. The entropy state is now high. Sugar cube = low entropy. Sugary water = high entropy.

To imagine the 'disorganization' of molecules in a solid but flexible (non-crystalline) substance such as a rubber band, we need to create analogous mental construct. Imagine if you will an entire spool of dental floss pulled from its reel and randomly wadded into a tightly compacted ball without regard. This is analogous to the organization of molecules in the rubber band.Tightly packed, dense and disorganized and in a high entropy state.

If you pinch ahold of either side of this ball of waded-up floss and pull it in opposite directions, the general disorganization of the threads of floss will more-or-less start to become horizontally biased as the ball of string is elongated. You are changing the entropy of the ball of floss, you are imparting order. You are lowering the entropy by introducing an ordered elongated structure.

In Physics as entropy is reduced, heat is given off. There are mathematical formulas that will determine exactly how much heat is lost or gained with changes in entropy but that is outside of the scope of this article. It is enough to know that quantitative heat is lost or gained respective to quantitative changes in the entropy state.

In the case of a rubber band this can be demonstrated easily by stretching and relaxing the elastic band, causing rapid changes in entropy. By holding each end of the rubber band with the fingers of your hands, press your lips firmly to the middle of the rubber band and maintain contact as you quickly stretch the rubber band in opposite directions outward. Your lips you will feel a sudden heat from the rubber band. Reduced entropy (more orderly alignment of molecules) causes the rubber band to lose heat. This is why you feel heat against your lips.

Fun like SillyBandz and Physics!

Here is something really neat; when you quickly release the tension while still maintaining contact with the rubber band, entropy (disorganization) increases and heat is now absorbed from your lips. -You will feel a noticeable cooling! You can repeat this again and again with exactly the same results.

In short, stretching a rubber band decreases entropy by increasing order and causes it to expel (gives off) heat. And conversely allowing a stretched rubber band to quickly retract increases entropy which increases disorder. This in turn causes the rubber band to absorb (takes in) heat from its environment. In this case, it absorbs heat directly from your lips.

Therefore, these equations work even without stretching the rubber band; heating a rubber band with external ambient heat will increase entropy and cause it to shrink, while cooling the rubber band (such as in the refrigerator or freezer) will reduce entropy and cause it to expand.

This explains why the icy-cold rubber band on my freezie pop bundle was now so much looser than it was at room temperature.

-Entropy is sort of complicated but seriously, physics is fun don't you agree?

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Sy Kravitz
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