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I always used to wonder why this happens.. when one stretches a rubberband to nearly it snapping point holding it close to your skin - preferably cheek(helps feel the heat), it emits heat. While releasing the stretched rubberband holding it close to the skin produces a cooling effect on the skin. Can some one explain the physics behind this pls?

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This is a very interesting question with a very interesting answer. The key lies in the reason for the stretchiness of the rubber band.

Rubber is made of polymers (long chain molecules). When the elastic band is not stretched, these molecules are all tangled up with each other and have no particular direction to them, but when you stretch the elastic they all become lined up with one another, at least to some extent. The polymer molecules themselves are not stretched, they're just aligned differently. To a first approximation there's no difference in the energy of these two different ways of arranging the polymers, but there's a big difference in the entropy. This just means that there's a lot more different ways that the polymers can be arranged in a tangled up way than an aligned way. So when you release the elastic band, all the polymers are jiggling around at random due to thermal motion, and they tend to lose their alignment, so they go back towards the tangled state, and that's what makes the elastic contract. This is called an entropic force.

Now, I said earlier that there isn't any difference in energy between the stretched (aligned) and un-stretched (tangled) states. But it takes energy to stretch the elastic -- you're doing work to pull the ends apart, against the entropic force that's trying to pull them back together. That energy doesn't go into stretching the individual polymer molecules, but it has to go somewhere, so it ends up as heat. Some of this heat will stay in the elastic (making the polymer molecules jiggle around a bit faster) but some will be transferred to the surrounding air, or to your skin.

The reverse happens when you let the elastic contract. The molecules are jiggling around at random and becoming more and more tangled, which makes them contract. But to contract they have to do work on whatever's holding the ends of the elastic apart. That energy has to come from somewhere, so it comes from heat.

At first this might seem to run against thermodynamics - normally you can't just cool something down without heating something else up. But remember that the state with the tangled molecules has a higher entropy than when they're aligned. So you're taking heat out of the air, which reduces its entropy, but this reduction in entropy is countered by the increase in entropy of the elastic itself, so the second law is safe.

For further reading you can look into the ideal chain, which is an idealised mathematical model of this situation.

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  • $\begingroup$ Beat me to it by a couple of minutes! :P +1, very clear explanation. $\endgroup$
    – Kitchi
    Feb 22 '13 at 11:57
  • $\begingroup$ A good answer but my rubber bands only seem to get warmer when they are stretched and released. How can I do this to make it cool a lot? $\endgroup$ Feb 22 '13 at 13:56
  • $\begingroup$ @MarkRovetta it's a small effect and will depend quite a bit on the material the band is made of. To observe the cooling effect, you should fairly slowly stretch a rubber band until it's nearly at breaking point. Just do this once, not repeatedly. You should be able to feel it heat up slightly. Then wait for it to cool back down to room temperature, then fairly slowly release it. If you can't feel a cooling effect, experiment with different rubber bands. When you release it, it will absorb less heat than it gives off when you stretch it, so it will never cool by a huge amount. $\endgroup$
    – Nathaniel
    Feb 22 '13 at 15:20
  • $\begingroup$ Can't wait to get a rubber band now! Next question: How many contracting rubber bands would it take to make an ice cube?! $\endgroup$
    – Widor
    Feb 22 '13 at 17:31
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    $\begingroup$ @Widor I looked into this a bit and, while it probably can't reach zero degrees, I did find this NASA-designed device that uses rubber bands for cooling. The design uses 12 bands, although in principle it would work with just one, since it expands and contracts them repeatedly to produce a heat pump cycle. $\endgroup$
    – Nathaniel
    Feb 24 '13 at 2:23
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Rubber elasticity is an entropic phenomenon. When you stretch the elastic you force the constituent chains into a more ordered state, but you haven't stretched the chains themselves. Entropy cannot go down, so the entropy becomes thermal motion of the atoms, i.e. they get hot.

The same thing happens when you relax the elastic band.

Some neat consequences of this:

1) You can actually make a heat engine out of elastic bands and a light bulb (see youtube)!

2) Rubber contracts when heated and expands when cooled

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  • $\begingroup$ When you say "entropy becomes thermal motion," do you mean "energy becomes thermal motion"? $\endgroup$ Mar 12 '19 at 5:28
  • $\begingroup$ “The same thing happens when you relax the elastic band” – I am not quite sure this is the entire story. Your argument is essentially “molecules get more ordered, entropy cannot go down => they start moving faster”. However, when you release the band, molecules become less ordered, therefore entropy goes up... and? $\endgroup$
    – kirelagin
    Oct 19 '19 at 20:26
  • $\begingroup$ @kirelagin The whole cycle is assumed adiabatic so there is no change in entropy. Instead the entropy sloshes from one molecular disorder to atomic disorder. $\endgroup$
    – user16035
    Oct 25 '19 at 21:43

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