Re: [Phys-l] Rubber band and thermodynamics

[John Denker <jsd@av8n.com>]

On 01/24/2012 03:22 PM, Bernard Cleyet wrote:
> I am planning on doing activity today in which stretch a rubber band
> against their lips. It should feel colder when it's allowed to
> contract again. All the explanations I've found get into polymer
> chains and the chemistry. I was hoping to demonstrate conservation of
> energy as the elastic potential energy of the band becomes thermal
> energy. But as the rubber band contracts shouldn't it be releasing
> thermal energy as its losing it's elastic potential energy??

Here's a simpler answer:

Executive summary:  As usual, the analysis requires using 
the first law (not just the second law).

In more detail:  The rubber band is losing its potential
energy *to your hand*.  That is to say, it is doing work 
against your hand, thereby transferring energy across the
boundary of the system. This is important, because if it
contracted without sending energy across the boundary, the 
cooling would be much less (if any), because you would need 
to do something with all that energy.  If you deposited it 
in the heat capacity of the rubber, the net result would
probably be heating rather than cooling.