Thermodynamics of rubber bands

From: Benjamin Saks <bensaks_at_gmail.com>
Date: Wed, 23 Jun 2010 05:53:17 -0400

Found this on wikipedia and I thought it might be interesting to discuss how
our winding and flying(unwinding) affect the rubber. The physics is a little
heavy.

Thermodynamics

Temperature affects the elasticity of a rubber band in an unusual way.
Heating causes the rubber band to contract, and cooling causes
expansion.[9]<http://en.wikipedia.org/wiki/Rubber_band#cite_note-8>

An interesting effect of rubber bands in
thermodynamics<http://en.wikipedia.org/wiki/Thermodynamic>is that
stretching a rubber band will produce heat (press it against your
lips), while stretching it and then releasing it will produce an
endothermic<http://en.wikipedia.org/wiki/Endothermic>reaction, causing
it to become cooler. This phenomenon can be explained with
Gibb's Free Energy <http://en.wikipedia.org/wiki/Gibb%27s_Free_Energy>.
Rearranging ΔG=ΔH-TΔS, where G is the free energy, H is the
enthalpy<http://en.wikipedia.org/wiki/Enthalpy>,
and S is the entropy <http://en.wikipedia.org/wiki/Entropy>, we get
TΔS=ΔH-ΔG. Since stretching is nonspontaneous, as it requires an external
heat, TΔS must be negative. Since T is always positive (it can never
reach absolute
zero <http://en.wikipedia.org/wiki/Absolute_zero>), the ΔS must be negative,
inferring that the rubber in its natural state is more entangled (fewer
microstates<http://en.wikipedia.org/wiki/Microstate_%28statistical_mechanics%29>)
than when it is under tension. Thus, when the tension is removed, the
reaction is spontaneous, leading ΔG to be negative. Consequently, the
cooling effect must result in a positive
ΔG<http://en.wikipedia.org/wiki/Gibb%27s_Free_Energy>,
so ΔS will be positive
there.[10]<http://en.wikipedia.org/wiki/Rubber_band#cite_note-9>
[11] <http://en.wikipedia.org/wiki/Rubber_band#cite_note-10>

-- 
BEN SAKS
www.bensaks.carbonmade.com

Received on Wed Jun 23 2010 - 02:53:29 CEST