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Using a hot gas to drive a piston: entropy reduction?

  1. Nov 4, 2015 #1
    Suppose we have an insulated cylindrical container with a piston inserted from one end. Suppose the volume confined by the piston is full of a hot gas. Now let the gas drive the piston so that the volume is increased.

    Did the entropy of the system decrease because some of the energy of the disordered random-directional movement of the gas molecules (disordered movement) was converted to the linear (one-directional) kinetic energy of the piston (more ordered movement)?

    How does this not violate the second law of thermodynamics?
  2. jcsd
  3. Nov 4, 2015 #2


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    Gold Member

    If you consider the walls of the cylinder adiabatic, best performance you can achieve is an isentropic change of state and the entropy stays the same (all the internal energy would have been transformed into work). In a real application you would have friction increasing the entropy of the system by transforming internal energy also into heat at the surfaces between piston and cylinder. So no entropy decrease after all.
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