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

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SUMMARY

The discussion centers on the thermodynamic principles governing the behavior of a hot gas driving a piston within an insulated cylindrical container. It establishes that while the gas's disordered energy can be partially converted into the ordered kinetic energy of the piston, this process does not result in a decrease in entropy. The second law of thermodynamics remains intact, as real-world applications introduce friction, which increases entropy by converting internal energy into heat. Thus, the system cannot achieve a net entropy reduction.

PREREQUISITES
  • Understanding of the second law of thermodynamics
  • Familiarity with concepts of entropy and isentropic processes
  • Knowledge of adiabatic systems and their properties
  • Basic principles of kinetic energy and thermodynamic work
NEXT STEPS
  • Research the principles of isentropic processes in thermodynamics
  • Study the effects of friction on energy conversion in mechanical systems
  • Explore adiabatic processes and their applications in engineering
  • Learn about the implications of the second law of thermodynamics in real-world systems
USEFUL FOR

This discussion is beneficial for physicists, mechanical engineers, and students studying thermodynamics, particularly those interested in the practical applications of entropy and energy conversion in mechanical systems.

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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?
 
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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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