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Entropy always increases over time within a closed system, except in idealized interactions that leave entropy unchanged, which are not practically possible. Fr

Entropy always increases over time within a closed system, except in idealized interactions that leave entropy unchanged, which are not practically possible. From what we know, the only thing that might actually be a true closed system is the universe. If you've identified some system that has decreased in entropy, then you've failed to notice something outside of it that has increased in entropy by a greater amount. Is it possible that our understanding of entropy is flawed? Yes. But unless you have some novel physics experiment or mathematical model to share with us, I'll stick with the second law of thermodynamics. >But this does not by itself imply entropy of the system increased. The reason is the system got to a non-equilibrium state which has no macroscopic thermodynamic description and no unique way to assign entropy and detect its changes. Entropy is the tendency to equilibrium. Your earlier statement, that entropy is only a measure of systems that are in equilibrium, is false. If you were right, entropy would be a totally meaningless notion because there are no closed systems and true equilibrium (as opposed to the near-equilibriums that we work with in practical terms) can only exist in one.

marcoperaza 11y ago
My response here is actually overly-simplistic and somewhat wrong. effie is right about classical thermodynamics only defining a measure of entropy (and various other properties of a system) for idealized states of equilibrium. There's been some attempts to define entropy for non-equilibrium systems, but our understanding is incomplete. It's partially a philosophical debate too.
- effie 11y ago
  Admittedly, the concept of entropy density has been used somewhat successfully for description of continuum close to equilibrium. But there are problems with extending this to general non-equilibrium case. I think a mix of oil with water is rather awkward to describe thermodynamically, because it is unstable and it is hard to introduce thermodynamic description for a fluid consisting of many drops of oil of different sizes scattered in water. Second law of thermodynamics can be phrased in terms of entropy, but it does not really involve time at all. All it says is after process involving no heat transfer is finished, entropy of final equilibrium state is not lower than entropy of initial equilibrium state.

marcoperaza 11y ago

My response here is actually overly-simplistic and somewhat wrong. effie is right about classical thermodynamics only defining a measure of entropy (and various other properties of a system) for idealized states of equilibrium. There's been some attempts to define entropy for non-equilibrium systems, but our understanding is incomplete. It's partially a philosophical debate too.

effie 11y ago
Admittedly, the concept of entropy density has been used somewhat successfully for description of continuum close to equilibrium. But there are problems with extending this to general non-equilibrium case. I think a mix of oil with water is rather awkward to describe thermodynamically, because it is unstable and it is hard to introduce thermodynamic description for a fluid consisting of many drops of oil of different sizes scattered in water. Second law of thermodynamics can be phrased in terms of entropy, but it does not really involve time at all. All it says is after process involving no heat transfer is finished, entropy of final equilibrium state is not lower than entropy of initial equilibrium state.