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I was under the impression that the entropy of the universe ALWAYS increases. However in my lectures the lecturer stated the second law of thermodynamics as 'In a spontaneous process the entropy of the universe always increases', which seems to imply that this is not so? However when I looked up the second law of thermodynamics, for example here it states

The Second Law of Thermodynamics states that the state of entropy of the entire universe, as an isolated system, will always increase over time. The second law also states that the changes in the entropy in the universe can never be negative.

Which also seems to imply that the etropy of the universe always increases, not just for spontaneous processes.

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  • $\begingroup$ Your teacher is wrong $\endgroup$
    – Paul
    Feb 22, 2017 at 17:39
  • $\begingroup$ The entropy in a reversible adiabatic process is constant; for that reason it is also called isentropic process. Furthermore, there is not much physical meaning one can assign to the concept of "the entropy of the entire universe". $\endgroup$
    – hyportnex
    Feb 22, 2017 at 18:07

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I think what your book meant was that, at any given time, there are many spontaneous processes taking place in the universe, and they all cause the entropy of the universe to increase. I think what your professor meant was that each and every spontaneous process that occurs causes the entropy of the universe to increase. For a processes that is reversible for both a system and its surroundings, the entropy change of the universe is zero. There are no processes for the combination of a system and its surroundings where the entropy change of the universe is less than zero.

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I think that statement is wrong. The second law, in general, does now say that the entropy of a system increases, it says that it never decrease. At least not in the thermodynamic limit. If you assume that the universe is initially in a pure state following unitary dynamics the entropy will stay constant.

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  • $\begingroup$ Entropy of a system can decrease ,take your room to be a system .Once you cool your room with your AC the entropy of the room decreases but entropy of the whole universe increases. $\endgroup$
    – Paul
    Feb 22, 2017 at 17:52
  • $\begingroup$ I think you mean ,entropy of an isolated system ?right?If so ,please consider editing your answer. $\endgroup$
    – Paul
    Feb 22, 2017 at 17:53

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