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Entropy decrease

  1. Jun 3, 2008 #1
    Sorry but I suck at thermodynamics...

    Can entropy decrease LOCALLY, provided that the decrease is compensated by an increase in the rest of the universe (or whatever isolated system we are in), so that the total change of entropy is positive?

    Can you provide a sample phenomenon where this happens?
  2. jcsd
  3. Jun 3, 2008 #2

    D H

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    Yes, entropy can decrease locally at the expense of increased entropy elsewhere. An everyday example is an air conditioner.
  4. Jun 3, 2008 #3
    temporarily yes...
    take a room filled with gas.
    the density in any small given sample volume of the room will have varying number of molecules, thus varying randomess, or entropy.
    However the overall entropy of the isolated system (here, room) will be constant.
  5. Jun 3, 2008 #4

    Andy Resnick

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    Yes- nearly all of biology/biochemistry works using this principle.
  6. Jun 3, 2008 #5
    How easily can you guarantee that the entropy of a certain subsytem (e.g. an amount of water in a container) will be decreased by COOLING it?

    (Obviously, the refrigerator would cause an entropy increase somewhere else)
  7. Jun 3, 2008 #6
    to decrease entropy you must apply work, this work will coase an increase of entropy somewhere else. But I do not think there is an example of entropy decreasing spontaneously, unles you go back to the begining of life (for which we cannot say exactly what happened)
  8. Jun 3, 2008 #7


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    This is completely wrong. Whenever a hot object cools, its entropy decreases. No work is necessary. There is no problem with the entropy of an object or a system decreasing spontaneously. What the Second Law forbids is the tendency for the total entropy in the universe to decrease spontaneously.
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