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2nd law of thermodyanmics?

  1. Aug 10, 2005 #1
    Why is the definition different in physics and chemistry for the 2nd law of thermodynamics?
  2. jcsd
  3. Aug 10, 2005 #2

    Andrew Mason

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    Entropy is defined the same way. dS=dQ/T. But it is used in different ways.

    Chemists are concerned with the release of energy in a reaction. The entropy difference between two states tells the chemist about the direction of the reaction and the magnitude of heat energy that is releaseable. The physicist is concerned with heat flow between reservoirs and the amount of work required to achieve such flow (or obtainable as a result of such flow).

  4. Aug 11, 2005 #3
    is the kelvin-planck statement the same as the thomsen statement?
  5. Aug 11, 2005 #4

    Andrew Mason

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    They are not the same but they both express the second law. (Wm. Thomson was Lord Kelvin).

    Thomson originally observed that “it is impossible by means of inanimate material agency to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects” . The Kelvin-Planck statement of the second law is: "It is impossible to obtain a process that, operating in cycle, produces no other effect than the subtraction of a positive amount of heat from a reservoir and the production of an equal amount of work".

    So it was not just physics students who struggled with the definition of entropy.

  6. Aug 12, 2005 #5
    it'd be nice if there were just one simple definition of entropy...
  7. Aug 12, 2005 #6


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    The way that I saw it described about 35 years ago was:

    1) No matter how hard you try, the best that you can do is break even.
    2) Breaking even can only be achieved at absolute zero.
    3) Absolute zero is impossible to attain.
    4) Give up and go make a sandwich.

    Of course, that's pretty silly... but I like it. :biggrin:
  8. Aug 13, 2005 #7
    (4) is funny...
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