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Second Law of Thermodynamics

  1. Feb 13, 2010 #1
    What is the explanation behind this law? I've read tons of definitions for it and I still can't understand it. Will you please also provide examples for it?
  2. jcsd
  3. Feb 13, 2010 #2
    The second law of thermodynamics is an empirical law meaning we cant derive it some equations, it is a statement that can only be verified by experiment. It concerns heat engines and refrigerators, primarily Clausius's and Kelvin's statements and their equivalence.

    I hope this link will help you:

    http://theory.phy.umist.ac.uk/~judith/stat_therm/node18.html#1_7 [Broken]

    Check out the subsection as well.
    Last edited by a moderator: May 4, 2017
  4. Feb 13, 2010 #3
    That's not true, it can be derived from statistics (see the fluctuation theorem, or even information theory).

    If you have a million coins lying on the ground, and randomly choose one of them to flip over, chances are extremely good that this action moves the distribution closer to 50:50 heads face up. (Do you understand?)
  5. Feb 13, 2010 #4
    I was considering it in a purely thermodynamic sense. You are correct by saying it can be derived from statistical mechanics. Fluctuation theorem is essentially a statical form of thermodynamics.
    Last edited: Feb 13, 2010
  6. Feb 14, 2010 #5

    Vanadium 50

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    Anti-meson, that's a tautology - "you can't derive the 2nd law if you restrict yourself to starting points from which you cannot derive the 2nd law." That's completely unhelpful to the OP.

    This is twice now that, once your statements have been proven wrong, you have attempted to redefine your way out of your mistake. I would recommend that in the future you chose your words with more care, so we can all use the same definitions. In that way, communication will be facilitated.
  7. Feb 14, 2010 #6
    I don't see where I have committed a tautology, since "you can't derive the 2nd law if you restrict yourself to starting points from which you cannot derive the 2nd law" is not what I have said.
    You cannot provide a mathematical derivation of the 2nd law from a purely thermodynamic view - a view that was in the mindset of Clausius, Kelvin, and Planck all of whom originally formalised the second law. Statistical mechanics, can however give a mathematical derivation which Boltzmann provided sometime later.

    I would recommend to you, Vanadium 50, to avoid paraphrasing as most of the time it is incorrect.
  8. Feb 14, 2010 #7
    Your original words, "..we cant derive it [from] equations, it is a statement that can only be verified by experiment" (period), were false. Now, ex post facto, you ask us to reinterpret those words only from whatever different context in which they would not be false? But now you have the problem that such a context ("purely thermodynamic view" means what exactly?) is ill-defined, and doesn't even address the original question you purported to be answering. (Yes, we don't dispute that historically the precursor to today's modern thermodynamics was originally found empirically.)

    But enlighten me: How is Max Planck (the person remembered for reapplying an approach from statistical mechanics to light) representative of a viewpoint ignorant of statistical mechanics?
    Last edited: Feb 14, 2010
  9. Feb 14, 2010 #8
    cesiumfrog, I am not going to enlighten you, you can do that yourself. Read up on some history about Planck and the formalisation of the laws of thermodynamics before your next post and then you might understand my comments.

    On a side note, it has become clear that your choice of Latin is nonsensical. Everything is ex post facto as we live in the present.
  10. Feb 15, 2010 #9


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    As has been mentioned in this thread by cesiumfrog, the 2nd Law of Thermodynamics can be derived, both from the http://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/Laws_of_Thermodynamics/Second_Law_of_Thermodynamics" [Broken], and from statistical mechanics starting point. It is not merely an "empirical law".

    Refer to, for example, P.G. Nelson, J. Chem. Ed. v.65 p.390 (1988).

    I believe that this question has been satisfactorily answered. If the OP has more questions, please PM me and this thread can be reopened.

    Last edited by a moderator: May 4, 2017
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