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Thermodynamic entropy of system of any size.

  1. Aug 18, 2011 #1
    After a bit of calculation, I came up with the following quantity for the bit-entropy of a thermodynamic system.

    We have the following assumptions:

    1. System at thermal equilibrium.
    2. Ideal gas.
    3. Monatomic gas (i.e. no internal degrees of freedom for particles).
    4. All particles have equal mass.
    5. Units are such that k_B (boltzmann constant) normalized to 1.

    Using just information-theoretic arguments (no assumptions from thermodynamics!) I calculated the raw entropy of such a system to be:

    S = (Q/T)[(log(T/T0) - 1) + (log(V/V0) - 1) + log(Q/T)]

    (T=temperature, Q=thermal energy, V=volume of system, T0,V0=unknown normalizing constants).

    This can be simplified to:

    S = (Q/T)[log(Q/T0) + log(V/V0) - 2]

    Further, I suspect it works for any system size, even systems that wouldn't be called 'ensembles' in the thermodynamic sense (like just a single particle, in which case Q=0. In general, we take Q = total energy - kinetic energy of center of mass of system).

    In addition, we find that dS is proportional to dQ/T (i.e. Clausius law of entropy), in the limit where Q >> T (which is always true in thermodynamic ensembles) and volume is held constant.

    Yet another interesting thing about this is that the entropy is not zero at the limit of T=0 (because then Q=0 too). Thus it appears the third law of thermodynamics need not apply from a purely information-theoretic standpoint.

    Is my formula correct?
    Last edited: Aug 18, 2011
  2. jcsd
  3. Aug 18, 2011 #2


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  4. Aug 18, 2011 #3
    Thanks for that link, I didn't know about that. I was just trying to satisfy my own curiosity.

    The Sackur-Tetrode equation appears only to work under the assumption of the uncertainty principle, whereas I derived my formula without any QM assumptions. Therefore I don't think they can be directly compared. However, this part:

    It very similar to my own method (except for the QM part obviously).
    Last edited: Aug 18, 2011
  5. Aug 18, 2011 #4


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    If I am not wrong, Sakur and Tetrode derived their formula in a classical mechanics context (in 1912 QM was not yet discovered), i.e. without involving the uncertainty principle.
  6. Aug 18, 2011 #5
    No, the Sackur-Tetrode equation requires Planck's constant, which was of course discovered in 1899.

    From http://en.wikipedia.org/wiki/Ideal_gas :

    Φ is really just a convoluted way of introducing HUP. It's kind of interesting in itself that the HUP was already 'realized' 15 years before Heisenberg published his paper.
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