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Homework Help: Joule Coefficient Derivation

  1. Oct 10, 2015 #1
    The Joule coefficient, μJ, is defined as μ = (∂T/∂V)U. Show that μJ CV = p – αT/κ

    Relevant equations

    What I have attempted was to make μ = (∂T/∂V)U=(∂T/∂U)V(∂U/∂V)T=(∂U/∂V)T*Cv-1.

    Then α/κT=(∂V/∂T)p/(∂V/∂p)T because the 1/V cancels.

    Since (∂V/∂T)p=(∂V/∂p)T *(∂p/∂T)V
    p – αT/κ becomes p-T(∂p/∂T)V

    so μJ*Cv = (∂U/∂V)T (because Cv and Cv-1 cancel)
    (∂U/∂V)T= p-T(∂p/∂T)V

    I try to make the assumption that it is an ideal gas so (∂p/∂T)V becomes nR/V
    so then p-T(nR/V)= p-p= 0. But that would mean (∂U/∂V)T = 0.

    I don't know that this statement is true or if it is safe to assume an ideal gas. I am afraid that maybe I missed a negative or that I used properties of partial differentials incorrectly. Can anyone provide assistance?
  2. jcsd
  3. Oct 11, 2015 #2


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    Hello. Welcome to PF.

    You don't want to assume an ideal gas.

    You've gotten to μ = (∂U/∂V)T*Cv-1. So, you need to deal with the factor (∂U/∂V)T. The first law will be helpful. Then, maybe use a Maxwell relation.
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