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Homework Help: Final temperature of a gas passed through a porous plug

  1. Nov 22, 2015 #1
    1. The problem statement, all variables and given/known data
    A gas has the following equations of state: [tex] P=\frac{U}{V} \,\,\,and \,\,\,T=3B\frac{U^{\frac{2}{3}}}{N^\frac{1}{3}V^\frac{1}{3}}[/tex]
    where B is a positive constant. The system obeys the Nernst Postulate (S tends to zero as T tends to zero). The gas, at a initial temperature [itex] T_i[/itex] and initial pressure [itex] P_i[/itex], is passed through a porous plug in a Joule-Thomson Process. The final pressure is [itex]P_f[/itex]. Calculate the final temperature.

    This question is from Callen, Thermodynamics (1985). Question 6.3-2

    2. Relevant equations
    The fundamental equation of Joule-Thomson effect is [itex] dT=\frac{v}{c_p}(T\alpha-1)dP[/itex] (*)
    Alpha is the coefficient of thermal expansion and Cp is the heat capacity at constant pressure.
    I think that may be useful know the differential of enthalpy [itex]dH=TdS+VdP[/itex] (assuming N constant)

    3. The attempt at a solution
    I tried to use the equations of state and write [itex]T=3B(\frac{P^2V}{N})^\frac{1}{3}[/itex]
    Then, isolating V in this expression, I tried to find alpha: [itex]\alpha=\frac{1}{v} \frac{\partial v}{\partial T}[/itex] and replacing theese two values in the expression (*). My problem is that I didn't find the Cp.
    Is my logic right? I can conclude by this way or am I completely wrong?
  2. jcsd
  3. Nov 22, 2015 #2


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    Homework Helper
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    You can work it this way, but as you noticed, it can take some work to get a useful expression for Cp.

    Instead, you can try using what you know about enthalpy for the porous plug experiment. Can you use your equations of state to express enthalpy as a function of P and T?
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