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Homework Help: Gas pressure in gravitational field from the partition function

  1. Jul 26, 2011 #1
    1. The problem statement, all variables and given/known data

    Please see P2 in http://panda.unm.edu/pandaweb/graduate/prelims/SM_S09.pdf

    "Starting with [itex]\mathbb{Z} (z_1,z_2)[/itex] above, derive expressions for the gas pressure..."

    2. Relevant equations

    3. The attempt at a solution

    To find the pressure at the top and the bottom of the gas column, I tried to use [tex]F=-kT\ln \mathbb{Z}[/tex], where F is the free energy, and [tex]P=-\left(\frac{\partial F}{\partial V}\right)_T[/tex] by writing [itex]h=\frac{V}{A}[/itex] and got a horrible expression. So I don't think that is the way to go for a timed exam. Moreover, I'm not sure how I can get the pressure at a specific height from that.

    Now, I'm thinking about starting from the probability [itex]p=\frac{exp[-\beta (\frac{p^2}{2m}+mgz)]}{\mathbb{Z}}[/itex], which directly gives the number of particles at height z. But how can I get the pressure from that?

    The second part where I have to calculate the pressure at the top given the pressure at the bottom looks easier though, since I have the pressure at the bottom as a reference point.
  2. jcsd
  3. Jul 28, 2011 #2
    I think I got it. Integrate [tex]p=exp[-\beta (\frac{p^2}{2m}+mgz)][/tex] along all three momenta, x and y coordinates, and all particle but one. Then I get the probability of a particle to be at height z. From there, I can get the number of particles at height z, and then the force and the pressure.
  4. May 22, 2013 #3
    It seems like it's easier to just write Z as an integral over 6N dimensional phase space, do the integrals over p and q. All that's left is a function of z1 and z2, then try using the thermodynamic derivatives to give you the pressure.
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