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Pathria: Specific Heat in a gravitational field

  1. Apr 22, 2008 #1
    1. The problem statement, all variables and given/known data

    Pathria 6.8: Evaluate the partition function of a classical ideal gas consisting of N molecules of mass m confined to a cylinder of vertical height L, which is in a state of thermal equilibrium at constant T in a uniform gravitational field of acceleration g. Calculate the specific heat and explain why it is larger than the free space value. (The last part is my problem).

    2. Relevant equations

    PV=nrT
    partition function Z = sum (e^(-BE))

    3. The attempt at a solution

    I made it pretty far fr the first part, although I think I made a mistake somewhere. I'm really looking for help on why the specific heat is larger than the free space value. I am wondering if it has to do with pressure due to gravity.

    I used the barometric Boltzmann distribution, taking an arbitrary cross-sectional area A.

    So a cross-section of the column has a volume Adh and mass m=ro(h) Adh

    So the cross-section exerts a force mg ro(h) n the gass below it

    So the excess pressure at height h over the pressure at h+dh is: P(h) -P(h+dh) = -dp = mg ro(h) dh

    pV=nRT
    ro = N/V
    n=N/N(0)
    so P=ro k t
    dro(h)/dh = -(mg/kt) ro(h)
    ro(h)= ro(h(0)) exp[(-mg(h-h0)/kT]

    epsilon=mgh
    sum(n eps) = E
    sum (n) = N

    general partition function is: Q=sum [exp(-BE)]

    E=sum(n(sub h) mgh)
    assume h(0)=0

    Z=sum [exp(-B n(sub h) mgh)] = exp(-BmgNL^2/2) --> is this right??

    to get the specific heat:

    U = N partial ln Q / partial B = N exp (-BmgNL^2/2) (-mgNL^2/2)
    = -(mgN^2L^2/2) exp(-BmgNL^2/2)

    Cv = dU/dT = (-mgN^2L^2/2T)^2 (1/k) exp(-BmgNL^2/2) --> somehow I think this is wrong, it looks like it has the wrong units

    now how do I explain why the specific heat with the gravitational field (this answer) is larger than the free space value?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
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