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Homework Help: Statistical mechanics - Helmholtz free energy and Z

  1. Apr 5, 2008 #1
    1. The problem statement, all variables and given/known data
    The neutnral carbon atom has a 9-fold degeerate ground level and a 5-fold degenerate excited level at an energy 0.82 eV above the ground level. Spectroscopic measurements of a certain star show that 10% of the neutral carbon atoms are in the excited level, and that the population of higher levels is negligible. Assuming thermal equilibrium, find the temperature.

    3. The attempt at a solution

    Using the fact that P(s) = 0.1 = ..., I have found that T = 5900 K.

    Now I want to verify F = -k*T*log(Z) and P(s) = exp(-F / kt)/Z by plugging in the same values, but it doesn't make sense. This is my F:

    [tex]F = - k \cdot 5900K \cdot \ln \left( Z \right) = - k \cdot 5900K \cdot \ln \left( {9 + 5 \cdot e^{\frac{{ - 0.82}}{{k \cdot 5900K}}} } \right)[/tex]

    I insert this is in P(s) = exp(-F/kT)/Z:

    [tex]P(s) = \frac{1}{Z} \cdot \exp \left( {\frac{{ - F}}{{kT}}} \right) = \frac{1}{{9 + 5 \cdot e^{\frac{{ - 0.82}}{{k \cdot 5900K}}} }} \cdot \exp \left( {\ln \left( {9 + 5 \cdot e^{\frac{{ - 0.82}}{{k \cdot 5900K}}} } \right)} \right)[/tex]. I have NOT multiplied with 5 since we are using F.

    I can see that it gives one, since e takes the logarithm, but still - it doesn't make sense, since it should equal 0.1. Can you see where my error is?
    Last edited: Apr 5, 2008
  2. jcsd
  3. Apr 5, 2008 #2
    Is the problem that [tex]P(s) = \frac{1}{Z} \cdot \exp \left( {\frac{{ - F}}{{kT}}} \right)[/tex] only works when I write [tex]F=E-TS[/tex] and NOT [tex]F=-k\cdot T\cdot \ln(Z)[/tex]?

    What I do know is that there is something fundementally wrong in the way I use this expression. It would be great if you could point out my error.
    Last edited: Apr 5, 2008
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