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Homework Help: Entropy, energy levels and harmonic oscillation.

  1. Aug 19, 2008 #1
    1. The problem statement, all variables and given/known data

    A model to describe the vibrations of atoms in a solid is to assume that the atoms are isotropic harmonic oscillators and that the vibrations are independent of the vibrations of the other atoms. We use this model to describe the entropy and heat capacity of Bohrium (B). The entropy is given as 9.797 J/(mol K) at T = 400K

    What is the heat capacity of B at T = 600K?

    2. Relevant equations

    Energy level of a harmonic oscillator: En = hw(1/2 + n) (1 dimension)
    Z = the partition function = [tex]\sum[/tex]e^(-En/k*T) = 1/(2*sinh(hw/k*T))
    F = -k*T*ln(Z)

    The partition function above is valid for a single atom which is only moving in one dimention but to get the correct parition function for motion in 3D one simply does Zcorrect = Z³ which gives the free energy as

    Fcorrect = -3*k*T*ln(Z)

    S = -([tex]\partial[/tex]F/[tex]\partial[/tex]T)
    Cv = T([tex]\partial[/tex]S/[tex]\partial[/tex]T)

    3. The attempt at a solution

    From the partition function i can derive the function for S in a straight forward way. I am given the entropy at T = 400 and what i need to know to derive the heat capacity Cv from S is the value for hw. But i cant figure out how to get hw from S given the entropy. I have tried solving the function by the Newton Rhapson method using MatLab but it doesnt want to work. So im stuck and im wondering if im even on the right track. Is there a simple way to find the energy levels of a vibration as in this case when given the entropy?
    Last edited: Aug 19, 2008
  2. jcsd
  3. Aug 19, 2008 #2
    Ok i think i got a clue (while looking in a book of all places!). What i need is the Einstein temperature of Bohrium and all will be solved nicely. At least i think it will.
  4. Aug 19, 2008 #3
    I think you can make an approximation sinh(x)=x since x=hw/kT is generally very small for T=400K.

    This just reduces to the classical expression 3k.
    Last edited: Aug 19, 2008
  5. Aug 19, 2008 #4
    I also noticed that it is Boron im working with and not Bohrium as i wrote. I blame the fact that i had to translate the problem to swedish.
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