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Homework Help: Fermi Energy of Liquid He3

  1. Feb 19, 2012 #1
    1. The problem statement, all variables and given/known data

    The atom [itex]He^3[/itex] has spin 1/2 and is a fermion. The density of liquid [itex]He^3[/itex] is [itex]0.081g/cm^3[/itex] near absolute zero. Calculate the Fermi energy [itex]\epsilon_F[/itex] and the Fermi temperature [itex]T_F[/itex]

    2. Relevant equations

    [itex]\epsilon_F = \frac{\hbar^2}{2m}(\frac{3 \pi^2 N}{V})^{2/3}[/itex]

    [itex]T_F = \frac{\epsilon_F}{k}[/itex]

    3. The attempt at a solution

    In the problem I'm given the density is [itex]0.081g/cm^3[/itex], which is my N/V. Assuming that m is the mass of [itex]He^3[/itex], then [itex]m = 5.008*10^{-24}g[/itex]. I should then be able to do a straight forward plug-n-chug; however my units don't work out as I get:

    [itex]\epsilon_F = \frac{\hbar^2}{1.0016*10^{-23}g}(3 \pi^2 0.081g cm^{-3})^{2/3}=1.99*10^{-50} kg^{5/3} m^2 s^{-2}[/itex]

    Clearly, this is not a unit of energy. What am I doing wrong?
  2. jcsd
  3. Feb 19, 2012 #2
    I need to use number density, rather than straight density. Which should take care of my problem.
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