- #1
cox24
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Homework Statement
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One mole of 2-D ideal gas is confined in an isotropic cone potential:
[itex] U = \lambda |r| [/itex]
where [itex] \lambda [/itex] is a positive parameter and [itex] r [/itex] is the displacement vector (2-dimensional) from the origin. The mass of each molecule is [itex] m [/itex].
(1) Determine the Helmholtz free energy [itex] A [/itex] of this gas confined in the potential at temperature [itex] T [/itex].
(2) If you want to vary [itex] \lambda → \lambda + \delta \lambda [/itex], you must do some work [itex] \delta W [/itex]. Compute [itex] \delta W [/itex], following Einstein.
(3) Show, as thermodynamics tells us, that [itex] \delta W [/itex] in (2) agrees with the variation of [itex] \delta A [/itex] of the Helmholtz free energy [itex] A [/itex] due to the same parameter change in (2).
Homework Equations
[itex] A = -k_{B}TlogZ [/itex]
[itex] Z = \sum e^{\frac{-H}{k_{B}T}} [/itex][/B]
H = Hamiltonian
[itex]k_{B}[/itex]=Boltzmann constant
Einstein: [itex] \delta W = < \delta H > [/itex]
The Attempt at a Solution
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(1) For an ideal gas, the system hamiltonian (with the potential term added) is:
[itex] H = \sum \frac{p_{i}^2}{2m} + \lambda |r| [/itex]
so,
[itex] Z = \sum e^{\frac{-(p_{x}^2 + p_{y}^{2})}{2mk_{B}T} - \frac{\lambda |r|}{k_{B}T}} [/itex]
How is this sum computed for the canonical partition function of a 2-D ideal gas with the hamiltonian included?
Also, for (2),
[itex] \delta W = < \delta \lambda H > = \delta \lambda < |r| > [/itex]
this answer is wrong, but I really don't see what I'm missing here.
Any help would be appreciated, thanks.