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Physics 418: Statistical Mechanics I
Prof. S. Teitel stte@pas.rochester.edu ----- Spring 2004

## Problem Set 4

Due Tuesday, March 23, in lecture

• Problem 1 [15 points total]

In lecture we saw that the quantum N particle canonical partition function could be written as a series involving 0-particle, 2-particle, 3-particle, ...,and N-particle exchanges. The first two terms in this series were,

 QN(T,V) = (VN/ N! 3N) [ 1 ± V -2 i

where f(r)=e-r2/2 and is the thermal wavelength. The (+) sign is for bosons, and the (-) sign is for fermions. The first term above gives the classical partition function, while the second term can be viewed as the leading quantum correction (in the limit that quantum corrections are small). For the calculations below, you may assume that this quantum correction term is small.

a) Explicitly evaluate the integrals to compute the above partition function. [5 pts]

b) Find the corresponding Helmholtz free energy. [5 pts]

c) Using your result in part (b), find the corresponding equation of state. How does the leading quantum correction change the usual ideal gas law, pV = NkBT? [5 pts]

• Problem 2 [10 points total]

Consider photons of a given energy = .

(a) If <n> is the average number of such photons in equilibrium at temperature T, show that the fluctuation in the number of photons is

<n2> - <n>2 = - (1/) (d<n>/d)     where = 1/kBT
[5 pts]

(b) Using the forumula for the equilibrium value of <n>, apply the above result to determine the relative fluctuation in the number of photons

[<n2> - <n>2]/<n>2
Is this large or small? [5 pts]

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