

Home
Contact Info
Course Info
Calendar
Homework
Lecture Notes




Physics 418: Statistical Mechanics I
Prof. S. Teitel stte@pas.rochester.edu  Spring 2004
Problem Set 5
Due Tuesday, April 20, in lecture
 Problem 1 [15 points]
Consider our discussion of Pauli paramagnetism of the free, noninteracting, electron gas. We claimed that the chemical
potential at finite magnetic field differed from its value at
B=0 only to second order in the Zeeman energy, i.e.,
µ(B) = µ(B=0) [ 1 + O(µ_{B}B/_{F})^{2} ]
where µ_{B} is the Bohr magneton and _{F} is the Fermi energy.
Explicitly demonstrate this result by computing µ(B)
to second order in B, in the limit T=0.
 Problem 2 [15 points]
a) Find the chemical potential µ for an ideal (nonrelativistic) Fermi
gas at low temperature, to second order in T, at fixed pressure p.
Note, this is different from what we did in lecture  there we computed µ at fixed density
N/V. (Hint: what is E/V at fixed p?)
b) The Gibbs free energy is related to the chemical potential
by G(T,p,N) = Nµ(T,p). The entropy can be derived from the
Gibbs free energy, and hence from the chemical potential, by using,
S =  (dG/dT)_{p,N} =  N(dµ/dT)_{p}
Using your result from part (a), find the entropy of the Fermi gas
to lowest order in T. Does your result agree with that given in
Pathria §8.1 Eq.(41), as derived there from the Helmholtz free energy?
 Problem 3 [15 points]
N Fermions A of spin 1/2 are introduced into a large volume V at
temperature T. Two Fermions may combine to create a Boson with
spin 0 via the interaction,
A + A <> A_{2}
Creation of the molecule A_{2} costs energy _{o} > 0.
At equilibrium, the system will contain N_{F} Fermions and
N_{B} Bosons. Provide expressions from which the ratio
N_{B}/N_{F} can be calculated, and perform the
calculation explicitly for T=0. What would this (T=0) ratio be,
if the particles were classical (i.e. quantum statistics can be neglected).
Explain the difference.
 Problem 4 [15 points]
Consider a noninteracting ideal gas of nonrelativistic bosons (i.e. (p) = p^{2}/2m) in d dimensions, where d is any number.
Show that there is no Bose Einstein condensation for any d less than or equal to two.
