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Statistical Mechanics - Physics at Oregon State University

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A.6. SOLUTIONS FOR CHAPTER 6. 257<br />

For small values of B we have<br />

N ≈ λV<br />

For small values of x we have<br />

e−x ≈<br />

1 − e−2x and hence<br />

Hence<br />

1 − x<br />

2x − 2x 2 + 4<br />

3<br />

N ≈ λV<br />

M = kBT<br />

<br />

∂N<br />

∂B T,µ,V<br />

eB<br />

2π22 eB <br />

2mck e− B T 2πmkBT<br />

c<br />

1<br />

=<br />

x3 2x<br />

1<br />

eB − mck 1 − e B T<br />

1 − x<br />

1 − x + 2<br />

1<br />

≈<br />

3x2 2x (1−x)(1+x+1<br />

3 x2 ) ≈ 1<br />

2x (1−1<br />

6 (2x)2 )<br />

eB<br />

2π22 <br />

2πmkBT<br />

c<br />

mckBT<br />

2 1 eB<br />

(1 − )<br />

eB 6 mckBT<br />

N ≈ λV mkBT<br />

2π23 <br />

2πmkBT (1 − 1<br />

2 eB<br />

)<br />

6 mckBT<br />

M = kBT λV mkBT<br />

2π23 <br />

2πmkBT B<br />

2 e<br />

3 mckBT<br />

χ = kBT λV mkBT<br />

2π23 <br />

2πmkBT 1<br />

2 e<br />

3 mckBT<br />

and using the expression for N <strong>at</strong> T = 0:<br />

χ = kBT N 1<br />

2 e<br />

=<br />

3 mckBT<br />

N<br />

2 e<br />

3kBT mc<br />

A.6 Solutions for chapter 6.<br />

Problem 4.<br />

A quantum mechanical system is described by a Hamiltonian H = H0 + κV ,<br />

with [H0, V ] = 0. κ is a small constant. The Helmholtz free energy is Fκ(T ).<br />

Calcul<strong>at</strong>e the change in Helmholtz free energy, ∆F = Fκ − F0 for this system<br />

up to second order in κ<br />

kBT .<br />

Z = T re −βH = T re −βH0−βκV = T re −βH0 e −βκV

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