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Equation (9-27) gives the density of states for a system of oscillators but ignores spin. The result, simply one state per energy change ofbetween levels, is incorrect if particles are allowed different spin states at each level, but modification to include spin is easy. From Chapter 8, we know that a particle of spinis allowedspin orientations, so the number of states at each level is simply multiplied by this factor. Thus,

D(E)=(2s+1)/h0.

(a) Using this density of states, the definitionNh0/(2s+1)=1, and

N=0N(E)D(E)dE

calculate the parameterin the Boltzmann distribution (9-31) and show that the distribution can thus be rewritten as

N(E)Boltz=kBT1eE/kBT

(b) Argue that ifkBT>>,the occupation number is much less than 1 for all E.

Short Answer

Expert verified

a) The expression for B is B=EkBT.

b) The harmonic oscillators are uniformly distributed across individual energy levels. In such cases, the occupation number is effectively less than 1.

Step by step solution

01

Concept used

Maxwell-Boltzmann statistics:

N(E)=BeE/kBT,

02

Apply Maxwell-Boltzmann statistics 

(a)

To find B, we will make use of the normalization condition defined as:

N=0N(E)D(E)dE 鈥︹. (1)

Where , is the occupation number following the Maxwell-Boltzmann statistics:

N(E)=BeE/kBT, 鈥︹.. (2)

The density of states for this system of harmonic oscillators with intrinsic spin:

D(E)=2s+10 鈥︹. (3)

Put the equation (2) and(3) in equation (1), we get

N=0BeE/kBT2s+10dE=B(2s+1)00e-E/kBTdE=B(2s+1)0e-E/kBT-kBT0=B(2s+1)0kBT

Solving for B we obtain:

B=N02s+11kBT

According to the problem:

E=N02s+1

Thus, the expression for B is:

B=EkBT

Put this expression for B into eq. (2), the occupation number is thus:

N(E)boltz=EkBT1eE/kBT

03

Explanation of the occupation number

(b)

Assuming that,kBT>> the denominator of N becomes extremely large and the magnitude ofE pales in comparison with the size of kBT. As a result, the occupation number is much less than 1 for all possible values of E, and that N0holds true ifkBT

In high temperature systems, the harmonic oscillators are uniformly distributed across individual energy levels. In such cases, the occupation number is effectively less than 1.

B=EkBT

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Most popular questions from this chapter

Exercise 52 gives the Boltzmann distribution for the special case of simple harmonic oscillators, expressed in terms of the constant, N0/(2s+1)and Exercise 53 gives the two quantum distributions in that case. Show that both quantum distributions converge to the Boltzmann in the limitkBT.

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