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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

:In a certain design of helium-neon laser, the chamber containing these gases has a perfect mirror at one end. as usual, but only a window at the other, Beyond the window, is a region of free air space and then the second mirror, which is partially reflecting, allowing the beam to exit. The resonant cavity between the mirrors thus has a region free of the helium-neon gas-the "lasing material"-in which you can insert something. If you insert a sheet of clear plastic at any orientation in this region between the mirrors, the laser beam disappears. If the same sheet is placed in the beam outside the partially reflecting mirror, the beam passes through it, regardless of the orientation. Why?

Show that equation (9- 16) follows from (9-15) and (9- 10).

Show that. using equation(936), density of states(938)follows fromlocalid="1658380849671" (937)

When a star has nearly bumped up its intimal fuel, it may become a white dwarf. It is crushed under its own enormous gravitational forces to the point at which the exclusion principle for the electrons becomes a factor. A smaller size would decrease the gravitational potential energy, but assuming the electrons to be packed into the lowest energy states consistent with the exclusion principle, "squeezing" the potential well necessarily increases the energies of all the electrons (by shortening their wavelengths). If gravitation and the electron exclusion principle are the only factors, there is minimum total energy and corresponding equilibrium radius.

(a) Treat the electrons in a white dwarf as a quantum gas. The minimum energy allowed by the exclusion principle (see Exercise 67) is
Uclocimns=310(32h3me32V)23N53

Note that as the volume Vis decreased, the energy does increase. For a neutral star. the number of electrons, N, equals the number of protons. If protons account for half of the white dwarf's mass M (neutrons accounting for the other half). Show that the minimum electron energy may be written

Uelectrons=9h280me(32M5mp5)131R2

Where, R is the star's radius?

(b) The gravitational potential energy of a sphere of mass Mand radius Ris given by

Ugray=-35GM2R

Taking both factors into account, show that the minimum total energy occurs when

R=3h28G(32me3mp5M)13

(c) Evaluate this radius for a star whose mass is equal to that of our Sun 2x1030kg.

(d) White dwarfs are comparable to the size of Earth. Does the value in part (c) agree?

We claim that the famous exponential decrease of probability with energy is natural, the vastly most probable and disordered state given the constraints on total energy and number of particles. It should be a state of maximum entropy ! The proof involves mathematical techniques beyond the scope of the text, but finding support is good exercise and not difficult. Consider a system of11oscillators sharing a total energy of just5h蝇0 . In the symbols of Section 9.3. N=11andM=5 .

  1. Using equation(9-9) , calculate the probabilities ofn , being0,1,2, and3 .
  2. How many particlesNn , would be expected in each level? Round each to the nearest integer. (Happily. the number is still 11. and the energy still5h蝇0 .) What you have is a distribution of the energy that is as close to expectations is possible. given that numbers at each level in a real case are integers.
  3. Entropy is related to the number of microscopic ways the macro state can be obtained. and the number of ways of permuting particle labels withN0 ,N1,N2 , and N3fixed and totaling11 is11!(N0!N1!N2!N3!) . (See Appendix J for the proof.) Calculate the number of ways for your distribution.
  4. Calculate the number of ways if there were6 particles inn=0.5 inn=1 and none higher. Note that this also has the same total energy.
  5. Find at least one other distribution in which the 11 oscillators share the same energy, and calculate the number of ways.
  6. What do your finding suggests?

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