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# In you computed the partition function for a quantum ISBN: 9780201380279 40

## Solution for problem 42P Chapter 6

An Introduction to Thermal Physics | 1st Edition

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Problem 42P

In Problem 1 you computed the partition function for a quantum harmonic oscillator: where ϵ = h f is the spacing between energy levels.

(a) Find an expression for the Helmholtz free energy of a system of N harmonic oscillators.

(b) Find an expression for the entropy of this system as a function of temperature. (Don’t worry, the result is fairly complicated .)

Problem 1:

This problem concerns a collection of N identical harmonic oscillators (perhaps an Einstein solid or the internal vibrations of gas molecules) at temperature T. As in Section 2.2, the allowed energies of each oscillator are 0, hf, 2hf, and so on.

(a) Prove by long division that For what values of x does this series have a finite sum?

(b) Evaluate the partition function for a single harmonic oscillator. Use the result of part(a) to simplify your answer as much as possible.

(c) Use formula to find an expression for the average energy of a single oscillator at temperature T. Simplify your answer as much as possible.

(d) What is the total energy of the system of N oscillators at temperature T? Your result should agree with what you found in Problem 1.

(e) If you haven’t already done so in Problem 1, compute the heat capacity of this system aud check that it has the expected limits as T → 0 and T → ∞

Formula: Problem 2:

In Problem 3 you showed that the multiplicity of an Einstein solid containing N oscillators and q energy units is approximately (a) Starting with this formula, find an expression for the entropy of an Einstein solid as a function of N and q. Explain why the factors omitted from the formula have no effect on the entropy, when N and q are large.

(b) Use the result of part (a) to calculate the temperature of an Einstein solid as a function of its energy. (The energy is U = qϵ, where ϵ is a constant.) Be sure to simplify your result as much as possible.

(c) Invert the relation you found in part (b) to find the energy as a function of temperature, then differentiate to find a formula for the heat capacity.

(d) Show that, in the limit T → ∞, the heat capacity is C = Nk. (Hint: When x is very small, ex ≈ 1 + x.) Is this the result you would expect? Explain.

(e) Make a graph (possibly using a computer) of the result of part (c). To avoid awkward numerical factors, plot C/Nk vs. the dimensionless variable t = kT/ϵ, for t in the range from 0 to about 2. Discuss your prediction for the heat capacity at low temperature, comparing to the data for lead, aluminium, and diamond shown in Figure Estimate the value of ϵ, in electron-volts, for each of those real solids.

(f) Derive a more accurate approximation for the heat capacity at high temperatures, by keeping terms through x3 in the expansions of the exponentials and then carefully expanding the denominator and multiplying everything out. Throw away terms that will be smaller than (ϵ/kT)2 in the final answer. When the smoke clears, you should find Problem 3:

Use Stirling’s approximation to show that the multiplicity of an Einstein solid, for any large values of N and q, is approximately The square root in the denominator is merely large, and can often be neglected. However, it is needed in below Problem 4 (Hint: First show that Do not neglect the in Stirling’s approximation.)

Problem 4:

This problem gives an alternative approach to estimating the width of the peak of the multiplicity function for a system of two large Einstein solids.

a) Consider two identical Einstein solids, each with N oscillators, in thermal contact with each other. Suppose that the total number of energy units in the combined system is exactly 2N. How many different macrostates (that is, possible values for the total energy in the first, solid) are there for this combined system?

b) Use the result of above Problem 2 to find an approximate expression, for the total number of microstates for the combined system. (Hint: Traet the combined system as a single Einstein solid. Do not throw away factors of “large” numbers, since you will eventually be dividing two “very large” numbers that are nearly equal.

c) The most likely macrostate for this system is (of course) the one in which the energy is shared equally between the two solids. Use the result of above Problem 2 to find an approximate expression for the multiplicity of this macrostate.

d)You can get a rough idea of the “sharpness” of the multiplicity function by comparing your answers to parts (b) and (c). Part (c) tells you the Height of the peak, while part (b) tells yon the total area under the entire graph. As a very crude approximation, pretend that the peak’s shape is rectangular. In this case, how wide would it be? Out of all the macrostates, what fraction have reasonably large probabilities? Evaluate this fraction numerically for the case N = 1023.

Figure:

Measured heat capacities at constant pressure (data points) for one mole each of three different elemental solids. The solid curves show the heat capacity at constant volume predicted by the model used in Section 7.5, with the  horizontal scale chosen to best fit the data for each substance. At sufficiently high temperatures, Cv for each material approaches the value 3R predicted by the equipartition theorem. The discrepancies between the data and the sohd curves at high T are mostly due to the differences between Cp and Cv. At T = 0 all degrees of freedom are frozen out, so both Cp and Cv go to zero. Data from Y. S. Touloukian, ed., Thermophysical Properties of Matter (Plenum, New York, 1970). Step-by-Step Solution:
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Ch. 3 Quantum Theory and Electronic Structures 1. Forms of energy a. Kinetic energy – energy from motion i. Ek = 1/2m(v^2) ii. Kinetic energy = one half of the mass times velocity squared iii. Thermal energy (form of kinetic energy)– associated with the random motion of atoms and molecules b. Potential...

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