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Q. 6.51

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Found in: Page 256

### An Introduction to Thermal Physics

Book edition 1st
Author(s) Daniel V. Schroeder
Pages 356 pages
ISBN 9780201380279

# In this section we computed the single-particle translational partition function,${Z}_{\mathrm{tr}}$, by summing over all definite-energy wave functions. An alternative approach, however, is to sum over all position and momentum vectors, as we did in Section 2.5. Because position and momentum are continuous variables, the sums are really integrals, and we need to slip a factor of $1}{{h}^{3}}$ to get a unitless number that actually counts the independent wavefunctions. Thus we might guess the formularole="math" localid="1647147005946" ${Z}_{\mathrm{tr}}=\frac{1}{{h}^{3}}\int {d}^{3}r{d}^{3}p{e}^{-{E}_{\mathrm{tr}}}{kT}}$where the single integral sign actually represents six integrals, three over the position components and three over the momentum components. The region of integration includes all momentum vectors, but only those position vectors that lie within a box of volume $V$. By evaluating the integrals explicitly, show that this expression yields the same result for the translational partition function as that obtained in the text.

The translational partition function is given by ${Z}_{\mathrm{tr}}=\frac{V}{{v}_{Q}}$.

See the step by step solution

## Step 1. Given information

The translational partition function is given by

${Z}_{\mathrm{tr}}=\frac{1}{{h}^{3}}\int {d}^{3}r{d}^{3}p{e}^{-{E}_{\mathrm{tr}}}{kT}}...........................\left(1\right)$

## Step 2. Calculation

As the translational kinetic energy of the molecules does not depend on the position, the integrand is also independent of the position and the integral over the position variables only yields the total volume $V$ of the box.

The expression for the translational energy of the gas molecules is given by

${E}_{\mathrm{tr}}=\frac{{p}_{x}^{2}}{2m}+\frac{{p}_{y}^{2}}{2m}+\frac{{p}_{z}^{2}}{2m}..................\left(2\right)$

Here, x-,y- and z- denotes the three components of the momentum.

Substitute the expression of energy into equation (1) and solve to calculate the value of the translational partition function.

role="math" localid="1647147921072" $\begin{array}{rcl}{Z}_{\mathrm{tr}}& =& \frac{V}{{h}^{3}}{\int }_{0}^{\infty }d{p}_{x}{e}^{-{p}_{x}^{2}}{2mkT}}{\int }_{0}^{\infty }d{p}_{y}{e}^{-{p}_{y}^{2}}{2mkT}}{\int }_{0}^{\infty }d{p}_{z}{e}^{-{p}_{z}^{2}}{2mkT}}\\ & =& \frac{V}{{h}^{3}}\left(\sqrt{\pi ·2mkT}\right)\left(\sqrt{\pi ·2mkT}\right)\left(\sqrt{\pi ·2mkT}\right)\\ & =& \frac{V}{{h}^{3}}{\left(2\pi mkT\right)}^{3}{2}}........................\left(3\right)\end{array}$

The quantum volume of the gas molecules is given by

${v}_{Q}={\left(\frac{{h}^{2}}{2\pi mkT}\right)}^{3}{2}}........................\left(4\right)$

Substitute the value of the quantum volume from equation (4) into equation (3) to obtain the required translational partition function.

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