# Homework Help: Statistical mechanics: multiplicity

1. Mar 2, 2012

### SoggyBottoms

1. The problem statement, all variables and given/known data
We have a surface that can adsorb identical atoms. There are N possible adsorption positions on this surface and only 1 atom can adsorb on each of those. An adsorbed atom is bound to the surface with negative energy $-\epsilon$ (so $\epsilon > 0$). The adsorption positions are far enough away to not influence each other.

a) Give the multiplicity of this system for $n$ adsorbed atoms, with $0 \leq n \leq N$.

b) Calculate the entropy of the macrostate of n adsorbed atoms. Simplify this expression by assuming N >> 1 and n >> 1.

c) If the temperature of the system is T, calculate the average number of adsorbed atoms.

3. The attempt at a solution

a) $\Omega(n) = \frac{N!}{n! (N - n)!}$

b) $S = k_b \ln \Omega(n) = k_b \ln \left(\frac{N!}{n! (N - n)!}\right)$

Using Stirling's approximation: $S \approx k_B ( N \ln N - N - n \ln n - n - (N - n) \ln (N - n) - (N - n) = k_B ( N \ln N - n \ln n - (N - n) \ln (N - n)$

A Taylor expansion around n = 0 then gives: $S \approx k_B (- \frac{n^2}{2N} + ...)\approx -\frac{k_b n^2}{2N}$

c) I'm not even sure if the previous stuff is correct, but I have no idea how to do this one. Any hints?

2. Mar 3, 2012

### Mute

Part (a) looks good, but you made a couple of sign mistakes in part (b). In the terms in the denominator you forgot to distribute the negative sign to the second term in n ln n - n and (N-n) ln(N-n). I've corrected the signs here:

$$S \approx k_B ( N \ln N - N - n \ln n + n - (N - n) \ln (N - n) + (N - n) = k_B ( N \ln N - n \ln n - (N - n) \ln (N - n)$$

This change will result in some cancellations that help simplify your expression. Rewriting

$$(N-n)\ln(N-n) = \left(1-\frac{n}{N}\right)N\ln N + (N-n)\ln\left(1-\frac{n}{N}\right)$$