Statistical Mechanics · Problem 6.1

Ideal Bose Systems — R. K. Pathria

Physics — Statistical Mechanics · entropy-ideal-gas-bosons-fermions

Problem

Show that the entropy of an ideal gas in thermal equilibrium is given by the formula S=k∑ε[(⟨nε⟩+1)ln⁡⟨nε⟩+1)−⟨nε⟩ln⁡⟨nε⟩]S = k\sum_{\varepsilon}[(\langle n_\varepsilon\rangle + 1)\ln\langle n_\varepsilon\rangle + 1) - \langle n_\varepsilon\rangle\ln\langle n_\varepsilon\rangle] in the case of bosons and by the formula S=k∑ε[−(1−⟨nε⟩)ln⁡(1−⟨nε⟩)−⟨nε⟩ln⁡⟨nε⟩]S = k\sum_{\varepsilon}[-(1 - \langle n_\varepsilon\rangle)\ln(1 - \langle n_\varepsilon\rangle) - \langle n_\varepsilon\rangle\ln\langle n_\varepsilon\rangle] in the case of fermions. Verify that these results are consistent with the general formula S=−k∑ε{∑npε(n)ln⁡pε(n)},S = -k\sum_{\varepsilon}\left\{\sum_{n} p_\varepsilon(n)\ln p_\varepsilon(n)\right\}, where pε(n)p_\varepsilon(n) is the probability that there are exactly nn particles in the energy state ε\varepsilon.

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