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A particle can occupy either the ground state at E = 0 or an excited state at E \(\gt\) 0. At a temperature T, the probability of the particle being in the excited state is :

  • A.0
  • B.\(1 + \mathrm{exp} - E/K_{B}T\)
  • C.\(\mathrm{exp} - E/K_{B}T/(1 + \mathrm{exp} - E/K_{B}T)\)
  • D.\(1/(1 + \mathrm{exp} - E/K_{B}T)\)

Answer: C

The probability is exp(−E/kBT) / (1 + exp(−E/kBT)).

By the Boltzmann distribution, each state's population is proportional to exp(−E/kBT), divided by the partition function, the sum of these factors over all states. With levels at 0 and E,

\[ \begin{aligned} q &= 1 + e^{-E/k_{B}T} \\ p_{\mathrm{exc}} &= \frac{e^{-E/k_{B}T}}{1 + e^{-E/k_{B}T}} \end{aligned} \]

Option D is the probability of the ground state, option B is the partition function itself (which exceeds 1, so it cannot be a probability), and 0 holds only at absolute zero.

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