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Statistical Mechanics

Statistical mechanics uses probabilities over microscopic states to study macroscopic physical properties. Equilibrium ensembles encode the constraints under which those probabilities are assigned.

Type: Concept Domain: Physics Mathematics Chemistry Biology

Overview

In a discrete microcanonical ensemble, fixed energy, volume and particle number specify W equally likely accessible microstates, giving S = k_B ln W. In a canonical ensemble at fixed temperature, microstates need not be equally likely; entropy is S = -k_B sum_i p_i ln p_i, where p_i is the microstate probability.

Why it matters

Canonical calculations connect thermal fluctuations to measurable response: the variance of energy equals k_B T^2 times the heat capacity at the fixed external constraints. This is an equilibrium relation, not a claim that every macroscopic fluctuation follows the same scaling.

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