Microcanonical ensemble and state counting
For an isolated equilibrium system with fixed energy shell, volume and particle number, the microcanonical model gives equal probability to accessible microstates.
Key knowledge
For an isolated equilibrium system with fixed energy shell, volume and particle number, the microcanonical model gives equal probability to accessible microstates. If their count is Ω, S=k_B lnΩ; the density of states g(E) counts states per energy interval and differs from the dimensionless shell count.
Values and equations
p_i=1/Ω; S=k_B lnΩ; Ω(E,δE)≈g(E)δE
- p_i,Ω
- State probability and shell count
- S
- Entropy J K⁻¹
- k_B
- Boltzmann constant J K⁻¹
- E,δE
- Energy and shell width J
- g(E)
- Energy density of states J⁻¹
Conditions and scope
- Specified shell width and accessible constraints; continuum phase-space counting requires a dimensionless counting convention.
- Equal a priori probability is an equilibrium ensemble postulate, not proof that every isolated dynamics explores every state.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Richard Fitzpatrick / University of Texas at Austin2 sources
Evidence and review
Thermodynamics and Statistical Mechanics (2016): Fundamental statistical postulate: Accessible-state ensemble and equal a priori probabilities. Thermodynamics and Statistical Mechanics (2016): Partition function: Final comparison of a restricted state sum with the unrestricted partition sum.
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