Thermodynamic observables from the partition function
For canonical Z=Σ_i exp(−βE_i), F=−k_BT lnZ, U=−∂_β lnZ and S=k_B(lnZ+βU).
Key knowledge
For canonical Z=Σ_i exp(−βE_i), F=−k_BT lnZ, U=−∂_β lnZ and S=k_B(lnZ+βU). At fixed volume and amount, Var(E)=∂_β²lnZ=k_BT²C_V; pressure is k_BT(∂_VlnZ)_T,N when the Hamiltonian's volume dependence represents mechanical work.
Values and equations
F=−k_BT lnZ; U=−∂_βlnZ; Var(E)=k_BT²C_V; p=k_BT(∂_VlnZ)_T,N
- F,U,E
- Free energy, internal energy and microstate energy J
- Var(E)
- Energy variance J²
- β
- Inverse thermal energy J⁻¹
- k_B
- Boltzmann constant J K⁻¹
- T
- Temperature K
- C_V
- Fixed-volume heat capacity J K⁻¹
- p,V
- Pressure and volume Pa; m³
- Z,N
- Dimensionless partition function and particle count
Conditions and scope
- Equilibrium canonical ensemble, T>0, β=1/(k_BT); convergent differentiable sums with fixed external parameters for β derivatives.
- Count degeneracies and particle statistics correctly. The variance relation assumes a Hamiltonian independent of temperature; effective temperature-dependent models need extra terms.
Sources · 3
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Richard Fitzpatrick / University of Texas at Austin3 sources
- Thermodynamics and Statistical Mechanics (2016): Calculation of mean valuesRichard Fitzpatrick / University of Texas at AustinCourse notes
- Thermodynamics and Statistical Mechanics (2016): Partition functionRichard Fitzpatrick / University of Texas at AustinCourse notes
- Thermodynamics and Statistical Mechanics (2016): Helmholtz energyRichard Fitzpatrick / University of Texas at AustinCourse notes
Evidence and review
Thermodynamics and Statistical Mechanics (2016): Calculation of mean values: Equations 7.35–7.50: energy derivatives, energy variance, generalized mechanical force. Thermodynamics and Statistical Mechanics (2016): Partition function: Equation 7.56: canonical entropy; F=U−TS gives F=−k_BT lnZ by substitution. Thermodynamics and Statistical Mechanics (2016): Helmholtz energy: Equations 6.100–6.109: F=U−TS and its natural-variable differential.
2026-10-10 · Independent AI editorial review: All 136 unique authored Physics compact statements, all hypotheses and displayed equations/variable/units tables, all 192 Physics-origin semantic links, exact 165 author inspection matrix and mapped retained conceptual claims have been independently read. Three Physics-tagged Measurement realization records are delegated to root Measurement review. Scientific source verification combines exact author section inspection with independent whole compact-content/semantic review and targeted primary rechecks recorded below; it does not claim each full source body was independently reread. Independent AI editorial review is not human specialist certification or experimental replication. Field inventory is approved after independently reviewing the two added source-inspected band-construction methods. Receipt: physics-independent-content-inventory-review.1.
Entry details
Permanent link to this entry ↗- Record identity
- physics-partition-observables
- Revision
- university-core.1
- Structured reference record
- University core expansion