Grand-canonical particle exchange and potential
For a system exchanging energy and particles with reservoirs at T and μ, p_i,N=exp[−β(E_i,N−μN)]/Ξ and Ξ=Σ_N,i exp[−β(E_i,N−μN)].
Key knowledge
For a system exchanging energy and particles with reservoirs at T and μ, p_i,N=exp[−β(E_i,N−μN)]/Ξ and Ξ=Σ_N,i exp[−β(E_i,N−μN)]. The grand potential Ω_G=−k_BT lnΞ=U−TS−μ⟨N⟩ has dΩ_G=−S dT−p dV−⟨N⟩dμ; particle number fluctuates.
Values and equations
Ξ=Σ_N,i e^[−β(E_i,N−μN)]; ⟨N⟩=k_BT(∂_μlnΞ)_T,V; Var(N)=k_BT(∂_μ⟨N⟩)_T,V
- Ξ,N,p_i,N
- Grand partition function, particle count and state probability
- E_i,N,μ,Ω_G,U
- Energies and chemical potential per particle J
- β
- Inverse thermal energy J⁻¹
- T
- Temperature K
- k_B
- Boltzmann constant J K⁻¹
- V
- Volume m³
Conditions and scope
- One species, equilibrium, T>0 and a convergent grand partition sum; μ is energy per particle.
- Weak reservoir coupling and fixed volume define the ensemble; multiple species require a chemical-potential term for each.
Sources · 2
References
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Evidence and review
Statistical Mechanics 8.333: Lecture 23: Section VII.C, printed pages 144–145, equations VII.25–VII.31: grand weights, normalization and occupations. Logarithmic derivatives follow by differentiating these normalized sums. Statistical Thermodynamics: CHEM_ENG 404 review: Printed pages 35–36, sections 6.3.1–6.3.2: normalized grand weights and grand-potential differential.
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