Reversible adiabatic ideal-gas relations
The power law needs an ideal gas and constant heat-capacity ratio.
Key knowledge
A fixed amount of calorically ideal gas undergoing a reversible adiabatic process obeys pV^γ=constant and TV^(γ−1)=constant, with γ=C_p/C_V>1. The gas cools during such expansion and warms during compression. An insulated free expansion has Q=W=0 and unchanged ideal-gas internal energy; it does not follow this reversible power law.
Values and equations
p₁V₁^γ=p₂V₂^γ; T₁V₁^(γ−1)=T₂V₂^(γ−1); γ=C_p/C_V
- p₁,p₂
- Equilibrium endpoint pressures Pa
- V₁,V₂
- Endpoint volumes m³
- T₁,T₂
- Endpoint thermodynamic temperatures K
- C_p,C_V
- Consistently defined fixed-amount heat capacities J K⁻¹
- γ
- Heat-capacity ratio 1
Conditions and scope
- Equilibrium ideal-gas equation, only pressure–volume work and constant positive heat capacities over the range; no entropy production for the displayed power law.
- Insulation alone does not establish reversibility or the power law. Heat-capacity variation, real-gas interactions and irreversible flow require other relations.
Sources · 1
References
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Evidence and review
Read §3.6 free-expansion counterexample and reversible/quasistatic adiabatic derivation3.12–3.14. Do not generalize the reversible power law to every insulated expansion.
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Entry details
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- foundations-refresh.1
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- Foundational reference additions