Entropy change and reversible heat transfer
Use a reversible comparison path to calculate a state-function change.
Key knowledge
Between equilibrium states, entropy change is ΔS=∫δQ_rev/T along any reversible comparison path connecting those states. At constant temperature on that path, ΔS=Q_rev/T. An actual irreversible process can have the same endpoint entropy change even though substituting its actual heat into this reversible integral is invalid.
Values and equations
ΔS=∫_A^B δQ_rev/T; ΔS_isothermal=Q_rev/T
- S,ΔS
- Thermodynamic entropy and change J K⁻¹
- Q_rev,δQ_rev
- Heat transferred reversibly into the system J
- T
- Thermodynamic temperature along the comparison path K
- A,B
- Equilibrium endpoints
Conditions and scope
- T>0; equilibrium endpoints and a thermodynamically admissible reversible comparison path.
- Q is path-dependent energy transfer and δQ is not the exact differential of a state variable. The formula does not imply that every adiabatic process has constant entropy; irreversible isolated changes can increase it.
Sources · 1
References
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Evidence and review
Read §4.6 state-function definition, reversible isothermal4.8/integral4.10 and explicit irreversible-process comparison-path example4.12. δ notation makes the non-state-function heat distinction explicit.
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