adiabaticisentropic

Adiabatic and isentropic are terms used in thermodynamics to describe how a system exchanges heat and how its entropy changes during a process. An adiabatic process is one in which no heat is exchanged with the surroundings (Q = 0). An isentropic process is one in which the system's entropy remains constant (dS = 0). When a process is reversible and adiabatic, it is also isentropic, because a reversible path with no heat transfer does not generate entropy.

For an ideal gas, the reversible adiabatic (or isentropic) relation can be written as P V^γ = constant, where γ = Cp/Cv. Other equivalent forms include T V^(γ-1) = constant and P^(1-γ) T^γ = constant. The first law gives ΔU = Q − W; with Q = 0 and for an ideal gas, ΔU = n Cv ΔT = −W.

Adiabatic does not automatically imply isentropic. Irreversible processes with internal friction, rapid mixing, shocks, or heat transfer in practice increase entropy, so S changes even if Q = 0 is not guaranteed. Thus true isentropic processes are idealizations, most often used as benchmarks in engineering.

Isentropic processes are used as a reference in nozzles, compressors, turbines, and other devices to gauge efficiency. The isentropic efficiency compares the actual process to the ideal isentropic one. Real processes deviate due to irreversibilities and heat transfer.