Isentropic process

Thermodynamics
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An isentropic process is an idealized thermodynamic process that is both adiabatic and reversible.^{[1][2][3][4][5][6][excessive citations]} The work transfers of the system are frictionless, and there is no net transfer of heat or matter. Such an idealized process is useful in engineering as a model of and basis of comparison for real processes.^[7] This process is idealized because reversible processes do not occur in reality; thinking of a process as both adiabatic and reversible would show that the initial and final entropies are the same, thus, the reason it is called isentropic (entropy does not change). Thermodynamic processes are named based on the effect they would have on the system (ex. isovolumetric: constant volume, isenthalpic: constant enthalpy). Even though in reality it is not necessarily possible to carry out an isentropic process, some may be approximated as such.

The word "isentropic" derives from the process being one in which the entropy of the system remains unchanged. In addition to a process which is both adiabatic and reversible, this can also occur in a system where the work done on the system includes friction internal to the system, and heat is withdrawn from the system sufficient to compensate for it so as to leave the entropy unchanged.^[8] However, in relation to the Universe, its entropy would increase as a result, in agreement with the Second Law of Thermodynamics.^{[citation needed]}