Thermodynamic Process

a change in the state of a physical system because of heat transfer and performance of work.

If a thermodynamic process occurs so slowly that at every moment the system is in thermodynamic equilibrium, the process is called an equilibrium process; otherwise it is said to be a nonequilibrium process. A process that can proceed in either direction through the same sequence of intermediate states is said to be reversible; such a process must be an equilibrium process. All real thermodynamic processes are irreversible, since they occur at finite rates and with finite temperature differences between the heat source and the system and are accompanied by friction and heat losses to the surroundings.

Figure 1. Graphic representation of thermodynamic processes on a pressure-volume diagram: (1) isobar, (2) isotherm, (3) adiabat, (4) isochor; (p) pressure, (v) volume

A thermodynamic process may occur at constant pressure (isobaric process), temperature (isothermal process), or volume (iso-choric process). A process that occurs without heat exchange with the surroundings is said to be adiabatic. These four types of processes are compared in Figure 1. In a reversible adiabatic process the entropy of the system remains constant—that is, the process is isentropic. An irreversible adiabatic process is accompanied by an increase in entropy. A process in which the enthalpy, or heat content, of the system remains constant is said to be isenthalpic. A process in which the initial and final states are the same is called a cycle; such processes may be used to produce power, heat, or cold.