Thermodynamic Process

A thermodynamic process is the procedure by which a system's initial state is changed into a different, final state. During the process, heat may be transferred into the system (positive heat, e.g. when water is heated to boiling) or released from the system to its surroundings (negative heat); simultaneously, work may be done by the system (positive work, e.g. an expanding gas pushing a piston outward) or done on the system (negative work, e.g. a piston being pushed inward, compressing the gas).

All of the processes analysed in the rest of this chapter (with the deliberate exception of free expansion, Section 4.7.3.7) are assumed to occur quasi-statically — that is, infinitesimally slowly, so that the system passes through an unbroken sequence of equilibrium states and never departs from thermodynamic equilibrium (Section 4.6.1) at any intermediate instant. This is what makes such processes representable, at every stage, by a well-defined point on a p-V diagram. …