Classification of Thermodynamic Processes

With the general groundwork of Sections 4.7.1 and 4.7.2 in place, thermodynamic processes are now classified into named categories, based on which state variable is held fixed. Since a state is described by three variables — pressure $p$, volume $V$, and temperature $T$ — the most practically useful way to change a system, for the purposes of measurement and analysis, is to hold ONE of these three fixed while allowing the other two to vary. This single organising idea generates the seven categories covered in the following subsections: reversible/irreversible processes (a distinction that cuts across all the others), isothermal, isobaric, isochoric, adiabatic, and cyclic processes (each defined by holding one variable fixed, or by a special condition on heat/work), and finally free expansion, a distinctive uncontrolled special case.

Three simplifying assumptions apply to every process discussed from here through the rest of the chapter, unless stated otherwise:

(i) Almost all of the processes discussed are reversible — meaning quasi-static, proceeding through an unbroken sequence of infinitesimally-close equilibrium states, so the system remains in thermodynamic equilibrium throughout the entire change. (The deliberate exception is the adiabatic process, Section 4.7.3.5, which is often carried out rapidly.) …