A quasi-static process is a thermodynamic process carried out through an idealized sequence of infinitesimal changes such that the system remains arbitrarily close to equilibrium at every intermediate step, allowing macrostate variables (pressure, volume, temperature) to be well-defined throughout and a continuous path to be plotted between states on a PV diagram. A reversible process is a special case of a quasi-static process that additionally involves no dissipation of energy (e.g., no friction), meaning the system can be returned to its original state via the same infinitesimal steps in reverse without net loss. This distinguishes defined, path-describable equilibrium transitions from abrupt, non-equilibrium transitions in which macrostate variables cannot be specified at intermediate points, a foundational concept in classical thermodynamics underlying later treatments of heat engines and cyclic processes.
A quasi-static process is a thermodynamic process carried out through an idealized sequence of infinitesimal changes such that the system remains arbitrarily close to equilibrium at every intermediat…