Quasistatic equilibrium

Quasistatic equilibrium (also known as quasi-equilibrium) is the quasi-balanced state of a thermodynamic system near to thermodynamic equilibrium in some sense or degree. A process is called quasi-static when it follows a succession of equilibrium states; the surroundings may be irreversibly altered during the process so that after a return path, the system ends up in a final state which differs from its initial state.

The quasistatic equilibrium model facilitates or justifies the use of Gibbsian thermodynamic applications, i.e. equations of state, to systems characterized by slow change if measured on the adjunct time scale. In short, the quasistatic equilibrium model approximates change as a series of equilibrium processes.

In a quasistatic process, or equilibrium process, a sufficiently slow transition of a thermodynamic system from one equilibrium state to another occurs such that at every moment in time the state of the system is close to an equilibrium state. During a quasistatic process, the system reaches equilibrium much faster, almost instantaneously, than its physical parameters vary. A quasistatic process is not necessarily a reversible one.

For quasi-equilibrium, it is only required that the properties of the system be uniform at any instant during a process.

pdV-Work in Various Quasi-static Processes

(a)Constant pressure process (isobaric or isopiestic process)

               W1-2=∫pdV=p(V2-v1)

(b)Constant volume process (isochoric process)

               W1-2=∫pdV=0

(c)Process in which pV=C (isothermal process)

               W1-2=∫pdV       pV=p1V1=C

               W1-2 = p1V1ln p1/p2

References

• Perrot, P. (1998). A to Z of Thermodynamics (dictionary). New York: Oxford University Press.
• Engineering thermodynamics^{page 44} by P.K. Nag.