Quasistatic process

A quasistatic process is a thermodynamic process executed slowly enough that the system remains arbitrarily close to thermodynamic equilibrium throughout. Equivalently, the system can be described at every instant by a well-defined thermodynamic state , so macroscopic state variables (such as temperature and pressure) exist along the entire path.

Physical interpretation

“Quasistatic” is a controlled idealization based on separation of time scales: external driving is slow compared with the system’s internal relaxation times, so spatial gradients (in temperature, pressure, chemical potential, etc.) are negligible within the system at each moment. In practice, a process is quasistatic to the extent that the system remains near equilibrium and its state variables can be treated as meaningful functions of time.

Key properties and useful relations

Because a quasistatic process admits an equilibrium description at each step, differential expressions take their equilibrium form. For a simple compressible system, the mechanical work element can be written using the thermodynamic pressure $P$ and volume $V$:

with the overall sign determined by the $P\,dV$ sign convention .

A quasistatic process is not automatically a reversible process . Dissipative mechanisms (friction, viscosity, electrical resistance, inelasticity) can persist even in the slow-driving limit, making the process irreversible despite being quasistatic. Quasistatic motion ensures the state is well-defined along the path; reversibility is the stronger condition that the path can be retraced without net change to system and surroundings.

Many equilibrium identities are derived by considering quasistatic variations of state functions , including the Maxwell relations .