Quasistatikus

Quasistatikus, or quasi-static, describes processes that proceed slowly enough for a system to remain in near-equilibrium throughout the change. In thermodynamics, the state of the system is described by a sequence of equilibrium states, so variables such as pressure, temperature, and volume change smoothly along a path in state space. This requires that external conditions evolve slowly compared with the system’s internal relaxation times, keeping gradients in properties (for example, temperature or composition) small.

The quasi-static assumption is often used to simplify analysis by treating the process as reversible, meaning the path could be traced backward with no net entropy production. In practice, however, real quasi-static processes may still involve irreversibilities such as friction, finite temperature differences, or internal dissipation. Therefore, quasi-static is an idealization that allows the use of differential relations like dU = TdS − PdV and the plotting of state paths on P–V or T–S diagrams.

In mechanics and structural engineering, a quasi-static loading assumption neglects dynamic (inertial) effects, so equations of motion reduce to static equilibrium. This is appropriate when loading changes slowly compared with natural or structural response times.

Common examples include slow compression or expansion of a gas, gradual heating or cooling under near-equilibrium conditions, and slowly applied mechanical loads in testing. The concept helps separate slow, controllable changes from rapid, transient phenomena in both theory and experiment.