nonsteadystate

Nonsteady-state is a term used across disciplines to describe a system whose macroscopic properties change with time, meaning the system is not in steady state. In a steady state, variables remain constant despite ongoing processes; in a nonsteady or transient state, they vary as the system responds to initial conditions, external forcing, or changing boundaries.

In physics and engineering, nonsteady-state or transient behavior arises when a system experiences a disturbance or a step change. Examples include transient heat conduction after a sudden change in boundary temperature, or the charging and discharging of an electrical circuit component such as a capacitor. Analyzing these problems typically involves solving time-dependent differential equations with appropriate initial and boundary conditions to describe how quantities evolve toward a new steady state or continue to change.

In chemistry and chemical engineering, nonsteady-state conditions occur during reactor start-up, changes in feed composition, or after disturbances that alter reaction rates. Transient analysis tracks concentration and temperature as they evolve in time, often using quasi-steady-state approximations for intermediates when appropriate.

In biology and pharmacology, nonsteady-state dynamics describe how concentrations of substances or signaling molecules change over time. Steady state in pharmacokinetics refers to the balance between intake and elimination under repeated dosing; nonsteady-state conditions apply during the initial dosing period or following a change in regimen.

Mathematically, nonsteady-state behavior is modeled with time-dependent equations and initial conditions. It is distinct from equilibrium concepts, which imply no net change, and from quasi-steady-state, which approximates slow changes as effectively constant for certain variables.