# Steady State

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## steady state

*Physics*the condition of a system when some or all of the quantities describing it are independent of time but not necessarily in thermodynamic or chemical equilibrium

## Steady State

(in some contexts, stationary state), in physics, a state of a physical system in which some quantities significant for characterizing the system (different quantities in different cases) do not vary with time. For example, the state of flow of a fluid is a steady state if the rate of motion and other characteristics remain invariant at each point in space. In quantum mechanics, a state in which the energy has a specific and time-invariant value is called a stationary state. (*See*OPEN SYSTEMS and PRIGOGINETHEOREM regarding steady states in thermodynamics.) The state of a system is quasi-stationary (in thermodynamics, quasi-static) if those quantities that, if constant, would make the state a steady state change slowly with time. Here, the relations between various properties of the system remain approximately the same as in a steady state.

## Steady State

the state to which a mechanism or system returns after the occurrence of a transient produced by a perturbation or an initial deviation in the system’s coordinates.

Examples of steady states in linear systems include the rotation of an engine at some fixed rate with a constant load applied to the shaft, harmonic oscillations in an oscillatory circuit, and the operation of an automatic control system with constant perturbations and control inputs. The steady state of a dynamic system is characterized by having the forces acting on the system compensated by a corresponding counteraction. For example, the motion of a rotating mechanism is described by the equation *M*_{d} = *M*_{r}, where *M _{d}* is the driving torque and

*M*

_{r}is the resisting torque. For a body being heated,

*Q*

_{h}=

*Q*

_{d}, where

*Q*

_{h}and

*Q*

_{d}represent, respectively, the amount of heat absorbed by the body during heating and the amount dissipated to the environment. In an oscillatory circuit,

*W*

_{s}=

*W*

_{h}, where

*W*

_{s}and

*W*

_{h}are, respectively, the amount of energy supplied from a power source during one period of the oscillation and the amount evolved as heat in the resistance of the circuit.