There is no requirement that a system undergoing an actual process be in equilibrium during the process. Some or all of the intervening states may be nonequilibrium states.

For many such processes we are limited to knowing the state before the process occurs and the state after the process is completed. However, even if the intervening states of the system are not known, it is often possible to evaluate certain overall effects that occur during the process.

Typically, nonequilibrium states exhibit spatial variations in intensive properties at a given time. Also, at a specified position intensive properties may vary with time, sometimes chaotically.

Spatial and temporal variations in properties such as temperature, pressure, and velocity can be measured in certain cases. It may also be possible to obtain this information by solving appropriate governing equations, expressed in the form of differential equations, either analytically or by computer.

Processes are sometimes modeled as an idealized type of process called a quasiequilibrium (or quasistatic) process. A quasiequilibrium process is one in which the departure from thermodynamic equilibrium is at most infinitesimal.

All states through which the system passes in a quasiequilibrium process may be considered equilibrium states. Because nonequilibrium effects are inevitably present during actual processes, systems of engineering interest can at best approach, but never realize, a quasiequilibrium process.

Our interest in the quasiequilibrium process concept stems mainly from two considerations:  Simple thermodynamic models giving at least qualitative information about the behavior of actual systems of interest often can be developed using the quasiequilibrium process concept.

This is akin to the use of idealizations such as the point mass or the frictionless pulley in mechanics for the purpose of simplifying an analysis.  The quasiequilibrium process concept is instrumental in deducing relationships that exist among the properties of systems at equilibrium

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