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QUASI - STATIC PROCESS IN THERMODYNAMICS

Consider a system of  gas contained  in a cylinder. The system initially is in a equilibrium state , represented by the properties p1,v1,t1. The weight on the piston just balances the upward force exerted by the gas. If the weight is removed, there will be an unbalanced force between the system and the surroundings, and under gas pressure, the piston will move up till its hits the stops. The system again comes to an equilibrium state, being described by the properties p2,v2,t2. But the intermediate states passed through by the system are non-equilibrium states which cannot be described by thermodynamic co-co-ordinates.


Now if the single weight on the piston is made up of many very small pieces of weights, and these weights are removed one by one slowly from the top of the piston, at any instant of  the upward travel of the piston, if gas system is isolated, the departure of the state of  the  system from the thermodynamic equilibrium state will be infinitesimally small. So every state passed through by the system will be an equilibrium state. Such a process, which is but a locus of all the equilibrium points passed through by the system, is known as quasi-static process. (Quasi meaning almost. ). Infinite slowness is the characteristic of quasi static process and thus a succession of  Equilibrium states.

If  the same slices or very small pieces of weights are now placed slowly one by one on the top of  the piston, the piston will move down slowly from state 2 to state 1 by following the same quasi static path of the succession of equilibrium states. So a quasi-static process is also a reversible process. A reversible process is performed in both ways from 1 to 2 and from 2 to 1 involving infinite time in executing each of the processes.

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