Quasi Static Process or Quasi Equilibrium Process in Thermodynamics.

when a system in which matter in enclosed, undergoes a thermodynamic process, the thermodynamic equilibrium will no longer remain in the system. Although, it is impossible to have a system in thermodynamic equilibrium when it undergoes a process but there is an assumption with some considerations in which a process can be occurred while maintaining system in thermodynamic equilibrium. This process is called Quasi Static process or Quasi Equilibrium Process.

Define Quasi Static 

The words 'Quasi' literally means 'Almost' and "Quasi static" means 'Almost Static'. So the Quasi static process is one which is almost remains in a single state. This process shows very small or infinitesimal deviation from its original state.  due to this negligible change of state the process is seems to be in thermodynamic equilibrium.

Quasi Static process Analysis

In the process a piston-cylinder setup is taken in which gas is filled inside it.
when the heating of the cylinder or container is done the inside gas temperature will increase and the piston will be raised up. here the system will not be in equilibrium.

Quasi Static Process
Quasi Static Process

But if we put Some Weight on the piston such that the height at which piston is raised is covers back by this weight as the piston will go downward due to this weight. when we further heat the cylinder the piston will be raised again so we again put some more weight on it to compensate the raising of the piston. we will do it until the heat is added and we make sure the piston is at its initial position by putting appropriate amount of weight in it.

In this way we will keep the piston at static position and avoid the change of state in the system. the change of state will be infinitesimal

This is a short description about Quasi static or quasi equilibrium process. it is an experimental assumption to show the thermodynamic equilibrium in a system. To maintain thermodynamic equilibrium in a system is a difficult thing. but in this experiment system remains closed to the equilibrium state.


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