Quasi-Static Process

Consider a system of gas exists in a cylinder. The piston consists of many very small pieces of weights. Initially system is in an equilibrium state. When the gas system is isolated, the weights on piston are removed one by one slowly, at any instant of upward travel of the piston. So every state passes through by the system will be in an equilibrium state. Thus the system passes through the locus of all equilibrium points. This infinitely slow system is a quasi-static process. If the same small weights are now placed slowly one by one on top of the piston then the process will reverse in the same manner.

When a process is processing in such a way that system will be remained infinitesimally close with equilibrium state at each time, such process will be termed as quasi static process or quasi equilibrium process.

In simple words, we can say that if system is going under a thermodynamic process through succession of thermodynamic states and each state is equilibrium state then the process will be termed as quasi static process.

We will see one example for understanding the quasi static process, but let us consider one simple example for better understanding of quasi static process. If a person is coming down from roof to ground floor with the help of ladder steps then it could be considered as quasi static process. But if he jumps from roof to ground floor then it will not be a quasi static process.

Quasi static process: Example

Let us consider the assembly of cylinder and piston as shown in figure. Cylinder is contained with gas and system is in equilibrium condition initially. Let us see the state of the system initially is at state 1 and indicated by its thermodynamic properties P1, V1 and T1. At this state pressure will be high and specific volume will be less at a temperature.

Weight placed over the piston is just balancing the force which is exerted in upward direction by gas. If we remove the weight from the piston, system will have unbalanced force and piston will move in upward direction due to force acting over the piston in upward direction by the gas.

Piston will move in upward direction and will be stopped once it will strike the stops. This condition of the system is expressed as final state and indicated by state 2 and will have its thermodynamic properties P2, V2 and T2. At this state pressure will be less and specific volume will be high.

Initial and final state of the system displayed here with the help of thermodynamic properties as state 1 and state 2 respectively, but intermediate states could not be displayed here by thermodynamic properties as intermediate states by which system has arrived at state 2 were not in equilibrium condition.

Let us consider the above single weight, placed over the piston, replaced by few infinitesimally small weights and these infinitesimally small weights are placed over each other and also resultant weight of all infinitesimally small weights is equivalent to the single weight placed earlier over the piston.

If we remove very slowly infinitesimally small weights one by one from the piston, what will be happened? When we remove the first infinitesimally small weight from the piston, piston will move very slowly as well as with infinitesimally small amount and will secure its next equilibrium state. Due to removal of infinitesimally small weights, one by one and also quite slowly, system will process from one state to another state with succession of equilibrium states.

Hence we can say here that system will arrive to final state from initial state with various equilibrium intermediate states and these intermediate states are displayed in above figure.

Such a process, where system process in such a manner as studied above, will be termed as quasi static process or quasi equilibrium process in the field of thermal engineering.