When a system is taken from one equilibrium
state to another, the change is known as process. The series of intermediate states through
which a system passes during a process is called the path of the process. If all these intermediate states
are equilibrium states, the process is known as quasi equilibrium or
quasi-static process.
Consider
a certain quantity of gas taken in a frictionless piston cylinder arrangement . The system is in
thermodynamic equilibrium so that there is no unbalanced force acting on
piston.
The moment the weight is
removed from the piston, mechanical equilibrium does not exist and as a result
the piston is moved upward until mechanical equilibrium is restored again. Therefore the actual process occurs only when
equilibrium does not exist.
if the entire weight on the piston is removed at
once, the deviation from the equilibrium is high and the expansion is
rapid. For such a process the
intermediate states are not equilibrium states and hence the process
would be non-quasi-equilibrium.
If the weight is assumed to
be made of a large number of small pieces as shown in Fig.1.5.b and taken off
one by one, the deviation from equilibrium is less. The process could be considered
quasi-equilibrium.
A thermodynamic system is said to undergo a cycle, if it is taken
through a number of processes such that, the final state of the last process is
identical with the initial state of the first
process in all respects. For cycles net change in any property is zero.
