A thermodynamically reversible process is one in which all the changes occurring in any part of the process are exactly reversed when it is carried out in opposite direction.
Characteristic of the reversible process:
It has to be carried out in infinitesimal amount and hence require infinite time. Therefore changes in this process are very slow.
At all time during the process, the driving force for the change is opposed by a restraining force infinitesimally smaller than the driving force.
It can be reversed by an infinitesimal increase in opposing force.
The work produced in the reversible process is maximum.
The process which occurs suddenly or spontaneously without the restriction of occurring in the successive stage of infinitesimal quantities.
Characteristic of the irreversible process:
In an irreversible process work done in the forward direction and in the backward direction are not equal.
If the initial and final stages are specified, the internal energy change would always be the same, whether the process has been affected by reversibly or irreversibly.
In an irreversible process, since the work done(dw) in two opposite direction are unequal, the heat transfer(dq)would also be unequal.
All the natural processes are irreversible.
e.g.=> (1) flow of heat from high temperature to low temperature
(2) expansion of gas from high pressure to low pressure