1.10.1 Reversible Process
If a process can be completely reversed, it is called reversible process. Fig. 1.8 shows a process 1 – 2 in the forward direction, if the process is carried out in the opposite direction (backward direction) i.e., from 2 to 1 in the same succession of states as followed in the forward direction, it is called a reversible process. Thus, the system comes to initial state. In addition, the interaction between the system and surroundings are also identical but opposite in direction. Thus the surrounding also comes to its initial state. A quasi-static process is a reversible process.
1.10.2 Irreversible Process
A process which is not reversible is irreversible process. A quasi-static process with friction is irreversible process. Non-equilibrium process cannot be exactly reversed and the system cannot be brought back to the original state without leaving a net change in the surroundings.