Reversible and Irreversible Processes

A thermodynamic process that does not fulfill conditions of a reversible process is termed an irreversible process. Irreversibilities are the reasons causing the process to be irreversible. The irreversibilities can be termed as internal irreversibility and external irreversibility. Internal irreversibility is there because of internal factors whereas external irreversibility is caused by external factors at the system-surrounding interface.

Generic types of irreversibilities are due to:

1. 1. Friction: Friction is invariably present in real systems. It causes irreversibility in the process as work done does not show an equivalent rise in the kinetic or potential energy of the system. The fraction of energy wasted due to frictional effects leads to deviation from reversible states.
   2. Electrical resistance: Electrical resistance in the system also leads to the presence of dissipation effects and thus irreversibilities. Due to electric resistance dissipation of electrical work into internal energy or heat takes place. The reverse transformation from heat or internal energy to electrical work is not possible, therefore leads to irreversibility.
   3. Inelastic solid deformation: Deformation of solids, when of inelastic type is also irreversible and thus causes irreversibility in the process. If distortion occurs within elastic limits, then it does not lead to irreversibility as it is of the reversible type.
   4. Free expansion: Free expansion refers to the expansion of unresisted type such as expansion in a vacuum. During this unresisted expansion the work interaction is zero, and without the expense of any work, it is not possible to restore initial states. Thus, free expansion is irreversible.
   5. Heat transfer through a finite temperature difference: Heat transfer occurs only when there exists a temperature difference between bodies undergoing heat transfer. During heat transfer, if heat addition is carried out in a finite number of steps then after every step the new state shall be a non-equilibrium state. To have equilibrium states in between, the heat transfer process may be carried out in an infinite number of steps. Thus, infinitesimal heat transfer every time causes infinitesimal temperature variation. These infinitesimal state changes shall require infinite time and the process shall be of the quasi-static type, therefore reversible. Irreversible state changes accompany heat transfer through a finite temperature difference which practically occurs and thus makes processes irreversible.
   6. Nonequilibrium during the process: Irreversibilities are introduced due to a lack of thermodynamic equilibrium during the process. Non-equilibrium may be due to mechanical inequilibrium, chemical inequilibrium, thermal inequilibrium, electrical inequilibrium, etc., and irreversibility is called mechanical irreversibility, chemical irreversibility, thermal irreversibility, electrical irreversibility respectively. Factors discussed above are also causing non-equilibrium during the process and therefore make the process irreversible.

Comparison between reversible and irreversible processes shows the following major differences.
Table 1: Difference between reversible and irreversible processes

S.no	Reversible Process	Irreversible Process
1	Reversible process can not be realized in practice.	All practical processes occurring are irreversible processes.
2	A reversible process leaves no trace of occurrence of process upon the system and surroundings after its’ reversal.	The evidence of the process having occurred are evident even after the reversal of the irreversible process.
3	The process can be carried out in the reverse direction following the same path as followed in the forward direction.	Process, when carried out in the opposite direction follows the path different from that in the forward direction.
4	Such processes can happen in either direction without violating the second law of thermodynamics.	Occurrence of irreversible processes in either direction is not possible, as in one direction it shall be accompanied by the violation of the second law of thermodynamics.
5	A system undergoing reversible processes has maximum efficiency. So the system with reversible processes is considered as reference systems or bench marks.	System having irreversible processes do not have maximum efficiency as the wastage of energy accompanies it.
6	Reversible process occurs at an infinitesimal rate, i.e., quasi-static process.	Irreversible processes occur at a finite rate.
7	System remains throughout in thermodynamic equilibrium during the occurrence of such process.	System does not stay in thermodynamic equilibrium during the existence of irreversible processes.
8	Examples of the reversible process; Frictionless motion, controlled expansion, and compression, Elastic deformations, an Electric circuit with no resistance, Electrolysis, Polarization and magnetization process, etc.	Examples of the irreversible process; Viscous fluid flow, inelastic deformation and hysteresis effect, Free expansion, an Electric circuit with resistance, Mixing of different gases, Throttling process, etc.