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Types of Irreversibility In Thermodynamics

The process becomes irreversible if it occurs due to finite potential gradient like the gradient in temperature or pressure, or if there is dissipative effect like friction, in which work is transformed into internal energy increase of the system. 




Two types of  Irreversibility can be distinguished :

1. Internal Irreversibility 

2. External Irreversibility

The Internal Irreversibility is caused by the internal dissipative effects like friction, turbulence, electrical resistance, magnetic hysteresis, etc., within the system. 

The External Irreversibility refers to the irreversibility occurring at the system boundary like heat interaction with the surroundings due to a finite temperature gradient.

If the irreversibility of a process is due to the dissipation of work into the increase in internal energy of a system, or due to a finite pressure gradient, it is called mechanical irreversibility. 

If  the process occurs on account of a finite temperature gradient, it is called thermal irreversibility, and if  it is due to finite concentration gradient or a chemical reaction, It is called Chemical Irreversibility.

A heat Engine cycle in which there is a temperature difference,

i) between the source and the working fluid during heat supply, and 
ii) between the working fluid and the sink during heat rejection,

 exhibits external thermal irreversibility. 

If real source and sink are not considered and hypothetical reversible processes for heat supply and heat rejection are assumed, the cycle can be reversible. With the inclusion of  the actual source and sink, However, the cycle becomes externally irreversible.  
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