We were discussing various basic concepts of thermodynamics such as “Clausius theorem” in our recent post. We have also discussed the “Concept of Clausius inequality in thermodynamics and also “Steady flow energy equation for throttling devices” and “steady flow energy equation for heat exchanger” too in the field of thermal engineering.

We will see here change in entropy for a reversible and also for an irreversible process with the help of this post.

### Let us see now the change in entropy for a reversible process and also for an irreversible process

Let us consider the following figure, a system is going from state 1 to state 2 by following the path A, we have assumed here that path A is reversible process. Hence system is going from state 1 to state 2 by a reversible path A as shown in following figure.

Let system is returning back to its initial state via a path B, let we have assumed here that path B is also a reversible path and therefore system is returning back from state 2 to state 1 via one reversible path B as shown in figure. Hence we will have one cycle 1-A-2-B-1 and it will be reversible cycle.
Now let system may also come back to its initial state via a irreversible path C or we can say that system is coming back via following the irreversible path C as shown in figure and hence we have one cycle 1-A-2-C-1 and it will be an irreversible cycle.

Let us see here first entropy transfer in a reversible cycle i.e. change in entropy or entropy transfer for reversible cycle 1-A-2-B-1.
Let us see here now entropy transfer i.e. change in entropy for irreversible cycle 1-A-2-C-1 and we will have following equation
Let us consider above both equations and we will have following equation
As we know that Path B is a reversible process and we also know that entropy is a property of the system and therefore change in entropy during process B and during process C will be same.
Change in entropy for reversible process will be determined by following formula
Therefore we can write here change in entropy for reversible process and for irreversible process in following way
Do you have suggestions? Please write in comment box
We will discuss another topic i.e. "Temperature entropy diagram for water" in our coming posts.

### Reference:

Engineering thermodynamics by P.K. Nag