What is Entropy? What is thermodynamic entropy?
There are three important E’s in the study of the thermodynamics: energy, equilibrium and entropy. The word entropy was first used by Rudolf Clausius. It is taken from the Greek word ‘tropee’ which means transformation. Entropy is the extensive property of the system (depends on the mass of the system) and its unit of measurement is J/K (Joule per degree Kelvin). Entropy is heat or energy change per degree Kelvin temperature. Entropy is denoted by ‘S’, while specific entropy is denoted by ‘s’ in all mathematical calculations.
The property ‘entropy’ plays central role in the study of thermodynamics and it has been introduced via the concept of the working of the heat engine. The subject matter of entropy has been developed due to invaluable contributions from Sadi Carnot, James Prescott Joule, William Thomson (Lord Kelvin), Rudolf Clausius, Max Planck and others.
The entropy of the system is not measured in absolute terms; rather it is measured in relative terms. The entropy of the system is measured in terms of the changes the system has undergone from the previous state to the final state. Thus the entropy is always measured as the change in entropy of the system denoted by ∆S and not merely S. If at all it is necessary to measure the value of the entropy at a particular state of the system, then zero value of entropy is assigned to the previous chosen state of the system.
Isentropic Process
The process during which the entropy of the system remains constant is called as isentropic process. During isentropic process the value of entropy of the system at initial and final state remains constant. Thus during isentropic process the value of ∆S=0.
The isentropic process can be reversible or irreversible. For the isentropic process to be reversible, it is essential that it is adiabatic process also, if it is not adiabatic, then the isentropic process cannot be reversible. This also implies that the process which is reversible and adiabatic is always an isentropic process.
Causes of the Increase in Entropy of the System
In actual practice the reversible isentropic process never really occurs, it is only an ideal process. In actual practice whenever there is change in the state of the system the entropy of the system increases. Here are the various causes of the increase in entropy of the closed system are:

Due to external interaction: In closed system the mass of the system remains constant but it can exchange the heat with surroundings. Any change in the heat content of the system leads to disturbance in the system, which tends to increase the entropy of the system.

Internal changes in the system: Due to internal changes in the movements of the molecules of the system there is further disturbance inside the system. This causes irreversiblities inside the system and an increase in its entropy.
Reference
Book: Engineering Thermodynamics by P K Nag
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