Concept of Entropy Transfer

ENTROPY OF THE SYSTEM

The change in the total entropy of the system = (Entropy transfer) + (Entropy generation)

ENTROPY TRANSFER

There are 2 ways of entropy transfer

  1. Heat transfer
  2. Mass transfer

As heat is said can be transferred from 1 system to another  through a boundary.. so can be entropy.

As heat is transferred from system to surrounding at const temp..

the system loses heat and surrounding gains it.

∴system’s entropy decreases & surrounding’s entropy increases.

Heat is regarded as disorganised energy. Work as organised energy.

Entropy transfers along with heat flow but not with work. Entropy may be generated via less useful form of energy (e.g. irreversibilities like friction.)

ENTROPY GENERATION

Entropy is generated due to Internal Irreversibilities & Work being dissipated into internal energy increase.

The first law of thermodynamics makes no distinction between heat transfer and work; it considers them as equals. 
The distinction between heat transfer and work is brought out by the second law: an energy interaction that is accompanied by entropy transfer is heat transfer, and an energy interaction that is not accompanied by entropy transfer is work

E.g.

Work is delivered to a flywheel. It starts to rotate.. the molecules of the flywheel is rotating in an organised way. There is no dissipation and hence no entropy increase. Same with work transfer in spring compression and elongation. No entropy transfer with work.

But if work is dissipated adiabatically into internal energy increase of the system, there is entropy increase (Entropy Generartion) but no entropy transfer.

In the expansion of hot fluid in turbine, the heat is lost ∴ entropy decreases. But entropy might increase (entropy generation) due to internal irrevesibilities such as friction. It might also happen that these decrease and increase in entropy are equal & net entropy change of system=0 thus making the system ISENTROPIC.

∴An isentropic process need not be adiabatic or irreversible.

But if isentropic process is reversible it should be adiabatic.

An adiabatic process need not be isentropic. But if adiabatic & reversible it must be isentropic.

A system executing a process must conserve energy but entropy is generated internally.

CLOSED SYSTEM

No mass flow across the boundaries of a closed system. So, only heat entropy transfer + entropy generation.

Any closed system and its surroundings can be treated as an adiabatic system.

CONTROL VOLUME

Apart from heat entropy transfer, it involves 1 more mechanism of entropy exchange: mass flow across the boundaries. 

Mass possesses entropy
as well as energy, and the amounts of these two extensive properties are
proportional to the amount of mass.

2nd law — any thermodynamic process is accompanied by entropy generation.

Entropy generation (Sgen)is not a thermodynamic property. δSgen is an inexact differential.

Sgen =0 are considered reversible path.

Sgen >0 are considered irreversible path.

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