Question

Entropy flows out the system in form of a heat in an internally reversible process. What happened to the entropy of the system? Explain.

Answer 1

Physically , Entropy is a disorder of a system and surrounding.

Basically in microscopic level, it occurs when heat transfer occurs because heat is a energy when it moves some additional movements happens e.g. - Molecular friction ,molecular vibration, internal displacement of molecule,spin moment, kinetic energy etc which make loss of useful heat thus heat can’t transformed fully into work. This addition movements creates chaos in system and surroundings. That’s why some times Entropy is called the measure of chaos .

For this microscopic chaos results in macroscopic level which occurs because some unnecessary irreversibilities e.g-friction , unstained expansion,mixing of fluids,electric resistance,inelastic deformation of solids,chemical reaction and unnecessary heat transfer in finite temperature difference. Noted that such kind of of energy loss can’t be regained so system and surrounding can’t come to it’s initial state without extra work done on it . Therefore Entropy is called the measure of irreversibilities .For this cause heat can’t transformed fully into work .

In real life, all sort of process has this kind of macroscopic & microscopic loss. So in reality each and every thermodynamic process is Irreversible Process.

From Clausius Inequity ,

For Irreversible Process, ∫ (δQ/T) < 0

Therefore, for all irreversible process or any real life thermodynamic process

dS > ∫ (δQ/T)

for total entropy change , S2 -S1 > ∫ (δQ/T)

Since we know that each & every system have certain loss of energy and they all are Internally Irreversible.

(Note : Internally reversible is that for which no irreversibilities are present in with the system. Irreversibilities may be located with in the surroundings. Practically , No internal friction is defined Internally reversible.)

Each & every systems in a thermodynamic process, generates finite amount of Entropy σ of their own for some known or unknown cause.

thus for entropy change , S2 -S1 = ∫ (δQ/T) + σ where σ is called Entropy Generation.

Entropy change depends on state of process but entropy generation depends on irreversibility of process.

For any irreversible process, even for adiabatic irreversible process( there occurs no transfer of heat) σ > 0 so the entropy change can be greater than zero due to entropy generation.
. But for an reversible process, even for adiabatic reversible process, σ = 0 so the entropy generation is zero.