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mayank pathak

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Hi, I have been studying thermodynamics from Herbert Callen's "Thermodynamics : an introduction to the physical theories of equilibrium thermostatics and irreversible thermodynamics"

In Chapter 4, Section 4.4, he writes :

Now he also describes what a reversible heat source is :

I understand his argument. But I fail to understand why is the heat source required to be reversible ? According to me, as long as the heat source(or sink) is constrained to have constant volume and constant mole numbers, same heat input will lead to same rise in internal energy and hence same increase in entropy in accordance with the fundamental equation of the heat source. And that is all that we need to prove maximum work theorem. And we don't actually need the heat source to be reversible.

Am I missing something ?

Edit : I have uploaded the relevant text from the book.

In Chapter 4, Section 4.4, he writes :

**"**

all processes occurring between a given initial and a given final state of a system, the flux of heat to an associated reversible heat source is minimum and the flux of work to an associated reversible work source is maximum for reversible processes."

all processes occurring between a given initial and a given final state of a system, the flux of heat to an associated reversible heat source is minimum and the flux of work to an associated reversible work source is maximum for reversible processes."

Now he also describes what a reversible heat source is :

**"**

A reversible heat source is defined as a system enclosed by a rigid

impermeable wall and characterized by relaxation times sufficiently short

that all processes of interest within it are essentially quasi-static.A reversible heat source is defined as a system enclosed by a rigid

impermeable wall and characterized by relaxation times sufficiently short

that all processes of interest within it are essentially quasi-static.

**"**

I understand his argument. But I fail to understand why is the heat source required to be reversible ? According to me, as long as the heat source(or sink) is constrained to have constant volume and constant mole numbers, same heat input will lead to same rise in internal energy and hence same increase in entropy in accordance with the fundamental equation of the heat source. And that is all that we need to prove maximum work theorem. And we don't actually need the heat source to be reversible.

Am I missing something ?

Edit : I have uploaded the relevant text from the book.

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