# What makes procesess internally reversible? [closed]

So I read that processes where there is no thermal gradients in the system are internally reversible, but for other cases, do we have to consider the temperature of boundary to calculate entropy change? What does this model of using boundary temperature do? Does it account for heat generation inside the system or does it consider it thrown outside?

So how does temp gradient in the system relate to irreversibility quantitatively? And to calculate entropy generation as a whole, do we need to consider whole universe or only system and surrounding separately, I mean for heat transfer between 2 thermal reservoirs with large heat capacities ( infinite) at finite temp difference, the system heat gen is zero but in universe, entropy is generated. So how do we spatially account for entropy generation in any general case, or can we only find entropy generation in universe as a whole? I am quite confused about this.

So basically two questions: How to map entropy generation in a general case? Why does considering boundary temperature help in determining overall entropy generation inside the system(in case of non uniform temperature inside the system)?

• This question includes multiple questions in one, it needs more focus and should be re-written or closed. Spell checking would also not hurt.
– hft
Commented Oct 20, 2021 at 7:47

1. Entropy transfer across the boundary interface between the system and its surroundings. This is equal to the integral of $$dQ/T_B$$, where dQ is the amount of heat flowing across the system boundary from the surroundings and $$T_B$$ is the temperature of the portion of the boundary through which dQ flows. For a reversible process, this is the only mechanism for entropy change and, in this case, $$T_B$$ is equal to the system temperature (which is uniform throughout the system, including at the boundary).
• Yes, in all such cases where the entropy generation is not apparent where to ascribe those to? And how is taking the value of $$\delta s =\delta q/T_{b}+ s_{gen}$$ is a definition of entropy transfer accounting for all the entropy change occuring inside the system including entropy generation. Commented Oct 20, 2021 at 18:17