If entropy is a state function, how can it keep on increasing?

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Discussion Overview

The discussion revolves around the concept of entropy as a state function and its behavior in isolated systems, particularly focusing on the implications of reversible and irreversible processes on entropy changes. Participants explore theoretical and conceptual aspects of thermodynamics.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant expresses confusion about how entropy, being a state property, can yield different values for different paths (reversible vs. irreversible) between the same states A and B.
  • Another participant suggests that if entropy increases, the two paths do not connect the same points, indicating that distinct states B and B' can be reached depending on the nature of the process.
  • A participant proposes that an irreversible process cannot take the system to the same state B that a reversible process can, suggesting a unique outcome based on the process type.
  • There is a discussion about the definition of an isolated system, with one participant clarifying that it may refer to thermal isolation while allowing for work to be done, while another challenges this interpretation by referencing an authoritative source.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the implications of entropy as a state function in relation to reversible and irreversible processes. There are competing views on the definitions and interpretations of isolated systems and their effects on entropy.

Contextual Notes

Participants highlight potential ambiguities in the definitions of isolated systems and the conditions under which entropy changes occur, indicating that assumptions about thermal isolation and work may vary.

champu123
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I just have this confusion which is completely eating me up. They say entropy of a system is a state property. Then they say that for a completely isolated system, entropy either increases or remains zero depending on the process being irreversible or reversible.

So, let's say for an isolated system I go from A to B thru a reversible path then entropy is zero. And if I go thru an irreversible path, it's something else. But if entropy is a state property, how can it be different for the two paths between same points? This completely makes no sense to me.
 
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champu123;4224876 So said:
If entropy increases, the two paths will not link the same points. There is a set of points B which can be reached on reversible paths starting from A and there are other distinct states B' which can only be reached in irreversible processes.
 
Plot the state of the system using Entropy and Temperature as coordinates.
 
DrDu said:
If entropy increases, the two paths will not link the same points. There is a set of points B which can be reached on reversible paths starting from A and there are other distinct states B' which can only be reached in irreversible processes.

Thanks for the reply. That cleared up some confusion. I also found some long discussion here: https://www.physicsforums.com/showthread.php?t=313396. This thread poses the same question as my confusion.

Hence, I'd like to quote the above mentioned thread and answer the question posted in above thread based on my understanding. In the above thread, the user say:

If we consider an isolated system in which a process occurs, then according to the clausius inequality :
dS≥dQ/T

Since dQ = 0 , it follows that if the process occurs reversibly dS = 0 and irreversibly dS > 0. But entropy is a state function , how could this possibly be ?

My answer: The point here is that if the process occurs irreversibly it takes the system to a different state then when it'd have been reversible.
Now, if I say that the system is at state A, and perfectly isolated. Now, if there is a reversible process which takes it to state B. So, are you saying that it's impossible to design an irreversible process that can take the system to state B? By using an irreversible process, you could take it another state B' or B'' but not B. I think the answer should be yes (i.e. it'd be impossible). And in that case it's a very interesting conclusion.

So now tell me is the conclusion indeed true??
 
Last edited:
If you mean with perfectly isolated that the system is thermally isolated but can also not do any work (or that no work can be done on the system) then you are right, the state cannot change at all.
However, usually with isolated one understands only thermally isolated, so that work can still be done.
 

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