Why is the Definition of Entropy Restricted to a Reversible Process?

In summary: This is because for an open system, the Clausius inequality states that the total entropy change is always greater than zero for an irreversible process and equal to zero for a reversible one. The definition of entropy (eq. 1) can still have a non-zero value even for a reversible process, as it is not necessarily zero. This is due to the fact that a reversible process minimizes heat and maximizes work, which is related to engine performance. The concept of entropy is further explained in the Physics Forums Insights article, which also addresses common misconceptions and offers a clearer understanding of the topic.
  • #1
lee403
16
1
Even though entropy is a state function and thus independent of path why is it that the definition of entropy is restricted to a reversible process?

Eq 1: dS=dqrev/T

First of all, is this for an open system, closed system or isolated system? The Clausius inequality deals with the exchange between the surroundings and the system so the total entropy change is always greater than zero for a irreversible process and equal to zero for a reversible one.

Eq 2: dS≥dqsystem/Tsurr.

I thought that this is because a reversible process progresses so slowly that it is always in equilibrium and that the change in entropy at equilibrium is zero. But the definition above (eq. 1), although q is reversible, is not necessarily zero.

One explanation I have gotten was that is that a reversible process minimizes heat and maximizes work. I understand this concept with engine performance and that reversible processes maximize work, but how this pertains to the definition of entropy I don't understand.
 
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  • #2
The answers to all these questions are covered in the following short Physics Forums Insights article: https://www.physicsforums.com/insights/understanding-entropy-2nd-law-thermodynamics/

I wrote this article specifically for people like yourself who are struggling with these concepts (because they are presented to poorly in the available textbooks and online literature). Please don't hesitate to ask any followup questions that you may have.

Chet
 

Related to Why is the Definition of Entropy Restricted to a Reversible Process?

1. What is entropy in a reversible process?

Entropy in a reversible process is a measure of the disorder or randomness of a system. It is a thermodynamic property that describes the distribution of energy within a system.

2. How is entropy of a reversible process calculated?

The entropy of a reversible process can be calculated using the equation ΔS = Q/T, where ΔS is the change in entropy, Q is the heat absorbed or released during the process, and T is the absolute temperature.

3. What is the significance of a reversible process in terms of entropy?

A reversible process is one in which the system can be returned to its initial state by reversing the steps of the process. In such a process, the entropy remains constant, as there is no net increase or decrease in disorder.

4. How does the second law of thermodynamics relate to entropy of reversible processes?

The second law of thermodynamics states that the total entropy of a closed system can never decrease over time. In a reversible process, the entropy remains constant, but in irreversible processes, the entropy always increases.

5. Can entropy of a reversible process ever be negative?

No, the entropy of a reversible process can never be negative. This is because entropy is a measure of disorder, and it is not possible to have a negative amount of disorder or randomness in a system.

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