# Talk:Entropy (classical thermodynamics)

WikiProject Physics (Rated B-class, High-importance)
This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
B  This article has been rated as B-Class on the project's quality scale.
High  This article has been rated as High-importance on the project's importance scale.

## Inexact differential

What *is* an inexact differential. I believe I was the person that put that redlink up a long while ago, and I had hoped that someone would fill that space with an explanation. I still have no idea what the inexact differential symbol δ *means*. Does anyone know? Fresheneesz 09:34, 5 May 2006 (UTC)

Yes, loosely it means that the function may not exist; also in thermodynamics it means that that function is not a state function and that it is path dependent. This is just from memory, I may be off on a few technical points. We are working to build the inexact differential article. Help if you want. Thanks:--Sadi Carnot 13:54, 25 July 2006 (UTC)
WikiProject Physics (Rated B-class, High-importance)
This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
B  This article has been rated as B-Class on the project's quality scale.
High  This article has been rated as High-importance on the project's importance scale.

## Apparent contradiction in first paragraph (introduced by recent edits)

There is an apparent contradiction in the first paragraph. First it says, "the entropy is defined only for a system in thermodynamic equilibrium"; and then it says, "as the system moves toward equilibrium, the entropy of the system increases, becoming a maximum at equilibrium". But if entropy is defined only *at* equilibrium, it will be *undefined* *before* a system reaches equilibrium. Therefore, the current wording implies that a system that is moving towards equilibrium will have an undefined entropy for most of the time, and then, once it reaches equilibrium, it will have some entropy. Perhaps the wording of both sentences in this paragraph needs to be improved to remove the contradiction? —Preceding unsigned comment added by 76.117.93.253 (talk) 00:26, 29 August 2009 (UTC)

Agreed, this is a problem. Plastikspork ―Œ(talk) 04:49, 2 September 2009 (UTC)
Entropy is defined for a system that is at equilibrium. If a system that is not at equilibrium can be broken up into subsystems (A,B,C,D,...) each of which is at equilibrium. Then the entropy of the total system can be represented as the sum of the entropies of each subsystem. This often allows the entropy of a non-equilibrium system to be a well defined quantity. —Preceding unsigned comment added by Simpleliquid (talkcontribs) 18:02, 8 October 2009 (UTC)

## Ice melting example

In the Introduction there is reference to "the ice melting example", this phrase is linked to an apparent bookmark within the same article. But the link takes you nowhere. A little further on, the text says "However, as calculated in the example, the entropy of the system of ice and water has increased more than the entropy of the surrounding room has decreased." Again, the calculation is nowhere to be found. This is less than satisfying. Has the example with its calculation been deleted? If so, references to it should also be deleted. In either case, a modification of the text is called for. Wdanbae (talk) —Preceding undated comment added 10:05, 13 September 2010 (UTC).

## Some suggestions for improvement

I like the article but I think there is room for improvement:

1. I would prefer to change the order a bit. E.g. start with the definition of entropy.
2. Through the small arrow, leaving and entering the engine in the figure, it is suggested that the system is not in the steady state: more energy goes in than out!
3. The use of absolute bars is confusing (strictly speaking it is wrong). It assumes that the heat and work flows are in the positive directions indicated by the arrows. Application of the expressions with heat and/or work flowing in other directions would give false results.
4. The entropy production is not a function of state and should not be expressed as ΔS. Better Si, where the lower index i refers to irreversible processes?
5. The relation dS=δQ/T only holds for closed homogeneous systems. In these systems the processes are reversible (almost by definition).
6. The term "universe" should be clarified (or, better, avoided). It is not the universe of the astronomers. Thermodynamics is a down-to-earth science.
7. The Section on Entropy measurement only gives the expression for the temperature dependence of the entropy. Furthermore a more natural place for this Section would be directly after the definition. — Preceding unsigned comment added by Adwaele (talkcontribs) 13:57, 14 May 2012 (UTC)