Talk:Entropy

Merger proposal

The Entropy (classical thermodynamics) page is completely encompassed by this article. There is no sensible explanation about why we should have both pages.--200.109.197.133 (talk) 07:30, 12 February 2013 (UTC)

Proposal noted at WT:PHYSICS Jheald (talk) 12:53, 12 February 2013 (UTC)

*Support The reason seems apt. There may be other sections that can be factored out if the article gets too large.--Probeb217 (talk) 04:49, 13 February 2013 (UTC)

Vote struck out. Elockid ^(Talk) 04:58, 13 February 2013 (UTC)

• Oppose. I've deliberately tried to give time to let other people come in here, and I'm but disappointed that they haven't, because this proposal deserves a fair hearing and fair consideration. It's not clear that we do have the scopes of the various articles on entropy correctly set up at the moment -- for example, the proposal above that the Entropy article, as the main entry point for people typing the word into the search box, ought to be something more like the Scholarpedia article [1], presenting an overview of all notionss of entropy, essentially replacing our current Entropy (disambiguation) article, rather than being focussed specifically on thermodynamic entropy.

I am not sure about that proposal, because in the past people have very strongly made the argument that it is thermodynamic entropy that people are most likely to be looking for, either from general interest about the nature of the universe, or because they're specifically meeting the concept in Physics, Chemistry or Engineering classes. As a result the argument is made that the WP:PRIMARYTOPIC for entropy should specifically be thermodynamic entropy. But that may or may not be right.

However, I am also uncomfortable about the current proposal. Because I think there is still a role for what was the original intention of this article, namely to be an entry-point for all ways of thinking about the entropy of physics, chemistry and thermodynamics -- so introducing the entropy of classical thermodynamics certainly, but also introducing statistical mechanical entropy, and the von Neumann entropy of quantum statistical mechanics, and showing how they (may) all marry up consistently with each other; as well as introducing how people think physical entropy should be talked about qualitatively -- particularly whether or not it's useful to talk about the log of the number of microstates as being predominantly determined by the extent of energy "dispersal", or the pros and cons of more traditional language involving "order" and "disorder".

I think that's a useful scope for the article to have, to give people as much of a handle on the question "So, what is entropy?"; though it's not an easy scope to do well. It's a rather different scope to trying to write a fully detailed article just about entropy in classical thermodynamics, which is what Entropy (classical thermodynamics) sets out to do. In particular, I think what the merge proposer writes, that the scope of that page should be "completely encompassed by this article" is probably not true. For the scope above to be possible, I think this article has to be written WP:SUMMARY-style, knowing that not all the detail can be presented here, that this article itself can only present a "taster" of a full proper treatment of each domain of thinking and calculating about entropy. The approach also seems to me very much in line with WP:TECHNICAL -- to try to give people as much as you can that is accessible as an overview of the whole subject first (the skins of an onion approach), rather than expecting to march the reader in order through the full detail of each domain one after the next.

So that's what I think had been the rationale between the article as it was (though I think it's proved a difficult brief to deliver). I still think it's a rationale that makes some sense. But if others want to argue why a different model for the article would be better, I'm not going to stand in anybody's way. (And sorry if I've been WP:TLDR for everybody). Jheald (talk) 11:46, 13 February 2013 (UTC)

Also oppose. The thermodynamic entropy article is, within its limitations, quite coherent, careful, and clear. The general entropy article has some nice attempts to include a more modern, general definition. However, the organization is a mess and various idiosyncratic and sloppy ideas dangle off various parts of it. Today I removed one section that was entirely wrong. Until this article is in better shape, I think it would be a shame to mess with the well-constructed thermodynamic article.Mbweissman (talk) 03:27, 8 March 2013 (UTC)

P vs. p: Power, pressure, probability

Today Koen Van de moortel changed one P to a p with the edit summary "P=Power, p=pressure". So I skimmed the whole article with the thought of making the notation uniform and found that

1. power actually does not occur in this article
2. pressure is sometimes P and sometimes p; and also
3. probability is sometimes P and sometimes p.

So to be consistent we can choose Option 1 P = probability, p = pressure everywhere, or Option 2 P = pressure, p = probability everywhere. Opinions? Dirac66 (talk) 19:00, 21 May 2013 (UTC)

Yes, P for pressure and p for probability (Option 2) seems best to me but either seems reasonable. Go for it! MorphismOfDoom (talk) 12:32, 5 June 2013 (UTC)

I have now counted the uses of each symbol at the moment in this article. Pressure is P 9 times and p 4 times, and probability (by coincidence) is p 9 times and P 4 times. So option 2 recommended by MorphismOfDoom corresponds to what previous editors decided most often and I will revise the article to conform to that. As for other articles, their usage also seems to vary between P and p in an unsystematic way. Dirac66 (talk) 02:23, 10 June 2013 (UTC)

OK, now P is pressure and p is probability. Except that the section Entropy and other forms of energy beyond work contains two equations where P is pressure and p is ... momentum. So we have another reason to use P for pressure, to avoid confusion with momentum within the same equation. Dirac66 (talk) 02:45, 10 June 2013 (UTC)

Many classic texts use W for probability, this goes back to Boltzmann himself, since it stands for "Warscheinlichkeit".

Delete first paragraph?

It seems to me the article might be better off without the first paragraph, which reads:

"Entropy is a measure of the number of specific ways in which a system may be arranged, often taken to be a measure of disorder. The entropy of an isolated system never decreases, because isolated systems spontaneously evolve towards thermodynamic equilibrium, which is the state of maximum entropy."

The article concerns thermodynamic, not specifically statistical mechanical, entropy, but the first sentence is more applicable to statistical mechanical interpretation of thermodynamic entropy than to the thermodynamic concept itself, which is worth understanding independent of the statistical mechanical accounts that may be given of it. It is also a specific, Boltzmannian attempt to give statistical mechanical interpretation to entropy, and may be at odds with more Gibbsian versions of entropy, so again, is probably best not to lead with. The validity of the second sentence is highly dependent on your definition of entropy, and again, it is probably best not to lead with it, but to discuss it later in the article. It is a reasonable point of view that isolated systems do not evolve toward thermodynamic equilibrium (and many attempts to prove that this is always so have failed), but rather that thermodynamic equilibrium tends to be reached through interaction with an environment.

If there's no strong objection within the next couple of weeks, I may give this a try, also checking to make sure that the points made in these sentences are addressed, with more nuance, later in the article.

The next paragraph introduces a thermodynamic definition of entropy, which seems a better starting point.

MorphismOfDoom (talk) 12:30, 5 June 2013 (UTC)

biassed advocacy of "energy dispersal" point of view

The entire section on energy dispersal should be deleted. All but one of the sources it references are innapropriate for a wikipedia article since they are not authoritative or representative of a widely accepted approach. A retired professor's personal website should not be relied on. One stray article he wrote for an education journal, even though a respectable peer-reviewed publication, also is insignificant. An unsupported unsubstantiated and, I suspect from perusing google books, false statement about shifting trends in Chemistry textbooks is just too remote from the purpposes of this article. A direct quote from Atkins might be useful. This section is way too big to be in proportion to the importance of this hobby-horse point of view.98.109.238.95 (talk) 22:05, 28 June 2013 (UTC)