Talk:Entropy
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Vague criticism
Sometimes the article is frustratingly vague. In particular:
It doesn't always seem to distinguish between definitions (p=mv, by definition) and laws (momentum is conserved, by observation).
A thermodynamic transformation is defined as "a change in a system's thermodynamic properties, such as its temperature and volume" (emphasis added). What other properties are relevant? Is it a long list?
What does "equilibrium state" mean? Complete thermodynamic equilibrium--the entire system must be a single temperature and pressure throughout?
I'd work on it myself, but I'm too clueless. Jorend 05:18, 24 Apr 2005 (UTC)
- The reason for the vagueness is that thermodynamics applies to lots of different systems, and the thermodynamic parameters relevant to one system may not make any sense in another system. For example, in the thermodynamics of gases we talk about pressure and volume, whereas in the thermodynamics of ferromagnetic systems we talk about magnetization and susceptibility. The concept of "pressure" (or even "volume") doesn't make any sense for magnetic systems, while magnetization doesn't make any sense when we are looking at ordinary gases. There are only a handful of parameters that are common to all thermodynamic systems -- temperature, energy, entropy, heat capacity, and so forth. -- CYD
- Thanks for your comment, CYD, and thanks immensely for all your work on this article. I wonder if it might be worth adding two examples (a classic ideal gas one and a magnetic one) to illustrate what the ambiguity is all about. I'm not sure who the target audience for this article is, but it would be quite frustrating to have it aimed completely over my head, as I'm actually an engineer by training and had a semester course in thermo! -- 24.147.43.253 00:08, 25 Apr 2005 (UTC)
An intuitive understanding of entropy
I read this article hoping to obtain an intuitive understanding of entropy. Didn't get it. And I don't think the problem is that the article is too "hard"; I don't think it would provide an intuitive grasp of the subject to any reader, however sophisticated. Maybe the topic is too hard. If nothing else, more examples (and examples less wedded to chemical engineering) would be welcome.
Perhaps it is unclear what I mean by "intuitive understanding". I mean the ability to answer questions like these:
Consider my coffee cup as a thermodynamic system. I drop an ice cube into it. How does this affect the entropy? I have introduced a big difference in temperature between two parts of the system, so the entropy decreases, right? But coming at it from a different direction, it seems as though I have added heat to the system, by adding matter to it: even though the ice is frozen, the ice molecules have some thermal energy. So the entropy increases, right?
Is entropy meaningful outside the context of thermodynamic machines? For example, is entropy defined at non-equilibrium states? (The article sort of gives the impression that it isn't. But lots of real-world systems are essentially never at equilibrium, so this would be a serious limitation.)
I'm not clear on how entropy, energy, and work are tied together. How exactly is entropy related to the ability to do work? The article states that when you reach the maximum entropy state, no more work can be done--states it, that is, but doesn't explain. What is meant by "can't do work"--can't do significant, macroscopic work?
Can a supply of energy be referred to as "ordered" (i.e., capable of doing work)? It seems by definition that heat is the only form of energy that increases entropy. Are all other forms of energy equally "ordered" as far as entropy is concerned? After some thought, my guess is that all other forms of energy can be used to do work; only thermal energy can become dispersed and unable to do work, and entropy is a function of the distribution of thermal energy in a system. Am I getting warm?
The article talks about entropy of a system in relation to work done by the system. What about work done within the system (as, by one part of the system on another part)? Perhaps an equivalent way to put this question would be: How does entropy relate to non-thermodynamic properties (like potential energy) of parts of a system? If my previous guess is right, the only effect on entropy is when activity within the system converts other sorts of energy to "waste heat".
Entropy is defined as a function of the state of a system. Does it make any sense to compare the entropy of one system to the entropy of some other system that contains entirely different matter? For example, which has greater entropy: your average rock; or an ice crystal of the same mass at the same temperature? Is the question meaningful?
Kindly don't answer the questions here, unless you just want to talk them through. Improve the article instead. Thanks! Jorend 05:18, 24 Apr 2005 (UTC)
Revocation of additional definition was incorrect.
Entropy is defined for an irreversible expansion as:
<math>\triangle S_{universe}\ =\ \triangle S_{system}\ +\ \triangle S_{surroundings}<math>
<math>= q_{sys}/T\ +\ q_{surroundings}/T\ <math>
<math>= nRTln(V2/V1)/T\ + [-nRTln(V2/V1)]/T\ =\ 0\ for\ equilibrium.<math>
<math>=nRln(V2/V1)\ >\ 0 for\ irreversible\ process!<math> because delta <math>S_{surroundings}<math> = 0 and <math>q_{surroundings} = 0<math>.
Therefore, <math>\triangle S_{universe}\ =\ \triangle S_{system}\ +\ \triangle S_{surroundings} \ge 0.<math> -- Tygar
- That is not a meaningful definition of entropy, since the entropy of the universe is an arbitrary quantity. In any case, your argument relies on several assumptions, such as the fact that entropy is a state function, derived from the original definition based on reversible transformations. The article does talk about the entropy change associated with irreversible transformations further down. -- CYD
Hmm, math seems to be borked right now, except for cached equations and text translations. Just in time for my big landing. Oh well. -- CYD
"The thermodynamic entropy S is a measure of the amount of energy in a system which cannot be used to do work"
- i think this is wrong - the work that cant be done depends on the temperature too - the TS product matters.
I have never seen such definition - where is it from - Terse.
- That sentence is from the original incarnation of the article by Tobias_Hoevekamp. It doesn't strike me as a particularly bad way to describe the entropy, although it is of course not absolutely accurate. It seems to me that the thermodynamic (as opposed to statistical) concept of entropy is difficult to define succintly. Can you think of a better description? -- CYD
- That depends on whether or not the system is in a heat bath... Phys 15:10, 22 Aug 2003 (UTC)
- Perhaps we can make it a bit more precise, by mentioning that. It is a good intuitive interpretation of entropy, but it could also be misleading. Terse 22:38, 31 Aug 2003 (UTC)
- Entropy and work (energy) are different things. They are related through temperature. Jellyvista
Hm, no mention of symmetry here. It struck me tonight that what's meant by order in entropy discussions is broken symmetry, and by disorder, symmetry. So entropy should be measuring some sense of the amount of symmetry. A little Googling indicates I'm on the right track, but my IB Chem in highschool totally ruined me for understanding entropy =p so I'm just gonna try to prompt someone not buffaloed by the shebang into checking out entropy-symmetry stuff. I'm fairly sure there's something important there
- The whole "disorder" thing is a bit misleading. What people typically mean by an "ordered" state is that the positions and identities of the parts are well known. If a deck of cards is "ordered", then you know exactly where each card is, and therefore the deck is in exactly one state. If you shuffle the deck, or make it "disordered", the position of the cards is unknown, and therefore there are more possible states in which the deck could be. The possible number of states (or the log thereof) is what entropy measures. In your question, "symmetry" by definition means that we know something about the state, which reduces its entropy. Jellyvista 07:42, Apr 12, 2004 (UTC)
Counting of microstates
think you should mention that in classical statistical mechanics infinite states/coarse graining isn't a problem when you are talking about change in entropy because difference in logs = log of a ratio which can be looked at as a ratio of volumes in phase space or the limit of ratios using coarse graining. currently the statistical formula is introduced here but immediately implied to be useless without jumping to quantum considerations
I can't touch it because I'm a total wikinewbie and don't know anything about thermo either
wtf
this article not written very clearly at all. The definition of absolute temperature is especially sketchy, and I don't like how it comes a good ways after the concept is first mentioned. And according to the current definition of "microstate", it would seem that there's an infinite number of them in any case. A good physics article should be accessable to anyone with competent knowledge of math.
- I'm not certain what your complaint about the temperature is. If you feel that you have a better way of presenting it, go ahead and give it a shot. Note that temperature is defined in terms of entropy in statistical mechanics, and vice versa in classical thermodynamics.
- As for the microstates: this subject is closely linked to the theory of measurement in quantum mechanics, which is (AFAIK) still dimly understood. The number of microstates would be infinite if we stick strictly to classical statistical mechanics. You have to introduce a "cut-off" to get rid of the infinities, essentially saying that two microstates that are "close" enough to one another should be treated as the same state. In classical mechanics, there's no way to justify this procedure, so it's an entirely arbitrary (though not unreasonable) requirement. In quantum mechanics, you can partially justify it using the Heisenberg uncertainty principle. -- CYD
- What is geometrical frustration?
Page move
I'm not entirely sure what's happened here. The article is at Entropy, but Talk:Entropy redirects to Talk:Thermodynamic entropy. As far as I can see, the article used to be at Thermodynamic entropy; after Entropy was moved to Entropy (disambiguation), Thermodynamic entropy was moved to Entropy. Looks like the talk page was left behind. I'm going to delete the current Talk:Entropy (which is just, and has always been, a redirect) and move Talk:Thermodynamic entropy there to match the article. — Knowledge Seeker দ 06:13, 12 May 2005 (UTC)