Talk:Thermodynamic system

open system
The present article's account of an "open system", with a beguiling diagram, is really an account of a special case, convenient for some engineering purposes, which makes it seem that heat and work can be defined for an open system. The sources acknowledge the special nature of the case, but this is not reflected in the article. The most general case of an open system does not allow a unique distinction between heat and work, and demands the concept of transfers of entropy and of internal energy. For closed systems, there is a unique distinction between heat and work.Chjoaygame (talk) 17:16, 11 December 2012 (UTC)

isolated system and gravity
The present article says that an isolated system is completely isolated. There are many cases in which one wishes to consider a system that is not allowed to see transfer of matter, heat, or pressure-volume work, but one does wish to allow a gravity field to permeate the working body. Sometimes people speak of an isolated system with no external long-range force field acting on it. I think the article should perhaps allow an isolated system with an external long-range force field acting on it? According to Crawford (1963), on page 5, referring to this, "there is no insulator against gravitational fields. Hence we must rule out gravitational effects altogether." But the Maxwell-Boltzmann distribution does consider external long-range force fields. This calls for some thought, and a look at the literature, to see how to deal with it in the article, I think.Chjoaygame (talk) 17:35, 11 December 2012 (UTC)

open system edit
An edit has been made to the section 'Open system'. As I read it, the model in the section allows differences of temperature and pressure between the inflow and outflow fluids, and perhaps even difference in bulk flow kinetic energy, and even indeed difference in bulk potential energy.Chjoaygame (talk) 18:56, 1 February 2013 (UTC)

more on open system
It seems that editor PAR has a project going, based on his personal intuition, to contradict reliable sources that say that heat and work are not uniquely in general defined for open systems. I do not have time right now to try to tackle this. In line with comments above, apparently ignored by editor PAR, I can only say that the section on open systems here has serious errors and that editor PAR seems intent not on remedying them, but on making them worse and entrenching them for the sake of his project. Sorry to say I cannot recall the source of the diagram, but I can recall that its originally accompanying text explicitly notes that the diagram refers only to a restricted class of processes of interest to engineers and not to open system processes in general.Chjoaygame (talk) 11:03, 12 March 2013 (UTC)

Boundaries vs walls
There have been some changes to the article recently to use the word "walls" of a thermodynamic system, in place of "boundaries". The same change has been made in other articles, e.g. adiabatic boundary / wall. I thought that boundary was the standard term in this context? To me, "wall" is a subset of "boundary", it implies something which is rigid, made of a different material to the system, etc. Am I missing something here? Djr32 (talk) 21:25, 28 July 2014 (UTC)


 * Thank you for this valuable comment. I used the term wall in preference to the term boundary because the term wall has more of a physical flavor than the term boundary, which has a more purely mathematical flavor. I think wall is therefore preferable. You propose that boundary is the standard term in this context. Perhaps you can say in detail why you think that.


 * I accept that wall might be read to refer to something rigid, made of a different material to the system, etc. On the other hand boundary suggests something purely logical or mathematical with no material or physical property. Perhaps one might argue for separately defined notions, wall and boundary, to be considered in the article. At present I am inclined to think one is enough.


 * I was moved to make the change by a comment by Max Born, one of the more influential voices in twentieth century thermodynamics. He wrote "Why not apply the methods of Cauchy to thermal processes, by treating each volume element as a small thermodynamical system, and regarding not only strain, stress, and energy, but also temperature and entropy as continuous functions in space. This has of course been done, but with limited success. The reason is that thermodynamics is definitely connected with walls or enclosures. We have used the adiabatic and diathermanous variety, and mentioned semi-permeable walls necessary for chemical separations; but a volume element is not surrounded by a wall, it is in free contact with its neighbourhood." But a volume element has a boundary, I presume.


 * Callen is a widely cited thermodynamic text. The index of that text does not list 'boundary', but it lists five kinds of walls. I went to amazon and did a 'search inside this book' for the word 'boundary' since I have no electronic copy of the book. I got 16 hits.


 * 1) "The definition of the mole number refers explicitly to the "number of molecules," and it therefore lies outside the boundary of purely macroscopic physics."
 * 2) "... among all the atomic states consistent with the given boundary conditions ..."
 * 3) "A description of a thermodynamic system requires the specification the "walls" that separate it from the surroundings and that provide its boundary conditions. It is by means of manipulations of the walls the extensive parameters of the system are altered and processes are initiated." In the section which this sentence introduces, the word wall appears 12 times by my count by eye. The section does not speak directly of the 'boundary of the system', or use the word 'boundary' otherwise than once, in the phrase 'boundary conditions' as just quoted. The next section of the book has the sentence "An essential prerequisite for the measurability of the energy is the existence of walls that do not permit the transfer of energy in the form of heat." The word boundary does not occur that I saw on a visual check of the section.
 * 4) "This is not the most common boundary condition for chemical reactions, which are more often carried out in open vessels, free to interchange energy and volume with the ambient atmosphere; we shall return to these open boundary conditions in Section 6.4"
 * 5) "(with the boundary between these regions generally occurring at temperatures on the order of 103 K)." These boundaries are between regions on a graph, not between thermodynamic systems.
 * 6) "... this is the equation of the liquid-vapor boundary curve in a P-T diagram ..."
 * 7) "... Calculate the functional form of the boundary of the two-phase region in the P-T plane ..."
 * 8) "This restriction on the form of boundaries of phase stability ..."
 * 9) "This agreement between entropy and disorder is preserved for other boundary conditions–that is for systems in contact with reservoirs, with particle ..."
 * 10) "Nevertheless the fundamental equation is an attribute of the thermodynamics system, independent of boundary conditions, so that the preceding formalism..."
 * 11) "... that the particular system being studied in the laboratory may have different boundary conditions–it may be closed, or it may be in diathermal contact only with a ..."
 * 12) "It follows from our discussion that this division occurs when the fugacity is of the order of unity
 * $$e^{\beta \mu}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\mathrm{(classical-quantum\,boundary)}\,\,\,\,\,\,\,\,\,(18.29)$$"
 * 13. "... (classical–quantum boundary)..."
 * 14. "Again, the variables to be held constant in the differentiation reflect the boundary conditions of the fluctuating system ..."
 * 15. "The boundary B separates the permissible states ("inside") for the non-permissible states ("outside")." Again a region in an abstract space, not the spatial delimitation of a thermodynamic system.
 * 16. "This variation may arise from inherent inhomogeneities in the properties of the system, or it may result from demagnetization effects of the boundaries of the system."

In Tisza's listing of postulates on pp. 108–109 includes:
 * "[Postulate] D a5 Wall: physical system idealized as a surface forming the common boundary of two systems, say $a$ and $b$, and preventing transfer of some of the quantities $X_{k}$, ... The wall is said to be restrictive ... Walls separating two subsystems are called partitions: those completely inclosing a system are called enclosures. The set of walls in a system is referred to as the constraints."

There are other references given in my edit.

My summary of this is that walls are important for thermodynamic systems and deserve a section heading in the article. The word 'boundary' is a word of the ordinary language, but 'wall' is a special term in thermodynamics.Chjoaygame (talk) 02:17, 29 July 2014 (UTC)

"generic" open system representation
A new diagram has been added, which is purported to represent a "generic" open system. It doesn't look generic to me. A generic system would not have distinguishable input and output paths. To me, it looks like something that an enthusiast has made up off the top of his head. It is unsourced and is hardly close to anything in the text of the article. Unless some very convincing justification for it is offered very soon on this page, I will probably remove it.Chjoaygame (talk) 13:39, 19 October 2014 (UTC)

Closed system
I've never come across this idea that a closed system can allow energy transfers. It doesn't even make sense, since if you added energy to a system from all directions, then it would increase in mass.

You can't say that you allow energy but not mass into a system, since E=mc^2 86.18.215.195 (talk) 09:20, 12 May 2015 (UTC)


 * Thank you for this comment. What you say is reasonable and cannot be faulted. Nevertheless, people use words in contexts, and word usage is not uniform. In classical thermodynamics, the word 'closed' is used by some writers to refer to a system that cannot interact with any other. Other writers call such a system 'isolated', and by 'closed' they mean 'able to transfer energy but not matter'. In classical thermodynamics, interconversion of matter and energy is usually not considered, and mass and energy are regarded as separately conserved. In other contexts, as you say, interconversion or equivalence of mass and energy are routine. The article is written on the basis of a fair sample of texts of classical thermodynamics.Chjoaygame (talk) 12:15, 12 May 2015 (UTC)


 * I see you have asked for references for the choice of words in the article. I have offered four. Since they each cover all three, 'isolated', 'closed', and 'open', I have placed them at the end of the paragraph, not exactly at the site of your request. I think they cover your request.Chjoaygame (talk) 14:24, 12 May 2015 (UTC)

It's ambiguous that this section refers to the section Closed system as the main page, while the other section refers back to this section as the main page. XerusLord (talk) 02:02, 24 April 2020 (UTC)

Please review, encyclopedic problem
The redirection of Open system (thermodynamics) to this page caused a reduction of relevance and conceptual problems (see also Wikidata)... See solution of Closed system, isolating and furthering the open system concept in another article.

SUGESTION:


 * 1) to create another (independent) article Open system.
 * 2) to create a sidebar template (like template:Crystallization) to be used as "table of contents" (ToC) of Thermodynamic system topics.

--Krauss (talk) 10:07, 6 December 2015 (UTC)


 * Forgive me, this is a little too telegraphic for my slow wits. I don't know what you mean by "reduction of relevance and conceptual problems". Perhaps you will kindly spell it out for me. I am not thrilled by the idea of creating an article [Open system#In thermodynamics], nor by the idea of creating a sidebar template.Chjoaygame (talk) 12:43, 6 December 2015 (UTC)

Simple vs. complex systems
The article states that a system must be in internal equilibrium and implies that the system must be internally uniform. Such a simple system would be a dull thing. In fact, systems can be complex, with multiple parts that are each internally in equilibrium but that may not be in equilibrium with each other. By removing constraints ("walls") within the system, the overall state of the system changes as it comes to a new equilibrium state.

A simple example is the Joule expansion in which the system consists of both the high pressure compartment and the low pressure compartment and changes when the constraint separating the compartments is released.

This is acknowledged implicitely in the section on closed systems: "For a simple system, with only one type of particle (atom or molecule), a closed system amounts to a constant number of particles. However, for systems undergoing a chemical reaction, there may be all sorts of molecules being generated and destroyed by the reaction process." It needs to be addressed systematically. Retired Pchem Prof (talk) 02:27, 22 January 2016 (UTC)


 * One of the commoner indicators of condescension is a sentence beginning with 'actually'. A variation is a sentence beginning with 'in fact'. We here have a post from an editor who may be indicating his authority by his user name Retired Pchem Prof.


 * Yes, he does point to a problem. Quite a problem for us. The difficulty is that the articles are expected to be more or less independent or self-standing, and ready to be read cold, no warm up, by a newcomer, and are written as an endless sequence of patches by mostly independent editors.


 * Planck, who doesn't have this particular problem, nevertheless has his own partial solution. Without using such terms as 'thermodynamic equilibrium', he writes on page 3 of the first edition:


 * § 6. In the following we shall deal chiefly with homogeneous, isotropic bodies of any form, possessing throughout their substance the same temperature and density, and subject to a uniform pressure acting everywhere perpendicular to the surface.


 * In contrast, Landsberg systematically gives a more elaborate account in terms of compound and complex systems, with recognition of the possibility of adiabatically separated complex systems; I forget the details, and don't have a copy of his text here to check.


 * Editor Retired Pchem Prof writes about complex systems with sub-systems that would not be in mutual thermodynamic equilibrium the moment its internal walls were made less obstructive by a thermodynamic operation. (I note his use of "" marks for walls.) He regards such systems as not necessarily being in equilibrium. Quite a lot could be written about this.


 * Editor Retired Pchem Prof would like a systematic response to this problem. I have often thought about it, and each time, I have felt it is difficult for us. The articles are written catch-as-catch-can by practically countless editors, to regiment whom would make herding cats look easy, even if it were felt desirable.


 * I think a case-by-case piecemeal approach, with occasional evasion, is our likeliest option.Chjoaygame (talk) 04:31, 22 January 2016 (UTC)

Mechanically isolated system - two articles disagree
The section Mechanically isolated system of this article now claims that such a system may exchange ... mass with its environment. However the article Mechanically isolated system claims that such a system does not permit any mass flows in or out of the system. Which is correct please? And can someone provide a source? At the moment neither article provides a source for the point in question. Dirac66 (talk) 02:08, 4 March 2016 (UTC)


 * Good point. Needs investigation. My instinct is that mechanical isolation precludes matter transfer. It would be silly otherwise. An isochoric process is NOT required to be mechanically isolated. The classic example is Joule's paddles. Will try to check.Chjoaygame (talk) 03:33, 4 March 2016 (UTC)


 * Some clues. The present article, until I deleted it just now, carried the wrong statement that mechanical isolation is equivalent to constant volume. Forgetting shaft work, rubbing, and bending.


 * The wrong statement is found also in the article Mechanically isolated system, this time with a sharply located reference. Either the reference is wrong at source, or is being misquoted. The reference is not available to me right now, nor readily available. It would cost me money and time to get it. That article was created by PAR, and at next edit, also by PAR, it then said "In thermodynamics, a mechanically isolated system can exchange no mass or work energy with its environment, but may exchange heat energy with its environment. The internal energy of a mechanically isolated system may therefore change due to the exchange of heat energy. For a simple system, mechanical isolation is equivalent to constant volume and any process which occurs in such a simple system is said to be isochoric. "


 * Copies from Wikipedia are a kind of cancer of the internet.


 * I think it will prove very difficult to find an explicit definition of the term 'mechanically isolated system' in a reliable source. I think the very term, as if standardly defined as a term of art, is a figment of OR. (Yes, you can find it here. It is indicated with dN = 0. And here. In this one 'open' means no matter transfer, but perhaps heat or work. Not classical thermodynamics. Truesdell believes in non-equilibrium entropy!) Even more do I think "the opposite of a mechanically isolated system" is OR in spades. Some time ago I searched for a reference for a systematic term for a system with work precluded. The best I could do was to find that Partington says in a footnote on page 183: "Rankine calls the curves representing changes without performance of work, adynamics."


 * Wikipedia is not a dictionary, nor a guide to every vagary of ordinary language.


 * My preferred solution: (a) delete the article Mechanically isolated system, (b) delete the section in this article, as  unreferenced.Chjoaygame (talk) 05:37, 4 March 2016 (UTC)


 * Just another comment: how an error in a Wikipedia article can spread like a cancer! Next we will be citing the above-linked witness page as a source for the definition of 'mechanically isolated system'!!Chjoaygame (talk) 08:21, 4 March 2016 (UTC)


 * Reliable sources do talk about systems with rigid walls. I suppose that has the effect of mechanical isolation, but I don't feel like writing an article about it. In particular, I seem to recall, Planck requires this for his radiation law derivation.Chjoaygame (talk) 08:28, 4 March 2016 (UTC)
 * Thanks for your replies. I had not realized that this point is so ill-defined in the literature. One would expect there to be an accepted definition, but if I understand you correctly most reliable sources do not use or define the term.
 * My own instinct would agree with yours that a mechanically isolated system exchanges neither work nor matter. That seems a more useful definition since it implies that mechanically isolated systems are a subset of closed systems, so that all results for closed systems are automatically true for mechanically isolated systems. However this argument is (for now) original research and so should not be inserted into Wikipedia articles without a source.
 * Deleting all references to mechanically isolated systems may be too controversial, since the term is used in some literature even if the definition is not rigorous. So perhaps we could retain (in both articles) the statement that such a system can exchange heat but not work, and just remove all statements about exchanging matter or not exchanging matter.
 * And yes, there are too many Wikipedia mirror sites out there. In Google searches it is best to ignore such sites and look at only real books with identified authors, or refereed articles in scientific publications. Dirac66 (talk) 15:44, 4 March 2016 (UTC)


 * I think the motive behind these "isolated" systems lies in the fundamental equation dU = T dS - P dV + &mu; dN. Material isolation is equivalent to dN = 0, "mechanical" isolation is dV = 0, and adiabatic isolation is dS = &delta;Q/T = 0. So the paddles are creating &delta;Q and are violating adiabatic isolation, not mechanical. Mechanical isolation does not imply material isolation. If you want to describe a system that is mechanically isolated with no loss of particles, then you say it is both mechanically and materially isolated. A closed system is then equivalent to a materially isolated system. An isolated system is then equivalent to a materially, mechanically, and adiabatically isolated system. I'm not saying these are good words to use, paddles certainly feel mechanical to me. But there is a distinct difference between heating the system with paddles and working the system by compressing it. I think the various isolations of importance are described mathematically above, and need to have specific terms associated with them. I don't especially like the present terms, but lets make sure that the "isolations" have well-defined meanings with regard to the equations of thermodynamics. In particular, don't lump paddles and compression under one term.


 * Consider also the terms isobaric (dP=0), isothermal ( dT=0), isochoric (dV=0), isentropic (dS=0). I suppose there ought to be an iso term for dN=0 and d&mu;=0, but I don't know what they are. PAR (talk) 17:56, 4 March 2016 (UTC)


 * On the point of sources, of course I agree with Editor Dirac66.


 * A system might have rigid walls permeable to matter. This might be relevant for osmotic pressure studies. In that Wikipedia article, I see no mention of 'mechanical isolation', nor of rigid walls. In Voet et al.on page 28 there is mention of a rigid cell wall that can withstand osmotic pressure, but I didn't find 'mechanical isolation'. No mention of this in the section on page 154 on osmosis in Atkins et al.. But they do talk of thermally conducting rigid walls, not mechanical isolation, on page 5.


 * I am now distinctly unhappy to call 'mechanical isolation' a 'term'. It is a phrase that is sometimes used, but in various ways, as part of ordinary language, and then is defined more by context than custom. I think we should strenuously avoid inventing it as a term of art. To do so would be WP:OR of a particularly bad sort, the carcinogenic sort. I note that the above comment by Editor PAR offers no source, but blatantly suggests how we should innovate. I still favour deletion of both article and section. If there is controversy, let's face it. I am not proposing deletion of all references to it, just of the article and section. Wikipedia is not a dictionary. I have been forced, more or less, into semi-inventing the "term" "Planck–Einstein relation" by the relentless insistence of a determined editor to use the "term" "Planck relation". Horrible. We could resist a harmful push, even form a rigid wall against it. If the pressure is too great, perhaps Editor Dirac66 is wise, as usual, to suggest that we should "just remove all statements about exchanging matter or not exchanging matter".Chjoaygame (talk)


 * I don't mean to advocate innovation, I just think we owe it to the readers to be very clear about what the various isolation terms mean mathematically. We need to be very clear, if you assume a system is x-isolated, then the following modifications to the mathematical statements of thermodynamics are exactly such and such. As it stands, this is the case. Forget calling it a mechanically isolated system. Call it a V-Isolated system. Then mathematically dV=0. An N-Isolated system has dN=0. An S-isolated system has dS=0. The question of what terms we want to use for X-isolated is now mathematically unimportant. We need to search the references for the most accepted terms for these isolations and go with that. If we have references that muddy the waters, don't use them. I cannot see the value of well-referenced mud.


 * Regarding "mechanical isolation", I don't believe the quote is misread: the quote from "Basic Thermodynamics" by Evelyn Guha (available on GoogleBooks) is:


 * "Isolated system: The system is thermally isolated &delta;Q=0, and mechanically isolated, dV=0..."


 * I was wrong to say adiabatic isolation amounts to &delta;Q=0, that's thermal isolation. Adiabatic isolation is thermal and material isolation.


 * I don't think Guha made up the term, so I expect it can be found elsewhere. Maybe not. Who cares? If you don't like "mechanical isolation", fine, find a reference which calls dV=0 something else. I don't care what its called, as long as its clearly defined (dV=0) and supported by a decent reference. PAR (talk) 20:25, 4 March 2016 (UTC)


 * Thank you for this. I think 'isochoric', that you have listed above (" isochoric (dV=0) "), will do what you intend, with good references, for which I will look.


 * As for 'mechanically isolated', it is used occasionally, not universally. I don't think it needs to be defined as a special term. Its meaning is obvious enough in ordinary language, with the context usually saying if matter transfer is allowed, perhaps though a distinct wall. It is very different from 'isochoric'.


 * 'Isochoric' allows shaft work (paddles), rubbing, and bending, at constant volume. It would, I suppose, allow matter transfer. It refers to a process as distinct from a wall. 'Rigid walls' I think excludes all mechanical work, but as noted above I think it would allow osmosis, though I don't know if that is routine in thermodynamics. Routinely, I think, rigid walls are allowed, but not required, to conduct heat or pass light.


 * I think it wise, indeed important, to remember that a system can have several different walls at once. I think it wise, therefore, to define kinds of wall in preference to kinds of isolation. For example, a chamber might have a wall that is rigid but selectively permeable to water, and another wall that is impermeable to heat and matter,  but transmits work with change of volume, in other words a piston. The chamber might even have a third kind of wall, that is rigid and impermeable to matter,  but conducts heat.


 * Since you mention it. Isentropic is, in reliable sources, defined to mean adiabatic and reversible = quasi-static. It looks as if it ought to mean $dS = 0$, and that is implied by its definition, but, in reliable sources, the converse is not part of the definition, i.e. in reliable source definitions, $dS = 0$ does not imply that the process is isentropic, distressing perhaps, wicked perhaps, but not ours to rectify, however much we might wish to do so. Perhaps surprisingly, the term 'isentropic' does not appear in every text.Chjoaygame (talk) 22:29, 4 March 2016 (UTC)

Though it cannot be decisive, it may be useful to quote the Oxford English Dictionary (2009), Second Edition on CD-ROM (v. 4.0.0.3), Oxford University Press Oxford English Dictionary Second Edition on CD-ROM. It lists "'isochor (-kɔː(r)) [Gr. χώρα space], a curve connecting points corresponding to equal volumes, on a diagram denoting relations between pressure and temperature; so isochoric (-'kɒrɪk)".

Likewise, in this dictionary on the web is found "isochor: a line representing the variation of pressure with temperature when the volume of the substance operated on is constant".

There is in Wikipedia this article: Isochoric process, but it is hardly supplied with references.

Found 'isochore' in the text, not necessarily in the index:


 * Atkins, P.W., de Paula, J. (2006). Atkins' Physical Chemistry, eighth edition, W.H. Freeman, New York, ISBN 978-0-7167-8759-4, p. 7: "The lines in this diagram are isochores, or lines showing variation of properties at constant volume."
 * Callen, H. B. (1960/1985), Thermodynamics and an Introduction to Thermostatistics, (first edition 1960), second edition 1985, John Wiley & Sons, New York, ISBN 0–471–86256–8, p. 23: "we draw ... an isochor ($V = constant)$;" p. 129: "The working fluid is cooled isochorically"; p. 130: "alternating with isochoric and isobaric steps"; p. 174: "a family of isochores is overlaid on the graph"; p. 177: "intersection of the corresponding isochore and adiabat"; p. 199: "along the isochore $v = v''_{0}$," etc., etc..
 * Kestin, J. (1966). A Course in Thermodynamics, Blaisdell Publishing Company, Waltham MA, p. 184: "Two constant-volume (also known as isochoric) processes are illustrated in Figure 5.21"; p. 516: "only the isochore $v/v^{*} = 1$ need be drawn}}"; p. 523: "isochores (broken lines)".
 * Blundell, S.J., Blundell, K.M. (2006). Concepts in Thermal Physics, Oxford University Press, Oxford UK, ISBN 978-0-19-856769-1, p. 165: "We also have that for isochoric processes (where isochoric means that $V$ is constant)"; p. 452: "• Isochoric = at constant volume."
 * Silbey, R.J., Alberty, R.A., Bawendi, M.G. (1955/2005). Physical Chemistry, fourth edition, Wiley, Hoboken NJ, p. 33: "isochoric step (heat flows out)."

No listing for 'isochor-' in index.
 * Adkins, C. J. (1968/1983). Equilibrium Thermodynamics, (first edition 1968), third edition 1983, Cambridge University Press, ISBN 0-521-25445-0.
 * Baierlein, R. (1999). Thermal Physics, Cambridge University Press, Cambridge UK, ISBN 0-521-59082-5.
 * Bailyn, M. (1994). A Survey of Thermodynamics, American Institute of Physics Press, New York, ISBN 0-88318-797-3.
 * Buchdahl, H. A. (1966), The Concepts of Classical Thermodynamics, Cambridge University Press, London.
 * Denbigh, K. (1954/1981). The Principles of Chemical Equilibrium. With Applications in Chemistry and Chemical Engineering, fourth edition, Cambridge University Press, Cambridge UK, ISBN 0-521-23682-7.
 * Guggenheim, E.A. (1967). Thermodynamics. An Advanced Treatment for Chemists and Physicists, fifth revised edition, North Holland, Amsterdam.
 * Haase, R. (1971). Survey of Fundamental Laws, chapter 1 of Thermodynamics, pages 1–97 of volume 1, ed. W. Jost, of Physical Chemistry. An Advanced Treatise, ed. H. Eyring, D. Henderson, W. Jost, Academic Press, New York, lcn 73–117081.
 * Kirkwood, J.G., Oppenheim, I. (1961). Chemical Thermodynamics, McGraw–Hill, New York.
 * Münster, A. (1970), Classical Thermodynamics, translated by E. S. Halberstadt, Wiley–Interscience, London, ISBN 0-471-62430-6.
 * Ziegler, H. (1977). An Introduction to Thermomechanics, North-Holland, Amsterdam, ISBN 07204-0432-0.

The term 'at constant volume' is widely used. Also, 'with the volume kept constant'. Perhaps this is because 'isochoric' seems recherché?

For 'rigid wall'.


 * Adkins, C. J. (1968/1983), p. 4: "Then a rigid wall prevents a system from changing its volume or shape so that no work of a mechanical nature may be done on it"; p. 218: "rigid containers each of constant volume";
 * Callen, H. B. (1960/1985), p. 15: "a closed, rigid cylinder. If the position of the piston is rigidly fixed the "wall" prevents the redistribution of volume ... said to constitute a wall restrictive with respect to volume"; p. 17: "a rigid adiabatic impermeable wall"; that several times; p. 54: "a rigid and diathermal wall, permeable to one type of material"; p. 55: "a diathermal rigid membrane, permeable to the first component but impermeable to the second"; [Callen is weakly deviationist in that he is not red hot on the general rule that if the wall is permeable to matter it must also be permeable to internal energy, the transfer of which across that wall cannot be resolved into heat and work];

work in progress, need to do other things for the moment.Chjoaygame (talk) 01:40, 5 March 2016 (UTC)

I just noticed above this: "So the paddles are creating &delta;Q and are violating adiabatic isolation, not mechanical." With much respect I think I need to thoroughly reject that as a matter of high principle. The paddles are causing friction or viscous drag inside the matter of the system. That is not heat transfer. By all the right books it is work transfer. Yes, the paddles are causing &delta;S > 0, but no, not &delta;Q > 0. That is isochoric work. A system can have isochoric work done on it, but cannot do isochoric work on its surroundings in a simple process. It can do PV work on its surroundings. Planck seems to have assumed that to do isochoric work it would need to be able to drive a shaft, which would require it to be able to organize a rotation, which he thought could not happen. Perhaps (and this is not recognized by standard texts that I know of, and thus is OR that I not proposing to put into the article) the system could be a bent spring that would be released from a strained position; I think this one is not recognized by the authorities when they are talking about such things. For that matter, they think only of PV work and straightforward shaft rotation. Joule's barrel of water did not start driving the paddles so as to raise a weight in the surroundings. On the other hand, the lowering of the weight drove the paddles.Chjoaygame (talk) 03:41, 5 March 2016 (UTC)

Added three positives. I think that should be enough for now?

I think 'mechanical isolation' as used by Guha as cited is not good for 'at constant volume'. It fails to observe shaft work, rubbing, and bending, which are constant-volume ways of transmitting work. If one word is needed, I think 'isochoric' will do. I should put some references into that article.Chjoaygame (talk) 07:44, 5 March 2016 (UTC)


 * Yes, you are right. I have to brush up on this. By my notes, dU=TdS-PdV+µdN and TdS = δQ = δQh+δQx where δQh is δQ by heating, and δQx is by external forces, paddles, electric current thru a resistor, etc. These are work terms so they are legitimately called -δWx as well, but they are work that is invariably turned into heat. We can use the work that is done -δWx, or the heat that is created by that work δQx, either way the books balance. If your reference search indicates -δWx is the preferred notation, then thats what we should use. Also, yes, adiabatic refers to, by my notation, δQh=0, not δQ=0, or by your notation, adiabatic refers to δQ=0 but not dS=0 since TdS=δQ-δWx.


 * Also, thank you for the search of references. i am still not sure how to say what an an isochoric process is "isolated" from. It's clearly isolated from the effects of external pressure by virtue of the rigidity of the container. Guha says mechanically isolated, but this glosses over the -δWx aspect of things. However, if it is the only term that has been used, we should go with it. Do you find a better term in the references to replace "mechanically isolated system"? PAR (talk) 07:30, 5 March 2016 (UTC)


 * I didn't find a term to replace 'mechanically isolated system' apart from the rarely used 'adynamic' that means 'no mechanical work'  I still think it better to define terms for kinds of wall than to try to define terms for kinds of isolation, because a system can have several walls. An isochoric process is isolated from PV work, but not from shaft work. I think mechanical isolation does not mean the same as 'no PV work'. It would mean 'no mechanical work, including no shaft work, no other isochoric work, and no PV work' but leave it open what other transfers (heat, matter) are permitted.Chjoaygame (talk) 08:04, 5 March 2016 (UTC)


 * Thinking it over, and googling a little. Yes, ok, 'mechanically isolated system' is used in reliable sources that one can find by googling (examples given above). It means 'no mechanical work', not necessarily specifying about heat and matter transfers.Chjoaygame (talk) 08:46, 5 March 2016 (UTC)


 * By "no mechanical work" did they specify whether it was reversible mechanical work (-PdV) or the sum of reversible and irreversible mechanical work (-PdV-δWx)? In the Wikipedia thermally isolated system article, for example, it allows for work, and  its not explicitly clear until they declare dS=0, that they are talking about reversible work. These ambiguities need to be ironed out. PAR (talk) 11:07, 5 March 2016 (UTC)


 * I think by default convention work is reversible in the surroundings, that is to say, idealized for convenience, because the surroundings are not of close concern, but by default in the system it is always irreversible. You would say (-PdV-δWx) . If it happens to be quasi-static, then it may also turn out to be practically -PdV. I didn't check what they thought about that, and I think they are not obliged to say. Work is defined by the forces and distances moved by the idealized working object in the surroundings. What it does to the system is up to the system to work out for itself. I would be cautious about analyzing into components such as (-PdV-δWx). What if there is also matter transfer through another wall? That would affect things in a way not uniquely analyzable by (-PdV-δWx) as work. I haven't checked the Google sources in detail, I leave that to you if you want to quote them. I am guessing they don't say. I don't know if they have thought about it. The orthodox sources that I keep hardly use the term 'mechanically isolated'.


 * The principle remains that work is defined by the idealized force and distance of the surroundings work reservoir. As far as the system is concerned it is an added quantity chosen depending on the independent state variables. When it's all done, the system analyzes it according to the constraints and the independent state variables. The state variables analyze the added quantity, they don't define it. The definition is done by the surroundings. The several walls pass their respective contributions as defining quantities. The system analyzes what it gets in total.Chjoaygame (talk) 12:22, 5 March 2016 (UTC)


 * I have tried to access Guha but failed. It is quoted above as follows
 * Regarding "mechanical isolation", I don't believe the quote is misread: the quote from "Basic Thermodynamics" by Evelyn Guha (available on GoogleBooks) is:


 * "Isolated system: The system is thermally isolated &delta;Q=0, and mechanically isolated, dV=0..."
 * I would be grateful for a clarification about this. The quote above quote ends "..." It is not clear from this whether Guha has considered the possibility of isochoric work. I have found an internet file that seems to be by Guha that seems to indicate that he is denying the possibility of isochoric work. The file reads "No work is performed during this process since ΔV = 0". I would say this simply makes him an unreliable source. Perhaps I am getting it wrong here? Further detail of the Guha source would settle the question, I suppose.Chjoaygame (talk) 08:59, 6 March 2016 (UTC)


 * Try this link: https://books.google.com/books/about/Basic_Thermodynamics.html?id=oEfVJJC4mpsC PAR (talk) 16:17, 6 March 2016 (UTC)


 * Yes, but they want me to buy it. I'm not at all keen to do that.Chjoaygame (talk) 16:30, 6 March 2016 (UTC)


 * I'm sorry, I knew it was searchable and saw the search button, but it doesn't seem to work. Try this link: https://books.google.com/books?id=oEfVJJC4mpsC&pg=PA150&lpg=PA150&dq=Evelyn+guha+thermodynamics+%22mechanically+isolated%22&source=bl&ots=FIGM5zTy5_&sig=ks5miOVjMUzMBAaQMGzs3_MWHlc&hl=en&sa=X&ved=0ahUKEwirzeiL1qzLAhXCVz4KHSQvDAEQ6AEIIjAB#v=onepage&q=Evelyn%20guha%20thermodynamics%20%22mechanically%20isolated%22&f=false


 * If that doesn't work, google this: Evelyn Guha Thermodynamics "mechanically isolated". It shows up in google books there too. PAR (talk) 18:22, 6 March 2016 (UTC)


 * I have tried various things. I haven't succeeded in entering the text of the book. My concern is the following from the article Mechanically isolated system:

For a simple system, mechanical isolation is equivalent to a state of constant volume and any process which occurs in such a simple system is said to be isochoric.


 * I think it obvious that that is not good as it stands. It seems to say that Guha thinks that mechanical isolation is equivalent to a state of constant volume. That forgets stirring, rubbing, and bending, all of which are kinds of mechanical work at constant volume, more or less. Perhaps, if you still have access to Guha, you can say whether Guha means to say that?Chjoaygame (talk) 04:38, 7 March 2016 (UTC)

Time to delete reference to mass flow
I am amazed at the amount of discussion on what I naïvely thought was a simple question: does a mechanically isolated system permit mass flow or not? Since I asked this one week ago, the length of this talk page has expanded from 16K to 53K, and the question has become much more complicated but still with no real sources.

I have succeeded in seeing p.150 of the book by Guha (perhaps my browser is more compatible with the document than Chjoaygame's?), but the brief relevant paragraph doesn't really answer the question as there is no mention whatsoever of mass transfer. This suggests to me that the author is thinking of a closed system, but we cannot be certain. As for mechanical work at constant volume, again Guha simply does not discuss it. Rather s/he simply writes dU + PdV - TdS ≥ 0 and says that in a mechanically isolated system, dV = 0 so the second term disappears. This is not really a statement that other types of work do not exist; rather they are just not considered in the brief discussion.

I also spent 30 minutes in my university library looking through the indexes of about 25 thermodynamics books in both physics and engineering. Result: NONE include mechanically isolated system in the index.

Conclusion: we have no reliable source (Guha or other) either for the statement in this article that a mechanically isolated system permits mass exchange, or for the contrary statement in the article Mechanically isolated system. So I will now attempt to wrap up the discussion by deleting both statements as unsourced, and removing this contradiction from Wikipedia. Dirac66 (talk) 22:36, 10 March 2016 (UTC)


 * Thank you for this.


 * Wikipedia is not a dictionary, nor a guide to every vagary of ordinary language.


 * My preferred solution: (a) delete the article Mechanically isolated system, (b) delete the section in this article, as  unreferenced. I have done (b). I would like to do (a), but consensus will be needed. I have examined Wachter et al.. It is not a reliable source in my judgment. Truesdell is eccentric.Chjoaygame (talk) 01:24, 11 March 2016 (UTC)

a puzzle
A body is contained by rigid walls that are selectively permeable to water. It contains salt dissolved in water. Its surroundings consist of pure water at specified temperature and pressure. It is in thermodynamic equilibrium with them. That means that water and internal energy have passed through the walls till a steady state has been reached.

Then a thermodynamic tampering takes place. An agent increases the surrounding water pressure at unchanged temperature.

Presumably the pressure inside the body will be increased in the new equilibrium.

Was the body mechanically isolated?

What if the body was pure water, no dissolved salt?Chjoaygame (talk) 10:55, 5 March 2016 (UTC)


 * Its not a closed system, dN≠0. All of the above iso-x and x-isolated systems have implicitly assumed a closed system, I think. For example isochoric process assumes a closed system.


 * Its clearly mechanically isolated, but its not isolated from the effects of outside pressure, due to the fact that its not closed. PAR (talk) 12:48, 5 March 2016 (UTC)


 * I agree. It was mechanically isolated. Only one wall. Matter was transferred. No defined work.


 * The article assumes a closed system but I think that is not necessary. The volume doesn't change, even if the system is open and matter is transferred. I agree that the original meaning of isochore assumes closed and a curve, but I don't think that matters. For the open system, the isochore is a surface.Chjoaygame (talk) 13:44, 5 March 2016 (UTC)

a thought
We are interested in processes. A process is defined by changes in walls. Changes in system walls + changes in non-immediate surroundings walls that generate changes in intensive variables in the surroundings that contact particular system walls. These define the relevant state variables for the system. Legendre transform to work in terms of those relevant state variables. Also need to decide whether to use an energy potential, or a Massieu-Planck (entropy-based) function.Chjoaygame (talk) 19:58, 5 March 2016 (UTC)


 * I found a half-finished table in my notes, so I finished it, and made a WikiTable out of it. Please note it is a work in progress, so I please feel free to object to any entry. Its on my user page at the bottom. PAR (talk) 06:58, 6 March 2016 (UTC)


 * I will look at this. I am much worried by the &delta;Wx and &delta;Qx symbols. Chjoaygame (talk) 16:38, 6 March 2016 (UTC)


 * The &delta;Wx is the  irreversible work done on the system (paddles, electrical resistors, etc.) and &delta;Qh is energy added by heating. TdS=&delta;Qh+&delta;Wx. Total work done on the system is &delta;W=&delta;Wx-PdV so dU=&delta;Qh + &delta;W. PAR (talk) 01:22, 7 March 2016 (UTC)


 * I had an idea that's what is intended by those symbols. At first glance that looks naughty. The work is supposed to be defined by force and distance outside the system. That's a big point of principle when one does away with the definition of quantity of heat by calorimetry. What the system does with the work done on it is the system's business, not predictable from the external definition. It depends on the constitution of the system. I don't see much about in the standard texts, except Haase. At first glance it defeats the aim of dealing with the first law without needing second-law analysis. I thought that it is well agreed that &delta;Q is energy added as heat. It is going back to Clausius to call anything else 'heat', and I think that's a very bad idea, not to say not done by standard texts. Guggenheim says of work done by a current in a resistor inside the system "We will now say that the work was converted into energy; to speak of its conversion into heat would be nonsense." Also I am unhappy about starting with infinitesimal increments, as if one intends to work only with processes that have a good continuity, excluding many reasonable processes such as explosions. I think the basic concepts should be initially developed for finite increments that cover a whole macroscopic process defined only by initial and final states. Infinitesimal increments should follow that, as special cases.


 * You write above the formula TdS=&delta;Qh+&delta;Wx. I see in that a quasi-static addition of heat and a finite-speed (non-quasi-static) addition of energy as work. Is that all in one physical process?


 * The first law, for me, for closed systems, is $&Delta;U = Q − W$ (Bailyn's sign convention). For open systems it is that plus $&Delta;U_{1} + &Delta;U_{2} = 0$. Haase, when it comes down to the wire, writes $Q = &Delta;E − W$ as a definition of heat, where $&Delta;E = &Delta;E_{kin} + &Delta;E_{pot} + &Delta;U = Q + W$ (his sign convention).


 * Haase likes to be one step more complicated than standard texts. On pages 19–24, he talks of $&delta;W_{l}$, quasi-static work, and $&delta;W_{diss}$, dissipative work, such that $&delta;W_{l} = &delta;W_{l} + &delta;W_{diss}$. For a system of several component subsystems he talks of $&delta;W^{*}$, the excess work, not $&delta;W_{diss}$, for a complicated reason. I think his $&delta;W_{diss}$ corresponds with your &delta;Wx ? But when it comes to stating the first law he talks initially of $W$ and $Q$. Guggenheim doesn't do that splitting of work thing. Münster doesn't do it. Bailyn doesn't do it. Callen talks of quasi-static work $đW_{M} = − P dV$, and later, after the second law, of a term he indicates he is inventing because it "does not have a familiar distinctive name", quasi-static chemical work, $đ W_{c}$ ≡ $$\sum_{j=1}^{j=r} \mu_j dN_j$$.


 * I am very keen to keep such things out of the mainstream of our articles, and to keep them in distinct specialized sections. It they find their way into the mainstream, I see that as opening the floodgates and inviting endless confusion, conflict, difficulty in finding sources that agree with one another, and misery for me. I don't know if I could cope with it.


 * I think one should separate two topics: (1) the differential geometry of the various equations of state, where one is interested in idealized fictive reversible or quasi-static "processes", and (2) the physics of general pseudo-real thermodynamic processes such as are conceived of for example by Lieb and Yngvason, who explicitly challenge Truesdell's view that the natural language for thermodynamics is the infinitesimal calculus.Chjoaygame (talk) 04:22, 7 March 2016 (UTC)

Well, I sat down trying to figure out what I disagree with in what you wrote above and came to the conclusion that I basically agree with everything you have said. It's true that the table on my talk page assumes incremental processes which account for a great majority of problems encountered. Most of the time, assuming a continuous path between two states is not a problem, since state variables do not depend on the path. I also wonder about the statement "What the system does with the work done on it is the system's business, not predictable from the external definition. " I cannot think of a case where non-PdV work is reversible. PAR (talk) 15:54, 9 March 2016 (UTC)


 * I suppose that if there are any perfectly insulating dielectrics, their polarization, apart from electrostriction, would be isochoric and reversible? I think piezoelectricity is said to be reversible? Likewise for losslessly magnetizable materials?


 * Thinking it over, I get the idea that processes that are irreversible and non-quasi-static, but still describable by infinitesimal increments, assuming the approximation, often enough very good, of local thermodynamic equilibrium, have "time rates of entropy production", and belong to non-equilibrium (pseudo-)thermodynamics. I now believe that the only genuine non-equilibrium thermodynamics is, or will be, built on many-time 'entropies', a new concept that I read of in the work of Phil Attard, still I think a work in progress, because he could not tell me how to measure them by macroscopic observations. Until I read that concept, I did not have a clear idea of what genuine non-equilibrium thermodynamics might mean. They are, I think, entropies in the sense that they have suitable definite statistical mechanical definitions.Chjoaygame (talk) 19:12, 10 March 2016 (UTC)


 * So what is the bottom line regarding "mechanical isolation"? I agree, the table on my talk page assumes infinitesimal increments, which may not always be appropriate, but assuming they are, we need words to specify when an increment is constrained to be zero. The fundamental differential statement is generally
 * $$dU=TdS-PdV+\sum \mu_i dN_i + \sum F_i dx_i$$
 * where the Fi are generalized forces (e.g. torque &tau; in the case of paddles, voltage drop for a resistor) and dxi is generalized displacement (e.g. angular displacement d&theta; for paddles, charge for a resistor). It seems to me we should have a term for a system "isolated" from changes in volume (dV), quantity (dN) at least, and lumping reversible work (PdV) along with, say, irreversible paddle work (&tau; d&theta;) is counterproductive. PAR (talk) 09:50, 12 March 2016 (UTC)
 * The question is not whether we need words or we should have a term, but rather whether this term is used often enough in the existing scientific literature to warrant inclusion in an encyclopedia. Dirac66 (talk) 11:49, 12 March 2016 (UTC)


 * Editor PAR may well be right that "we need words to specify when an increment is constrained to be zero." But this is Wikipedia. We are constrained by policy not to invent them if they are not already in reliable sources. If they are not there, we have to manage without them. There is a term, 'adynamic', invented by Rankine, and reported by Partington (who I think is mostly a reliable source), that seems hardly ever to be used, that means 'work is prohibited'. I think it is a fair term, but still few seem to find it useful, as far as I can see. To specify that a process occurs at constant volume, I think many reliable sources say 'the process occurs at constant volume'. The term 'closed system' very often means that transfer of matter is prohibited. The danger of original research is clear and present. Already above (Witness this) I have cited a website that has copied the article Mechanically isolated system with its errors. You may well be right that "lumping reversible work (PdV) along with, say, irreversible paddle work (&tau; d&theta;) is counterproductive", but if reliable sources do it, that's the way the cookie crumbles. Reliable sourcing is above all in the policies of Wikipedia. I think our best approach is to survey sources to seek out those that are reliable, and report them.


 * Editor PAR asks for the bottom line on 'mechanically isolated system'. I think it is not an established term of art in reliable sources, and consequently we should not define or report it as if it were such. The phrase doesn't have an established customary meaning, in particular as to the question of co-occurring matter transfer. It belongs to the ordinary language; that doesn't earn it a dedicated article. I think the article should be deleted.Chjoaygame (talk) 12:08, 12 March 2016 (UTC)


 * I certainly didn't mean to advocate making up terms. I mean to advocate that the terms that we do use are well defined, conceptually and mathematically. I don't mean to suggest we use unreferenced statements, but if we have a bunch of good, but possibly conflicting references and a bunch of good editors who have a decent understanding of the subject, I see it as our duty to try to form a coherent picture rather than present the reader with an incoherent mess of conflicting, well-referenced statements.


 * If the consensus is that the use of "mechanically isolated" as used by Guha is too vague and/or introduces too many complications, then I'm fine with that, but if it brings order and coherence to our thinking, then it should not be rejected because it cannot be found or clearly explained in other references. However, I object to Guha being rejected by anyone who has not read the relevant passages, or understands it only from second hand. The relevant pages are freely available at https://books.google.com/books?id=oEfVJJC4mpsC&pg=PA150&lpg=PA150&dq=Evelyn+guha+thermodynamics+%22mechanically+isolated%22&source=bl&ots=FIGM5zTy5_&sig=ks5miOVjMUzMBAaQMGzs3_MWHlc&hl=en&sa=X&ved=0ahUKEwirzeiL1qzLAhXCVz4KHSQvDAEQ6AEIIjAB#v=onepage&q=Evelyn%20guha%20thermodynamics%20%22mechanically%20isolated%22&f=false or by googling ( Evelyn Guha Thermodynamics "mechanically isolated" ) pages 149-153 and if for some reason you are unable to get this, I can email screenshots to anyone interested. Until one has read the relevant material, one cannot justify rejecting it.


 * Upon reading it, it is clear that "mechanically isolated" also includes material isolation (dN=0), so yes, the present statement in the "mechanically isolated" page is not correct. PAR (talk) 04:23, 13 March 2016 (UTC)


 * I feel rather foolish that I seem unable to read the Guha book from the internet. I have tried your link. I led me to a page for the book but for whatever reason it seemed not to offer me the possibility of reading inside it. Perhaps as suggested by Editor Dirac66 this is due to my browser. For this reason I have now installed Mozilla Firefox because my nephew thinks I ought to have done so long ago. Sad to say it still didn't seem to want me to read it. I guess I am stupid. Perhaps my internet security is causing the problem? There is on the page an icon that lets me read another book, by Yi-chen Cheng, in plenty of detail with no problem.


 * The problem is not that there is anything wrong with using a concept of mechanical isolation for some purpose or other, defining it as you will. The problem is posting things that make it seem that a 'mechanically isolated system' is a standard term of art, when so far, apart from Wachter & Hoeber, which I have objections to, we seem to have only one source for it, Guha. I think giving it an article of its own is posting something that makes it seem that it is a standard term of art. I think you can email me from Wikipedia, if you have time. But I have to say in advance that it being a nearly isolated source, for me it won't make much difference what it says, because one swallow doesn't make a summer.Chjoaygame (talk) 10:05, 13 March 2016 (UTC)
 * Re computer problems: You have now tried another browser which didn't work, so we can reject the hypothesis of browser incompatibility. Perhaps internet security as you say, or perhaps your PDF reader is incompatible with the document. Of course checking all hypotheses could consume much time and money, and is not worth it just to read one excerpt of one book. Probably the simplest solution is to log in on someone else's computer and follow the same links. Dirac66 (talk) 14:52, 13 March 2016 (UTC)


 * Thank you for this. Perhaps it's finger trouble or brain trouble. I have tried on two of my own computers. I use Adobe Acrobat and another reader. I'm not sure if the files are pdf. I just seem to have nothing to even try to read. I can see the book, but not get into it. Editor PAR intends to send me some material. Perhaps I should try an Internet café.Chjoaygame (talk) 15:52, 13 March 2016 (UTC)


 * I have just come from an internet café. It was apparently no different from my home computer. Perhaps it is to do with copyright in different nations? More likely something so obvious and in my face that I can't see it.Chjoaygame (talk) 08:28, 14 March 2016 (UTC)


 * Thanks to a friend I have now read some relevant parts of Guha.


 * I would say that the text does not clearly enough consider the possibility of shaft work, and therefore is not a good source for a definition of mechanical isolation. This is because I think mechanical isolation should explicitly exclude shaft work. Joule's experiment used shaft work.Chjoaygame (talk) 09:08, 14 March 2016 (UTC)

always enclosed by walls ?
Citation from the introduction: "The thermodynamic system is always enclosed by walls that separate it from its surroundings; these constrain the system" - question, is that really true? How about a drop of water in a cloud, which can be a model of a thermodynamic system? ArchibaldWagner (talk) 21:56, 23 February 2018 (UTC)

reason for edit
In this edit I have removed a one-sentence paragraph from the lead. The removed item was reading


 * Furthermore, the state of a thermodynamic system is described by thermodynamic state variables, which may be intensive, such as temperature, or pressure, or extensive, such as entropy, or internal energy.

The true situation is more subtle than is expressed in that sentence. I will shortly replace that sentence with a better one.

The problem with the sentence is that it fails to recognise the deep difference between the directly and concretely measureable state variables such as pressure, volume, and mass, and indirectly measurable variables such as entropy and internal energy. In further edits, I will clarify this point.Chjoaygame (talk) 13:36, 5 March 2021 (UTC)

Extensive property
The property of the system whose value depends upon the amount of substance present in the system is called extensive property for example:- Mass, volume, surface area, enthalpy, entropy, free energy etc... 2405:201:5001:7C6E:C525:58AE:B129:2570 (talk) 14:36, 30 January 2022 (UTC)

Internal variables
It seems to me that the author himself has no clue what these internal variables are. Weaky3 (talk) 23:51, 25 March 2023 (UTC)