Talk:Isentropic process

Could be more understandable

I would add to the article the following explanation to make everything more easy for understanding.

Entropy balance for closed system: dS = dQ/T + dσ, where

 dS is entropy change for the system, 
 dQ/T is entropy generation due to heat transfer to/from the system
 dσ is entropy generation due to internal irreversibilities in the system

=>

S2-S1 = Q/T + σ By definition isentropic process is S2-S1 = 0. Therefore, the next statements easily follow:

1) Isentropic adiabatic process must be reversible:

  0 =     0     +  σ => σ=0 => reversible process
  (S1=S2) (Q=0)

2) Irreversible isentropic process must be NOT adiabatic:

  0       =  Q/T   +   σ      => σ = -Q/T 
  (S1=S2)    (!=0)     (!=0)     ( heat must be removed from the system, minus sign )

IMHO, it's always better to see formulas instead of verbal explanations..

-

agree IlyaV 18:42, 28 October 2007 (UTC)

Removed statement about ideal gas

I removed this notion from the article because I didn't find that it was verifiable:

for an ideal gas γ is a function of temperature

- Ac44ck (talk) 23:40, 7 November 2008 (UTC)

For a monoatomic gas γ is 5/3 independent of temperature (neglecting ionization of the atom). For diatomic (or even larger) molecules γ does depend on temperature because more degrees of freedom (rotation, vibration) become available at high temperatures. See Heat capacity ratio. --Tukss (talk) 13:07, 17 June 2009 (UTC)

Use of variables in derivation

Is there any reason why a lower case p is used instead of an upper case P in the derivation of the relations? Upper case P is the usual variable for pressure and lower case p is just confusing. —Preceding unsigned comment added by Geoffhotchkiss (talk • contribs) 18:32, 9 July 2010 (UTC)

http://www.docstoc.com/docs/6233924/Pressure

"The upper case P is normally reserved for power."

- Ac44ck (talk) 02:04, 11 April 2011 (UTC)

Why Is Compressor Efficiency "Upside Down"?

Is this a mistake, or am I missing something? If it's not a mistake maybe it could be explained with a note. — Preceding unsigned comment added by 190.21.107.42 (talk) 23:47, 13 August 2014‎

Not a mistake. This is because a compressor applies work to the flow to impart a desired pressure ratio (or mass flow or velocity rise). Due to entropy, you need more work than you would need for an isentropic pressure rise. This means the isentropic compressor work is less than the actual compressor work. The isentropic efficiency of any machine that adds energy to the flow will be of the form (isentropic work)/(actual work).

In the case of the turbine and nozzle, these devices are concerned with extracting energy from the fluid. Due to entropy, you will extract less energy than you would from an isentropic process. This is why the compressor seems upside-down to the turbine and nozzle; the compressor performs the opposite process.

I don't whether or not this merits a note in the article, but a simple way to indicate the difference might be to group the equations as "Devices that impart energy to the flow" and "Devices that extract energy from the flow". Hopefully someone can think of a less wordy way to say the same thing. 209.87.255.222 (talk) 16:21, 25 November 2016 (UTC)

Simplified the long sentence in the lede

The previous lede sentence was heavily qualified and very long, warning the reader that "isentropic" (like most everything in engineering) is often only an approximation or an assumption. I figure people know this anyway, or can obtain this knowledge from another source. 178.38.79.250 (talk) 20:05, 19 January 2015 (UTC)

This article needs work

Parts of the article seem to be lists of equations copied from a handbook without much explanation. And the equations have a lot of redundancy.

I would like to see more conceptual material. In particular, I would like to see concretely some example that is adiabatic but not isentropic (i.e., entropy is generated internally and dS > δQ/T =0). Friction is mentioned; perhaps also viscosity in a fluid? But I want to see how the model is set up, the concrete math of where and how the entropy gets generated, how it is accounted for. It shouldn't just remain a black box (at least not in every situation).

178.38.79.250 (talk) 20:51, 19 January 2015 (UTC)

Another example of internal energy generation is sudden expansion of gas into a vacuum. If it takes place in an insulated container, it is adiabatic but not isentropic or reversible. 178.38.77.196 (talk) 20:12, 13 March 2015 (UTC)

Definition seems convoluted

In thermodynamics, an isentropic process is a fictive idealized thermodynamic process that is adiabatic and in which the work transfers of the system are frictionless; there is no transfer of heat or of matter and the process is putatively reversible. Consequently, the entropy of the system remains constant throughout.

Why isn't an isentropic process simply one where the entropy remains constant throughout?

The long list of conditions seems like it could be a re-charactization of isentropic in terms of concretely checkable properties, but not actually the real, conceptual definition. Also, it seems like the conditions might be sufficient but not necessary.

Also, the qualification putatively reversible bothers me. Is it really a part of the definition of adiabatic? "Putatively" seems too subjective to be part of a physics definition.

178.38.112.246 (talk) 03:15, 12 March 2015 (UTC)

I agree with your concerns. This change was made in the past day by User:Chjoaygame. In any article like this one, we can explain the meaning of the term, or we can explain all the background material that is known about the term. The former must come first in the article, and the latter can fill the remainder of the article. I think Chjoaygame has placed the background material first, and an explanation of the meaning of the term second. I will reverse the order of things. Dolphin (t) 06:11, 12 March 2015 (UTC)

I have made the change. See my diff.Dolphin (t) 06:20, 12 March 2015 (UTC)

"Why isn't an isentropic process simply one where the entropy remains constant?" Because this is the Wikipedia which is based on reliable sources, and reliable sources don't say that. Of course if they did as one wished, they would do so, but they don't.

The definition that has been removed was carefully constructed from sources. The new version is not supported by sources, however logical it might seem to someone who did not consult sources. The new version seems right because it seems logical, but is not what the reliable sources say. I note that there is no claim made here that sources were consulted for the new version. For example, no thermodynamic process is reversible. That is the second law. There is putative or fictive or imagined reversibility, but such is not actually physical.

There are other conceptually possible readings of 'isentropic'. For example, an isentropic process could logically be one in which the entropy increases for a while then decreases, leaving the final value the same as the initial. But sources don't say that.

People come to Wikipedia to find out what is reliably supported, not what is guessed as an apparently logical reading of a word, a guess they could easily have made for themselves.

I will be impressed if you find a concurrence of reliable sources for the "improvement". I can imagine some unreliable sources that would support it. But Wikipedia requires reliable sources.Chjoaygame (talk) 08:43, 12 March 2015 (UTC)

In Fundamentals of Classical Thermodynamics (SI version 2e), Gordon J. van Wylen and Richard E. Sonntag, John Wiley and Sons 1976, section 7.4 says:

From the definition of entropy, it is evident that the entropy remains constant in a reversible adiabatic process. A constant-entropy process is called an isentropic process.

Also, don't forget WP:Make technical articles understandable. Dolphin (t) 13:06, 12 March 2015 (UTC)

Thank you for your helpful reminder to make technical articles understandable. It doesn't mean that one should use unreliable sources or distort reliable ones.

When removing the former obvious guess-work "etymology", I noted the van Wylen and Sontag reference that was there before. It struck me as inadequate, and so I checked more reliable sources. As noted just above, no actually physical thermodynamic process is reversible. That is the second law. The new and "improved" first sentence gives no hint of that, and so is misleading. That's why I looked in reliable sources. They are mostly more particular than van Wylen and Sonntag, 1965 as given in the references section of the article, or 1976 as cited just above by you here on the talk page. I checked a number of them, and from them I wrote the version that has been removed. In particular I refer you to Lieb and Yngvason who have written in some detail about such matters. They say that a thermodynamic process can involve a wide range of intermediate stages, and that it is assessed only by the initial and final states of thermodynamic equilibrium, without regard to the intermediate stages. Indeed it is generally the case that the entropy is not defined during the intermediate stages. So van Wylen and Sonntag are definitely talking about a special imaginary scenario. The sources that I found are reported in the version I wrote, as cited in the article. In particular, that includes the 2009 edition of Borgnakke and Sonntag, p. 288. There they are talking about an ideally reversible Carnot engine, not a really physical thermodynamic process. Generally, statements should be as simple as possible, but not simpler. The unqualified "process" of the present first sentence is an over-simplification. It is a kind of would-be etymological gloss on the word, that might suit a dictionary that, without a physical check, was guessing the meaning, and it lacks physical content. Wikipedia is not a dictionary. I think it should be in the first defining sentence that such a fictive "process" is fictive.Chjoaygame (talk) 16:33, 12 March 2015 (UTC)Chjoaygame (talk) 18:39, 12 March 2015 (UTC)

More than my just previous comment is needed. The sources do not license the use of the word unrestrictedly. The new initial definition more or less deliberately does so, and thereby misrepresents the sources. The sources carefully restrict the context of the word and Wikipedia should show that clearly and upfront, not as an afterthought as in the new version. Wikipedia intends to make the reliable sources available, not to improve on their substance. The new initial sentence is altogether an oversimplification.Chjoaygame (talk) 18:45, 12 March 2015 (UTC)

You have written The definition that has been removed ... . You have also written I checked a number of them, and from them I wrote the version that has been removed. Please identify the text you wrote that has now been removed.

It is well established that, on any subject, the totality of reliable published sources is not entirely uniform and consistent. Different sources explain things in different ways, and different authors display different points of view. Some sources present a simple and basic view of a subject whereas other sources present a more comprehensive and complex view. Wikipedia doesn’t aim to elevate one reliable source and bury the rest. Providing we don’t give undue weight to a source or an author, Wikipedia aims to present the full spectrum of information on a subject, even though that is more difficult than presenting a sanitised, single-focus view of the subject.

Fundamentals of Classical Thermodynamics is a reliable published source. No-one should be surprised to see that it presents things in a different way, and at a different level, to some other reliable published sources. Providing undue weight isn’t given to van Wylen and Sonntag’s statements, those statements are entirely acceptable on Wikipedia.

You have written that the initial sentence is an oversimplification. The test of whether it has a place on Wikipedia depends entirely on whether it can be traced to a reliable published source. I believe it can because it is what van Wylen and Sonntag have published. Dolphin (t) 12:34, 13 March 2015 (UTC)

The test is not merely whether it can be traced to a reliable published source, as you beguilingly propose. More is needed. The Wikipedia entry should accurately represent a fair sample of reliable sources.

The first sentence of new edit fails this proper test, because it quotes the source you choose out of the context in which the source makes the statement. The context of your source is not a general account of "isentropic processes". It is, as I noted above, an account of a particular theoretical device, namely a reversible Carnot engine. The first sentence of the new edit is misleading because it seems to be a general definition and fails to indicate this source's special context.

It appears that the new first sentence was an inadequately surveyed guess, made without checking the sources [1,2,3,4] that were already provided in the article. In particular, reference 3 is a recent edition of the text that used to be van Wylen and Sonntag, updated to Borgnakke and Sonntag 2009. The reference cited just above is the 1976 edition of the text. The situation hasn't changed, but one would expect also a note on a more recent edition. Checking the four sources that are cited, and surveying for others, would have revealed that the term "isentropic" is not universally used, and that when it is used in reliable sources, it is mostly in a restricted context. The new edit's first sentence fails to reflect the reliable sources in this important respect.Chjoaygame (talk) 13:15, 13 March 2015 (UTC)

(1) Here is another source: Kittel and Kroemer, Thermal Physics, 1980, p. 173: A process without a change of entropy is called an isentropic process or an adiabatic process. The term "adiabatic" has the specific meaning that there is no heat transfer in the process. For simplicity, we stick with "isentropic".

This seems misleading or false for adiabatic. Don't we have adiabatic ==> (isentropic <==> reversible <==> no internal entropy production) ? So suggesting that adiabatic is more specific than isentropic is belied by the many adiabatic systems with internal entropy production. But I like the take on isentropic.

(2) I think the constant emphasis on "no real processes are reversible" is slightly overdone. Our vocabulary is as much about our models as it is about the physical, real world. There are plenty of reversible processes in the world of models -- the (ideal) Carnot engine is just the start. And we need sharply defined vocabulary as much for the models as we do for the real world. So I would say that certain processes are isentropic, reversible, and so forth, without constantly qualifying the adjective. The logical relations between adiabatic, isentropic, and reversible hold exactly. It's just that certain processes are realized only approximately in the real world. An analogy would be saying that Newton's space-time is putatively flat.

NB The question of whether a process is realistic or idealized is handled pretty crisply in adiabatic process, where there is even a whole paragraph devoted to discussing the role of approximations and idealizations.

178.38.77.196 (talk) 20:05, 13 March 2015 (UTC)

As usual, Kittle & Kroemer have it each way. Not a good source. They are believers in a pedagogical approach that they call "thermal physics", a cross between thermodynamics and kinetic theory and statistical mechanics. Confusing for Wikipedia.

It is not a matter of what we like to think or say. It is about what the most reliable sources say.Chjoaygame (talk) 00:45, 14 March 2015 (UTC)

The difficulty associated with trying to base Wikipedia on the most reliable published sources, as opposed to all published sources that qualify as reliable, is that determining which are the most reliable is entirely subjective. The sources that one User regards as the most reliable will be different to the sources chosen by another User. I'm not aware of any WP guideline that gives credence to the idea that Wikipedia aims to elevate the most reliable sources and bury the rest. WP:NPOV begins by saying "All encyclopedic content on Wikipedia must be written from a neutral point of view (NPOV), which means representing fairly, proportionately, and, as far as possible, without bias, all of the significant views that have been published by reliable sources on a topic." Note that WP:NPOV says "all of the significant views", even though all of them won't be mutually consistent.

What do you think of the following alternative wording of the first sentence?

If the entropy of a system at the end of a thermodynamic process is equal to the entropy at the start of the process, the process is called an isentropic process.

It avoids asserting or implying that any constant-entropy processes actually exist. It does so by saying "If the entropy of a system ... is equal ... the process is called an isentropic process." It more closely matches the statement by van Wylen and Sonntag by actually using the same last six words.Dolphin (t) 06:28, 14 March 2015 (UTC)

• I think you go too far in your statement that "The difficulty associated with trying to base Wikipedia on the most reliable published sources, as opposed to all published sources that qualify as reliable, is that determining which are the most reliable is entirely subjective." It seems to work from an assumption that a source is either reliable or not, ignoring degrees of reliability. And it goes too far in saying "entirely" subjective, for obvious reasons. And it assumes that subjectivity per se is objectionable, which I think also goes too far. And it doesn't fit too well with sticking to one 1976 edition of one source.Chjoaygame (talk) 18:04, 14 March 2015 (UTC)

As for your proposal. What matters is not what I think. It's what a fair survey of reliable sources yields.Chjoaygame (talk) 22:23, 14 March 2015 (UTC)

And what does a fair survey of reliable sources yield? Dolphin (t) 06:40, 15 March 2015 (UTC)

I posted my result. Now it's your turn.Chjoaygame (talk) 08:20, 15 March 2015 (UTC)

• I think the requirement that the entropy remain constant in the system at each time is much better -- it's consistent with adiabatic, where there is no heat flux across the boundary (at any time), isothermal, where the temperature remains constant at all times, and isochoric, same thing.

(Of course, if the system is thermally isolated, the only way to have the entropy the same at the final time and the initial time is that it be constant the whole time.)

For me Kittel and Kroemer is a great source. Clearer and more logical than Wikipedia articles on thermodynamics, though less broad. But the quote I gave illustrates a pervasive problem in thermodynamics exposition -- it waffles back and forth about the meanings of terms. "Isentropic" is defined cleanly in Kittel and Kroemer, but "adiabatic" isn't. He brings it into semantic contact with "isentropic", then demurs. Whereas "adiabatic" is defined cleanly in Wikipedia as "no thermal contact with outside world". Hence no heat exchange or entropy transport across the boundary. I can work with that.

"Isentropic", on the other hand, appears everywhere, in all kinds of sources, as a description of lines or surfaces of constant entropy on phase diagrams. It is understood than an isentropic process is one whose trajectory on the phase diagram stays on one of these lines. That is, its entropy remains constant. If the system is thermally insulated, this can only happen if there is no internal entropy production. The details of why this is true don't matter to the definition. If you start listing the reasons -- no viscous stirring, no friction, no expansion into a vacuum, no energy transport across a temperature difference -- first of all, you are giving an explanation rather than a definition, and second, to make it equivalent to the definition, you should end by saying "or other source of internal entropy production".

Even here you won't get the full definition, because of the background assumption that the system is thermally insulated. There is an example in this very article: For an irreversible process, the entropy will increase. Hence removal of heat from the system (cooling) is necessary to maintain a constant entropy for an irreversible process in order to make it isentropic. Thus an irreversible isentropic process is not adiabatic. Here, isentropic is being used as a synonym for maintaining the same entropy at all times even though the entropy under consideration is that of a system that's not thermally isolated -- entropy can flow across its boundary, and this capability is exploited in the example to keep the system entropy constant.

Incidentally, this process is not adiabatic, yet adiabatic is mentioned in the lede in a way that makes it seem part of the definition, instead of, for example, the most common and important way to get an isentropic process.

I do agree that statistical physics and thermodynamics are distinct subjects. Another thing I find particularly confusing is when quantum considerations are used on an ad-hoc basis to justify derivations in an otherwise classical statistical physics situation. Physicists rely on the real world to have more coherence and integrity than their own mathematical models.

89.217.4.12 (talk) 11:17, 14 March 2015 (UTC)

@89.217.4.12: I like what you have written. I particularly agree with your comment that you are giving an explanation rather than a definition. The whole article is available for explanation but the first sentence or the first paragraph should be confined to definition. The lede should be meaningful to young people and others new to the science.

We see three excellent contributions to this thread from IP addressees: 178.38.79.250, 178.38.77.196 and 89.217.4.12 - was that you each time? It makes it difficult trying to communicate with someone who appears as a mercurial IP address. Please give consideration to registering as a user so you can contribute under your own username, as Chjoaygame and I do. It costs nothing and actually confers more privacy than using an IP address. Dolphin (t) 11:44, 14 March 2015 (UTC)

They are all me. The address changes frequently by itself. I even noticed earlier comments of mine and responded to them before slowly remembering who wrote that ! I'll give some thought to the username idea. 89.217.4.12 (talk) 14:12, 14 March 2015 (UTC)

Minimal pairs

In linguistics the idea of minimal pairs exists, which can inter alia help to see a contrast between two letters/sounds without any other confusion. For example, "tea" and "pea" could illustrate the distinction between "t" and "p" (better than contrasting "table" and "pomelo"). If the distinction being noted between "adiabatic", "reversible adiabatic" and "constant-entropy" processes is significant, then could some directly contrasting examples of such (perhaps with a graph of overload curves) please be presented to illustrate such differences clearly? Otherwise the distinction may seem too obscure. —DIV (120.17.46.63 (talk) 01:48, 9 March 2018 (UTC))

edit with slighting summary

This edit by Editor Bubaff is covered by the following edit summary: "Conservative opinions based in a classical, limited, and conservative understanding of physical systems only garnish condescension. The notion of an ideal machine was not a theorist reaching for a phantom limb. To use words like fictial, or idyllic (or ...)".

The words used were "fictive idealized". According to the second law, reversible processes do not occur in nature. The term 'isentropic' is not widely used in good quality reliable sources, and when it is used in them, it is usually defined rather narrowly. It originated in the writing of Gibbs.Chjoaygame (talk) 13:44, 3 September 2015 (UTC)

If any of the cited sources use the phrase "fictive idealized", it would be a good idea to make that clear in the Notes. --50.53.59.40 (talk) 21:15, 6 September 2015 (UTC)

Isentropic process

In thermodynamics, an isentropic process is an idealized thermodynamic process that is both adiabatic and reversible.[1][2][3][4][5][6] The work transfers of the system are frictionless, and there is no net transfer of heat or matter. Such an idealized process is useful in engineering as a model of and basis of comparison for real processes.[7] This process is idealized because reversible processes do not occur in reality; thinking of a process as both adiabatic and reversible would show that the initial and final entropies are the same, thus, the reason it is called isentropic (entropy does not change). Thermodynamic processes are named based on the effect they would have on the system (ex. isovolumetric: constant volume, isenthalpic: constant enthalpy). Even though in reality it is not necessarily possible to carry out an isentropic process, some may be approximated as such. The word "isentropic" can be interpreted in another way, since its meaning is deducible from its etymology. It means a process in which the entropy of the system remains unchanged; as mentioned, this could occur if the process is both adiabatic and reversible. However, this could also occur in a system where the work done on the system includes friction internal to the system, and heat is withdrawn from the system in just the right amount to compensate for the internal friction, so as to leave the entropy unchanged.[8] However, in relation to the universe, the entropy of the universe would increase as a result, in agreement with the Second Law of Thermodynamics.

Male voice 2400:1A00:B111:16A9:48CC:4BA8:9E6D:95B3 (talk) 14:52, 13 September 2022 (UTC)

Definition of isentropic fundamentally wrong

"In thermodynamics, an isentropic process is an idealized thermodynamic process that is both adiabatic and reversible."

This is only true in the converse: "In thermodynamics, an isentropic process is an idealized thermodynamic process where entropy remains constant. An adiabatic and reversible process is by definition isentropic (see Second Law of Thermodynamics)."

A square is a rectangle, but not all rectangles are squares. 208.91.55.70 (talk) 14:54, 5 October 2023 (UTC)

Thanks for taking an interest in our article. The topic you have raised has been raised before, although I suspect it didn’t ever reach a conclusion that satisfied everyone. See above at #Definition seems convoluted. Dolphin (t) 00:18, 6 October 2023 (UTC)

Thank you for your quick response. I strongly disagree with the conclusion from that discussion to keep this wording of the definition. As a thermodynamics student, speaking on behalf of others, Wikipedia returning this statement (which I will argue is false) is detrimental to our understanding of the material and our success in the class. This by itself is not grounds to change the wording, but please understand my background and context for this comment. Regardless of whether this is a fictive process, the point still stands that an isentropic process need not be adiabatic or reversible. For instance, logic says that if one has a Carnot engine, that is reversible, yet by no means adiabatic. It is reversible because it returns to its original state, but it is not adiabatic because we have two heat flows across the boundary - one from the hot source and one to the cold source. True, the individual processes may not be isentropic, but the overall behavior is such. Let us assume a different scenario where a temperature gradient is present across a gaseous medium between a hot source and a cold source. Temperature gradients beget entropy, yet within the boundaries of the gaseous medium at equilibrium, despite entropy being generated constantly, there is a constant entropy across the whole system (or, rather it can be modeled as such). In the case of an infinitesimally small distance, this is by all means true. The entropy being generated is transferred to the cold source alongside the entropy moved from the hot source. In this case, we have an isentropic process yet again, which is neither adiabatic nor reversible. Truthfully, neither process exists in the real world as conceptually phrased, but for the purposes of modeling it is important to approach scenarios with the correct assumptions and methods (all models are wrong, but some are useful). This is why I find issue with the current wording - it imposes a false assumption and a false model, which makes it useless as a modeling tool. I admit that I have no sources for this line of thought, and I don't care enough to collect some beyond seeing that all other top websites and the way that thermodynamics is taught at the university level disagree with Wikipedia. At some point, I think we have to get with the times and concede that the word may have evolved past its original usage and now unmistakably refers to something broader. 208.91.53.187 (talk) 03:14, 12 October 2023 (UTC)

" that an isentropic process need not be adiabatic or reversible" Yes, it does. I'm pretty comfortable that's the definition of an isentropic process. I can't follow your thought experiment, but it is pretty important that some principles be established. Arguing on the basis that "neither process exists in the real world" is quite beside the point, as, in the real world isentropic processes do not exist. Just as in the real world, neither do adiabatic or reversible processes exist. Isentropic is simply shorthand for "adiabatic, reversible". Gpsanimator (talk) 22:01, 3 August 2024 (UTC)