Talk:Reversible process (thermodynamics)

Disagree with definition

The definition of reversibility in terms of infinitesimal changes acting over infinite time has always seemed to me something like defining 0/0. I think some mathematicians should be brought in to examine this language. In the Schaum's on thermodynamics, they put it as an "infinitesimal rate". That is not the kind of language I ever encountered in calculus, though Dirac delta functions were pretty out there. —Preceding unsigned comment added by 74.170.68.234 (talk) 20:26, 21 May 2008 (UTC)

I agree. The essential element of reversibility is that the process takes place without loss or dissipation of energy. If that can happen in a process that occurs at a finite rate then the process is still reversible, regardless that the process is not at an infinitesimal rate. Many processes take place with inevitable losses, so for those processes it is necessary to introduce the concept of an infinitesimal rate to avoid the complication of the losses, but that does not mean that the infinitesimal rate is an essential part of the definition of reversible process.

For example, consider an object falling freely in a vacuum. On the way down, the magnitude of the acceleration is the same as it is on the way up, only the sign is reversed, mechanical energy is conserved and the process is reversible even though it is most definitely not taking place at an infinitesimal rate. I know that an object falling is not normally considered to be a thermodynamic process but the concepts of reversible process and entropy are not confined to thermodynamics so the analogy of the object falling is a valid one. Does anyone agree with me? Dolphin (t) 00:14, 20 April 2011 (UTC)

The thermodynamic definition of "reversible processes" does not require that the "forward" process take place at an infinitesimal rate, nor does it require that the process pass only through equilibrium states. (Nor does it require that the energy in"the system" be conserved.) Instead it only requires that, conceptually, we can imagine a path that goes from any later state of the the process back to any earlier state of the process such that the path passes only through equilibrium states. The equilibrium states need not be states that he process actually went through. The definition doesn't say that there is a way to "turn the process around" so it would take such a backward path without further intervention.

I agree that expressing this idea in terms of "infinitesimal changes" is not mathematically rigorous, but the same can be said of expositions of other topics in physics that are analyzed using "dy" and "dx". The general idea is that the states of the process can be represented as points in a "space" and that there is a backward path through the equilibrium states that is continuous- i.e. it has no gaps in it.

This article is about the thermodynamic definition of a reversible process. The definition of a reversible mechanical process (e.g. a block sliding down a frictionless inclined plane or an object falling without air resistance) is usually only given by default. We are told that in real life, mechanical processes are not reversible since there is friction. That implies the definition of a "reversible" mechanical process is that it is a mechanical process that takes place without friction. This does not specify any connection between the mechanical definition of "reversible process" and the thermodynamic definition of "reversible process". The current Wikipedia doesn't have an article on "Reversible process (mechanical)".

The concept of thermodynamic entropy IS confined to thermodynamic processes. So if we wish to discuss an entropy for mechanical process then we must create a technical definition for it. Shannon entropy is defined for probability distributions. If a falling object is viewed as undergoing a deterministic process, we don't have any handy probability distribution to use.

It is common to read that a system undergoing a reversible mechanical processes doesn't experience any change in entropy. That is true in the thermodynamic sense of entropy if we consider the thermodynamic entropy of the substances that make up the parts of the system (e.g. the material of the block and the material of the inclined plane when a block slides down a frictionless inclined plane) - with the tacit assumption that these materials are in thermal equilibrium with each other. That definition of entropy doesn't depend on whether the material in the block is in relative motion to the material in the inclined plane.

Tashiro~enwiki (talk) 16:41, 5 February 2017 (UTC)

Maybe a definition in terms of a limiting process would be clearer; a reversible process is the limit of some sequence of processes where $\mathrm {d} S$ approaches 0 (or something like $\delta W\rightarrow -P\mathrm {d} V$ if you define $\mathrm {d} S$ using reversibility). I got the idea from [1], but it's not worked out there. Yodo9000 (talk) 19:59, 8 November 2022 (UTC)

I agree with you, never have i found, until now, such a clear and understandable definition of reversibility. 93.87.195.63 (talk) 13:17, 7 February 2012 (UTC)

Is that what's essential? Free expansion is non-dissipative but is irreversible. Plenty of processes give off heat but are reversible.— Preceding unsigned comment added by 12.29.151.161 (talk) 08:42, 24 November 2014‎

The statement: "In a reversible cycle, the system and its surroundings will be exactly the same after each cycle." is not correct. One cycle of a Carnot engine leaves the system in the same state, but heat flow occurs from the hot to the cold reservoir so there is a change in the surroundings. This should be changed to: "In a reversible cycle, the system and its surroundings will be returned to their original states if the forward cycle is followed by the reverse cycle." AMSask (talk) 04:10, 30 August 2015 (UTC)

A reversible process is likely to reverse immediately, thus taking an indefinite amount of time to end up in a final state that is also in equilibrium, because the system would be in disequillibrium every step of the way. You can go from one equilibrium to a different equillibrium within a finite amount of time, but that process would be irreversible. Of course, the system may continue to interact with its environment, but the initial process itself wouldn't be reversible (spontaneously).204.89.11.242 (talk) 02:28, 8 November 2016 (UTC)

Totally reversible vs. internally reversible

In at least one textbook I've seen (Engineering Thermodynamics by Cengel and Boles), they use two terms to discuss reversibility: totally reversible and internally reversible. The Wikipedia article here is only the "totally reversible" half of the equation. Internally reversible is given a similar definition, but without the requirement that the reverse path must be done without the environment changing. The upshot is that an internally reversible path is basically any path that can be drawn on a P-V diagram because it's in thermal equilibrium the whole time (as opposed to a "free expansion", for example).

That may be useful to add to this article--that some people refer to what's presented in this article as being "totally reversible", and that there's sometimes a separate "internally reversible" condition. Johncolton (talk) 21:47, 19 September 2012 (UTC)

I agree. Do you mean Thermodynamics, An Engineering Approach by Çengel and Boles? Reading the book (5e), I think internally reversible is the same as quasi-static, but the book doesn't state it that clearly; "The quasi-equilibrium process is an example of an internally reversible process." Yodo9000 (talk) 18:47, 8 November 2022 (UTC)

Inconsistency: allow entropy to be a state function OR allow process to be irreversible???????

In here, it says: " Reversible processes are a means of allowing entropy to be a state function, where a state function is something that does not rely on the pathway that it took to determine its quantitative value. "

In irreversible process, it says: " However, because entropy is a state function, the change in entropy of a system is the same whether the process is reversible or irreversible. "

Entropy is strong in this one — Preceding unsigned comment added by 78.162.22.9 (talk) 06:14, 19 March 2014 (UTC)

Entropy is determined by a reversible pathway. In the irreversible case, given some initial and final states, the change in entropy would be decided (the values would be identical) by any reversible process with the same initial and final states. Any irreversible process may differ from one another of from any reversible process in terms of heat, work done, etc., but the change in entropy has been set already (by any of the reversible processes).204.89.11.242 (talk) 02:33, 8 November 2016 (UTC)

Not the same as quasistatic

The article currently states "Reversibility ... refers to performing a reaction continuously at equilibrium". No; this is what is called a quasistatic process. You can have dissipation (say, friction) and still be in continual equilibrium by going slowly; such a process is quasistatic but not reversible, since entropy is generated / heat is dissipated.— Preceding unsigned comment added by 12.29.151.161 (talk) 08:32, 24 November 2014‎

Change to system or surroundings

Following Klaus Schmidt-Rohr's edit which described the process as being done on the surrounding rather than the system, I thought I'd open up a discussion here. I already edited the page in a manner that somewhat reverts this, but still leaves a reference to the surrounding, as I think the main focus in any process is the system (by definition of what we call the system). Would be happy for input. Kwikwag (talk) 14:27, 5 May 2016 (UTC)

I agree with Klaus Schmidt-Rohr's edit. He did not actually say that the process is done on the surroundings, and in fact the process involves changes in both system and surroundings. What he did say is that the reversal of process direction is due to a small change in the surroundings rather than of the system. Actually the direction of a process can be reversed by either type of change: for example if initially the system pressure is 1.000 atm and the external pressure is 0.999 atm, then we have an expansion which can changed to a contraction either by changing the system to 0.998 atm or by changing the external pressure to 1.001 atm. However what is of interest is reversing the direction at the given state of the system, which is accomplished only by changing the external pressure. Therefore I believe the word surroundings is required after infinitesimal changes in some property of the. Dirac66 (talk) 20:47, 5 May 2016 (UTC)

It would? Why?

In the first paragraph, "Since it would take an infinite amount of time for the reversible process to finish, perfectly reversible processes are impossible."

I do not see anywhere prior to that statement the supporting claim or explanation that it would take an "infinite amount of time". Why would it? How do we know that it would? The timing of this process needs to be established before this claim -- which lacking any context appears absurd -- can be made. — Preceding unsigned comment added by 70.89.176.249 (talk) 01:29, 22 February 2020 (UTC)