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I recently proposed the tag , and wrote a usage guidance for it; however, it has been almost immediately set as a synonym of .

I am wondering, however, whether we should have a separate tag for free-energy. I feel that the tag is really too vague, and as a matter of fact is one of the most used, because it can apply basically to anything, from general relativity to quantum field theory to fluid dynamics.

Free energy, on the other hand, belongs to a quite specific domain —thermodynamics/statistical mechanics—, and it is a very important concept there (Helmholtz free energy being the natural thermodynamic potential of the widely used canonical ensemble, and Gibbs free energy being fundamental in the study of phase transitions).

I think free energy deserves a tag of its own: is just too vague sometimes.

Examples of questions were I think that the tag would apply:

...and many others.

These questions are specifically about (Gibbs or Helmholtz) free energy, not about energy in general.

I know that we should not encourage an uncontrolled proliferation of useless, too specific tags. However, being someone who is mostly active in thermodynamics and statistical mechanics posts on the site, I strongly feel that would be useful.

Of course, if we finally decide to undo the synonym, I volunteer to do the retagging of old questions and check when the tag can be applied to the new ones.

Addendum (to address the comments)

  • I think that adding or to is not enough to clarify, since in thermodynamics/statistical mechanics there is a thermodynamic potential called "(internal) energy" which is different from the free energy.

  • It's true that the tag may be misused by people asking questions about perpetual motion machine and other crackpottery. The best we can do on this front is write a clear usage guidance and correct tag mis-usages (after all there are other examples of tags which are useful but frequently misused, like ). However, we must also note that such questions are not so common on Physics SE and also usually they get closed pretty quickly.

  • I understand that we want to keep the tagging system simple. However, we also want it to be specific enough, and I think that this is one of the cases where additional specification is needed. Also, there are many examples of very specific tags that haven't been synonymized, like ,,, etc.

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    $\begingroup$ A variant of this would be to add the tag [thermodynamic-potentials], so as to also include the thermodynamic meanings of internal energy, enthalpy, and the grand potential. $\endgroup$
    – Semiclassical
    Feb 28, 2018 at 21:34
  • $\begingroup$ @Semiclassical Good idea, even if I feel that [thermodynamic-potentials] has a lower probability of being used by a user that doesn't already know about the existence of the tag. $\endgroup$
    – valerio
    Feb 28, 2018 at 22:05
  • $\begingroup$ Possibly. But one could have thermodynamic-potentials as the main tag and “free-energy” as a synonym. $\endgroup$
    – Semiclassical
    Feb 28, 2018 at 22:53
  • $\begingroup$ @Semiclassical That's also an interesting possibility. $\endgroup$
    – valerio
    Feb 28, 2018 at 23:07
  • $\begingroup$ Or just add appropriate other tags (e.g., thermo, stat-mech, QM, etc) and have no confusion about what energy is being referred to. $\endgroup$
    – Kyle Kanos
    Mar 1, 2018 at 0:25
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    $\begingroup$ @KyleKanos The point is that adding the tag [thermodynamics] or [statistical-mechanics] to the tag [energy] is not enough to clarify, since in there is already a thermodynamic potential called "energy" in these fields. $\endgroup$
    – valerio
    Mar 1, 2018 at 10:05
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    $\begingroup$ I disagree with the synonymizing. The cluster of concepts around the various free energies and other thermodynamic potentials does warrant a dedicated tag (but I'm not sure whether free-energy or thermodynamic-potentials (or having both) would be better). $\endgroup$
    – Emilio Pisanty
    Mar 1, 2018 at 10:41
  • $\begingroup$ Free-energy may end up being abused by the various perpetual motion and magnets-are-weird questioners. $\endgroup$
    – Jon Custer
    Mar 1, 2018 at 18:00
  • $\begingroup$ @JonCuster I thought about this actually. What we can do is write a clear usage guidance ("do not use for...") and then of course re-tag the questions where the tag is used inappropriately. We will never be able to completely prevent the misuse of the tag, but this applies to any other tag. There are lots of tags which are useful but often misused, like mathematical-physics. $\endgroup$
    – valerio
    Mar 1, 2018 at 18:07
  • $\begingroup$ @valerio - plus those questions never last too long (hopefully). But, as a tag to follow the signal to noise might be a bit low. $\endgroup$
    – Jon Custer
    Mar 1, 2018 at 18:10
  • $\begingroup$ I lean towards thermodynamic-potentials. If you try free-energy you might get swamped by people asking about perpetual motion machines. $\endgroup$
    – knzhou
    Mar 2, 2018 at 10:53
  • $\begingroup$ @knzhou thermodynamic-potentials is a good option. However, questions about perpetual motion machines are not so common and usually they get closed pretty quickly. $\endgroup$
    – valerio
    Mar 2, 2018 at 12:11
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    $\begingroup$ Similar questions could be asked about other thermodynamical potentials, heat, internal-energy, kinetic-energy, etc., etc. FYI, there are currently around 20 tags that containing the word 'energy'. Don't forget that the more refined we make the tags, the more fragmented and arbitrary they are applied by users. If tags are not used consistently, a simple non-tag keyword search would do a better job. We need to keep the tag system simple. $\endgroup$
    – Qmechanic Mod
    Mar 2, 2018 at 13:04
  • $\begingroup$ @Qmechanic What do you think about Semiclassical's proposal to introduce a [thermodynamic-potentials] tag, and make [free-energy] a synonym of that? $\endgroup$
    – valerio
    Mar 2, 2018 at 13:12
  • $\begingroup$ @Qmechanic I understand your point of view, and I agree that the tag system should be simple. However, it should also be specific enough, and I think that this is one of those cases where some more details are needed (and healthy). We have some examples of very specific tags (random example), so I really wonder why a concept like that of free energy wouldn't deserve a tag of its own. $\endgroup$
    – valerio
    Mar 2, 2018 at 13:20

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