Talk:Perfect gas

Re-writing Article

I am starting my rewriting of these gas-related articles.

I've completed Gas so far. I am working my way towards Ideal Gas

Difference from Ideal Gases?

I don't understand how a Perfect Gas is any different from an Ideal Gas? I was under the impression that ideal gases assumed no intermolecular forces, as well? —Preceding unsigned comment added by 75.71.68.254 (talk) 07:43, 3 September 2008 (UTC)

An ideal gas does not ignore intermolecular forces. A perfect gas does. Katanada (talk) 04:53, 28 October 2008 (UTC)

Do you have a reference for that? I don't think it's correct. Not only the Wikipedia article on ideal gas says that ideal gases have no intermolecular forces, but every book and article I've read uses the terms interchangeably. I think perfect gas should redirect to ideal gas, as it used to. --Itub (talk) 15:07, 31 October 2008 (UTC)

How many references would you like? I can compile an entire list for you. They're all related to aerospace engineering because that's what I do so its going to be a lot of books on aerodynamics and combustion thermodynamics and a lot of NASA stuff but if you want the list of books I can get you a list. The Wiki article on Ideal gas needs a LOT of work. It has a LOT of incorrect information at the conceptual level. The equations are fine but they're mostly thermo eqns that have no reason to be there. Katanada (talk) 19:19, 7 November 2008 (UTC)

A couple should be enough. If they are available in Google Books or amazon.com "Look Inside" it would be very convenient, but if not I would be thankful if you could give a quote from each book. Like I said in my talk page, I can also find plenty of books in the areas I'm familiar with (physical chemistry and related fields) that say that the two terms are synonymous or that say quite explicitly that ideal gases have no intermolecular forces. It may just be a case of different fields using different definitions for the same term, but in that case that needs to be stated in the article ("In aerospace engineering, an perfect gas is... In other fields, a perfect gas is an ideal gas.") --Itub (talk) 05:36, 8 November 2008 (UTC)

Hi, I'm a professor who's been teaching thermodynamics to engineers and scientists for decades with many published articles in the field, and I'd like to weigh in here. Now, an "ideal gas" is widely accepted to be a gas obeying the equation of state pV=nRT. A little elementary calculus can be used to show that the internal pressure of an ideal gas is equal to zero, as are the Joule and Joule-Thompson coefficients, solely using pV = nRT and the first and second laws - no other assumptions need to be made (for example, consult the Wikipedia page on internal pressure, and apply the formula they call the "thermodynamic equation of state" to pV=nRT to show that the internal pressure is zero, and therefore U does not depend on volume). These results means that for any ideal gas - that is, any gas obeying pV = nRT - the molar internal energy and enthalpy are at most a function of temperature (and are never a function of pressure or volume). This directly requires that the molar heat capacities of ideal gases be at most a function of temperature. I do agree that it can be useful to distinguish the usual "perfect" gas model - constant heat capacities - from the "semi-perfect" model in which the heat capacities are functions of T (btw, that temperature dependence is well known to arise from intramolecular forces causing highly quantized molecular vibrations which may or may not be thermally accessible at a given T, not intermolecular forces - see any elementary textbook on statistical mechanics). But if you decide to combine pV=nRT with heat capacities which vary with both P and T, you're not using an "ideal gas model." It's, well, something else that the scientific and engineering communities have never given a name to. Anyway, both the perfect and semi-perfect gas models are correctly understood as special cases of the ideal gas model, so I think the table in this article needs some edits. I see this is a very old discussion (2008!) so I am tempted to forge ahead, but I await your comments. KeeYou Flib (talk) 17:15, 16 February 2019 (UTC)

I have forged ahead after a long wait. I hope that the page as edited makes more sense to everyone. KeeYou Flib (talk) 16:36, 13 May 2021 (UTC)

@Qflib Is it consistent to combine pV=nRT with heat capacities varying with both P and T (or even just with P)? I mean: are there consistent models or systems with such behavior? And would you know some references in the scientific literature that develop or study such models or systems? The article says "It can be proved that an ideal gas (i.e. satisfying the ideal gas equation of state, PV=nRT) is either calorically perfect or thermally perfect." which would rule out either of the possibilities... 2A01:E0A:179:4500:B9E3:4442:1AD5:F6D3 (talk) 12:15, 28 November 2024 (UTC)

It’s not consistent mathematically, but people do sometimes use inconsistent models for systems just because it’s convenient. I’m unaware of a reference for someone doing so, though.

Basically if PV=nRT then the internal pressure is zero. This in turn requires the internal energy and enthalpy to be at most functions of T. Since the heat capacities are T derivatives, the same applies to them. Qflib (talk) 17:21, 28 November 2024 (UTC)

Restored 2008 version

This article was replaced by a redirect to ideal gas in 2008 since ideal and perfect gases are one and the same according to the vast majority of authors". The above discussion was also hidden since this talk page was replaced by a redirect. As far as I can tell from the above discussion, there was no consensus about this redirect, so I have reverted it. I came here because I was reading a paper that discussed a calculation assuming a perfect gas; it was mentioned in various places, with confusing links that turned out self-redirects.

I have added a sentence to Ideal gas along the lines "Sometimes, a distinction is made between an ideal gas, where cV and cp could vary with pressure and temperature, and a perfect gas, for which this is not the case." I hope everybody can live with that. I don't have the time to do an extensive literature survey to figure out which authors in which fields exactly make the distinction and which ones don't.

Han-Kwang (t) 11:09, 25 September 2011 (UTC)

Makes sense. It did need some editing to explain that these ideas are in widespread use and therefore a wikipedia article about it is needed, and also to connect the meaning of the different assumptions involved with respect to the whole idea that the internal energy and enthalpy of an ideal gas can at most be only a function of temperature. I have added some explanations and references to make these ideas clearer. KeeYou Flib (talk) 15:26, 13 May 2021 (UTC)

completely ionized plasma

[...] and molecules are not permitted to dissociate. The latter implies temperature limited to 2500 K.

Aren't the condidions for a termally perfect gas also fullfilled for a completely ionised plasma? This would then mean temperatures have to be eighter <2500K or a lot higher.

Can someone confirm, find a reference, and maybee look up at what temperature a H/He plasma is completely ionized to give as an example? must be above $10^{5}K$ — Preceding unsigned comment added by 193.197.56.41 (talk) 15:58, 10 November 2016 (UTC)