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The entropy change for an ADIABATIC process between two well defined equilibrium states IS ZERO!! (the one written there is valid just because Cp-Cv=R for an ideal gas, but zero is what should appear in the table!). If the process is irreversible and adiabatic, all we can say is that entropy increases (whatever the system) - and the same two states connected through the reversible adiabat (isoentropic) cannot be connected through an irreversible, adiabatic process. —Preceding unsigned comment added by 18.82.6.138 ( talk) 19:13, 25 September 2008 (UTC)
The b term is an excluded volume term. It's there because gas molecules have a certain size, and two cannot coexist at the same place. Van Der Waals gas is very important because its theory is also the base for the description of liquid mixes User:ThorinMuglindir ( 81.57.151.89) 11:53, 23 October 2005 (UTC)
"Generally, deviation from an ideal gas tends to decrease with lower temperature and higher density (i.e., higher pressure)[...] The ideal gas model tends to fail at lower temperatures or higher pressures[..]"
Maybe i am misunderstanding but if the deviation from the model decreases with temperature then it shouldn't mean that it will fail, actually the opposite.
Ptsneves (
talk) 18:24, 8 November 2010 (UTC)
(First and Foremost)
With this in mind, it was rather foolish not to have even mentioned Charles's Law and Boyle's Law in this article, and so, I have added the following. One needs to realize that many people have heard of these laws in high school chemistry and general sciences courses - even if they never went to college, or perhaps majored in the humanities or the social sciences there.
"Among other things, an ideal gas would follow Charles's Law and Boyle's Law exactly at all conceivable temperatures and pressures, and an ideal gas would be impossible to liquify at any temperature or pressure." 98.67.106.251 ( talk) 19:22, 4 May 2009 (UTC)
for ideal gas, pv/rt=1. for most of the real gas, (a)p<500atm, pv/rt<1;(b)p greater 500atm, pv/rt greater 1. for (a), it's intermolecular attraction factor; can some one explain the factor of (b) molecular volumn factor? I don't understand....
"Could please someone clarify whether a noble gas such as helium which I understand is normally monoatomic could be considered as an Ideal gas. As written in this article, an Ideal Gas is defined as molecules... L.L.
Perfect Gas vs. Ideal Gas
Perfect Gas should NOT redirect to this page and the first statement in this article is incorrect. Perfect gases and Ideal gases are NOT the same thing! Katanada ( talk) 21:19, 11 January 2008 (UTC)
THEY ARE NOT THE SAME!!
-- yes they (usually) are (see below). —Preceding unsigned comment added by 146.6.143.48 ( talk) 03:38, 20 February 2008 (UTC)
I suspect they are sometimes used interchangeably on some courses. However, a perfect gas is an ideal gas that has constant specific heat capacities. A semi-perfect gas is one which has specific heat capacites that are functions of temperature only. Specific heat capacities are in fact dependent on both pressure and temperature for real gases. The Perfect Gas page should be re-made. See http://www.roymech.co.uk/Related/Thermos/Thermos_fundamentals.html for example.
In many courses (especially for engineers) they are held to be very different things.
Tannoreth ( talk) 12:14, 19 November 2008 (UTC)
A perfect gas is one with specific heats that are independent of the temperature T.
The “perfect gas approximation” has nothing directly to do with whether a gas is ideal or not (though because non-ideal gas behavior can appear at very low temperatures, cp may be considerably different at those temperatures than at, say, 300K).
As long as the number of DOF’s of the molecules does not change with temperature T, then the specific heats Cv and Cp will be constant and thus the gas will be “perfect”. A perfect gas is simply one that has constant specific heats.
Katanada ( talk) 21:10, 11 January 2008 (UTC)
An ideal gas is one in which the separation between molecules is sufficiently large (i.e., the density is sufficiently low) that intermolecular (van der Waals) forces are negligible, so that the gas satisfies the ideal gas law. A gas ceases to be “ideal” when the density becomes very large (e.g., at very high pressures and/or low temperatures), so that the intermolecular forces become large enough to produce substantial departures from the ideal gas law. The gas then instead follows non-ideal behavior of the form PV = ZRT where Z is a “ Compressibility factor”
Katanada ( talk) 21:10, 11 January 2008 (UTC)
Why is there no article real gas? -- Lode 16:44, 19 Jul 2004 (UTC)
Well done, PAR. Just a few suggestions.
Shouldn't there be a list of the assumptions made when dealing with ideal gases?
I.e. no intermolecular attractions etc...
The widipedia entry "gas" seems to list properties that I think are Ideal gas properties not real gas properties, I think that this needs clarification between the two.
Hi PAR. If n is the amount of gas measured in mol, and the gas constant R is measured in J·K−1·mol−1, then nR is the amount of gas measured in J·K−1. Bo Jacoby 17:35, 8 December 2006 (UTC)
If some fixed amount of gas has the pressure P, measured in pascal, and volume V, measured in cubic meter, and temperature T, measured in kelvin, then the product PV is measured in joule, and the expression PV/T, measured in joule per kelvin, happens to be asymptotically constant for sufficiently low pressure and correspondingly large volume. This constant value is an expression for the amount of gas, because it is proportional to the volume at constant pressure and temperature. Chemists, however, divide the mass (kg) of some substance by the molecular weight (kg/mol) to get the amount of substance measured in mol. The gas constant R is simply the conversion factor between these two units of measurement: mol and J·K−1. If the mol was never invented, then molecular weight would be measured in kg·J−1·K, and the gas constant would disappear from all the formulas. A third unit of measurement is the molecule. The conversion factor between molecule and J·K−1 is the boltzmann constant, and the conversion factor between mol and molecule is avogadro's number. Please don't introduce a fourth unit of measurement, mol·m·s−1. Bo Jacoby 06:09, 9 December 2006 (UTC)
Yes, nc could be used if no better measure existed, but n is a little better, and nR is much better. The mass of the gas is unnecessary and should be erased from the theory by Occam's razor. So the mol is unnecessary, (and so is the speed of light in this context). The simplest formula for an amount of gas measured from P,V and T is PV/T, and the SI unit for this quantity is J·K−1. You may multiply by constants to get other units, but that only complicates matters. (The standard cubic foot of gas refers to non-SI units of volume, pressure and temperature). The ideal gas law says that amount does relate to energy, because the amount of an ideal gas is PV/T, which is energy divided by absolute temperature. Bo Jacoby 23:34, 9 December 2006 (UTC)
OK. It's a pity, though, because the WP article is not helpful for understanding. Even you did not understand the meaning of the gas constant. Why not leave it to people who do understand? Bo Jacoby 23:47, 9 December 2006 (UTC)
Your own writing says that Nk=PV/T where N is the number of molecules and k is measured in J·K−1·molecule−1, such that Nk is measured in J·K−1. What more do you want? I am merely clarifying what you wrote without quite understanding. Bo Jacoby 23:55, 12 December 2006 (UTC)
I changed the word 'particles' into 'molecules' because these are the free particles of a gas. We are not talking about the number of quarks and electrons of the gas, but about the number of molecules. User Sadi Carnot reverted my edit. Please explain here on the talk page. Also you claim that "N is not the number of molecules, the Nk = nR edit is false and OR; + other grammatical and factual errors." I disagree. Please discuss first and edit later. Bo Jacoby 16:49, 14 December 2006 (UTC)
As user Sadi Carnot does not answer, I am going to revert his deletion. Bo Jacoby 13:11, 15 December 2006 (UTC)
Why can't these people explain their point of view? I hope we agree to create high quality WP articles. That Nk=nR follows from the original statement of the gas law. Agreed? That the gas particles are called molecules follow from the article on molecule: "The concept of a single-atom or monatomic molecule, as found in noble gases, is used almost exclusively in the kinetic theory of gases, where the fundamental gas particles are conventionally termed "molecules" regardless of their composition". Agreed? Bo Jacoby 13:27, 15 December 2006 (UTC)
Hi PAR. Earlier, physicists were accustomed to use many units of measurements and to convert by means of conversion factors, but now the units are standardized and the conversion factors rarely occur in physical equations any more, and so a modern physicist doesn't recognize a conversion factor when he sees one. You yourself did not recognize R as a conversion factor in PV=nRT, even if it is the only constant amongst four variables. Traditionally there are many units of measurements for pressure, (pa, bar, at, atm, mmHg, psi) and for volume, (liters, cubic feet, barrels, gallons, pint, ounces ...) and absolute temperature ( kelvin, rankine, ...), but there are only two units of amount of substance around, the mol and the molecule. So the gas law is written PV/T=nR=Nk, converting the number of moles or the number of molecules into the appropriate unit. The modern approach is to chose units such that the conversion factors disappear. Then the gas law is PV/T=n, where n is the amount of gas measured in the appropriate unit. This is easily understood by an old physicist, but not for a young one, and so the WP article must explain rather than take it for granted. As by now neither the reader nor the author understood the ideal gas law! Bo Jacoby 15:14, 15 December 2006 (UTC)
I probably have the same standard references as you do. I understand it. You don't. Bo Jacoby 16:21, 15 December 2006 (UTC)
The article does not mention the difference between a thermally perfect and a calorically perfect gas.
The difference is important when dealing with large temperature ranges. In such cases one can assume the gas to be just thermally perfect but calorically imperfect. This allows the use of equation of state for ideal gas while also accounting for variation in the heat capacities and with temperature (which can be significant).
I am not sure where to include this in the article. So I am mentioning it here. - Myth ( Talk) 08:12, 25 February 2007 (UTC)
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Google search result for this reference.Found this statement :Gases are most ideal at high temperatures and low densities.
This is WRONG since gases at higher temperatures will have MORE collisions hence the "ideal gas" property is lost (as you may be aware molecules have inelastic collisions hence kinetic energy is lost). Also at lower temperatures the distance between particles is greater, hence there is more interaction between molecules. Therefore, a gas which is at extremely high/low temperatures (in comparison to standard labratory conditions) will not obey the ideal gas laws fully.
The low density may also be checked.
I have removed this nonsense. Thanks —Preceding unsigned comment added by 121.222.226.157 ( talk) 08:54, 23 October 2007 (UTC)
Isn't it wise to specify that p is the absolute pressure? Not only here, but virtually anywhere pressure is mentioned. Even if it were clarified somewhere else, which I don't know, this lack of information is unpractical to the average user. I'm asking because I'm new to WP and I don't want to mess around with articles! Podi74 ( talk) 12:19, 19 December 2007 (UTC)
By introducing the undefined concept of 'moderate temperature' an editor has made the text nonsensical. Bo Jacoby ( talk) 09:31, 20 February 2008 (UTC).
Perhaps RIPE would be a better acronym, rather than PRIE? PRIE is just a collection of letters, RIPE actually spells something, so it is easier to remember, especially for students. Just a thought. —Preceding unsigned comment added by 151.200.36.227 ( talk) 04:15, 2 May 2008 (UTC)
Shouldn't the units be given for each of the quantities in the equation. The formula only works with that gas constant if SI units are used. Plenty of engineers (and some Americans?) still do not use SI. So if we are going to give a value for the gas constant instead of just a link, then the units have to be defined too. Yobmod ( talk) 14:56, 10 February 2009 (UTC)
"...the classical thermodynamic ideal gas is based on classical thermodynamics..."? —Preceding unsigned comment added by Paranoidhuman ( talk • contribs) 02:45, 12 August 2009 (UTC)
I'm not sure I see the benefit of having two separate articles. Its not that they are redundant (although there is overlap) but they seem to be intrinsically talking about the same thing, and it is just confusing to have two places to look for one topic. An ideal gas is by definition anything which behaves according to the ideal gas law. So I think there is a case to merge them. This would make it much easier to find what one wanted w/o searching through both of them.
If there is not support for this, then I think we will need to more clearly define the scope of the two articles. Personally, if I needed to know something about an ideal gas it wouldn't be 100% clear which one I should look in first. Thanks for your thoughts! David Hollman ( Talk) 15:41, 9 September 2010 (UTC)
This is nonsense:
Constants do not depend on temperature. 3/2 does not depend on temperature. The meaning is that is constant for the simplified model of ideal gases but vary for real gases. So it should state that
Bo Jacoby ( talk) 09:07, 12 September 2010 (UTC).
Yup -- you're right :) ... Someone should change that if it hasn't been changed already. Cv is by no means a constant. Also, its not always equal to those numbers either... You can say that it is "generally" equal or something that is less restrictive... but then we'd have to have the quantum argument :( Katanada ( talk) 01:45, 11 November 2010 (UTC)
The last sentence in this section seems spurious or at least sits at the wrong place: "Ideal gasses are not found in the real world. So they are different from real gasses. There are basic assumptions made in the kinetic theory of gasses".
Second: in this sentence gasses is written with double S. http://www.collinsdictionary.com/dictionary/english/gas says both forms are allowed, but the other occurrences on this page have a single S. -- 46.115.56.114 ( talk) 00:07, 23 August 2012 (UTC) Marco Pagliero Berlin
Spherical particles are not necessary, only that the interparticle distance be large. What is needed that rotational energy is negligible. Spherical particles will have a rotational energy. If they are small enough, it will be negligible. I will change this soon. PAR ( talk) 16:24, 14 September 2012 (UTC)
Classical thermodynamic ideal gas description contradicts the definition of an ideal gas. The definition states "composed of a set of non-interacting point particles". Classical thermodynamic ideal gas description talks about spherical particles that collide. This sounds wrong and should be removed or explained properly. — Preceding unsigned comment added by 2401:FA00:0:3:BE30:5BFF:FEE1:EBF4 ( talk) 04:53, 30 October 2013 (UTC)
I'm still trying to understand the heat capacity section. If you compare to the article on Heat capacity, there's no point where the variables with bars over them are introduced. I can't see such an introduction in this article.
Now, there is a meaningful difference between the heat capacity and the specific heat capacity. But this is generally designated with upper case versus lower case. It's not clear at all what the bars above the variables mean. I think some description should be added. But I don't know what it means, so could anyone help me? - Theanphibian ( talk • contribs) 19:39, 21 March 2014 (UTC)
I have a very nice ideal gas simulation (in Java) available here: http://www.ics.uci.edu/~wayne/Gas . Does anybody I'm totally new to editing Wikipedia articles and don't know if it would have a place or if so where to put it. If anybody thinks there is a place for such a simulation please contact me at whayes@uci.edu (Wayne Hayes, UC Irvine) Waynehayes ( talk) 14:40, 15 April 2014 (UTC)
It should include that particles can collide, but only elastically; otherwise since they do not "interact" (and their volume is zero) it seems that they do not collide. — Preceding unsigned comment added by 84.120.147.21 ( talk) 07:45, 29 August 2014 (UTC)
Is it really necessary to have two equations of state in the "Classical thermodynamic ideal gas" section? Isn't the whole point of an equation of state that it completely describes the system? Any other equation of state would be redundant (although perhaps more convenient). In this specific case, the fact that the internal energy of an ideal gas is a function of temperature only comes straight out of the total differential of U(T,V) with the use of a few Maxwell relations and the first equation of state (PV = nRT). Consequently, the second equation of state isn't necessary to satisfy the property of being a function of temperature only. Also, is it really necessary to include all of the dimensionless quantities in this article? The article should, first and foremost, be intended for the lay audience, not specialists. Does anyone else think that dimensionless quantities are a bit to over-the-top for such a basic sections of the article? JCMPC ( talk) 00:10, 19 October 2014 (UTC)
I have undone a faulty good-faith edit that has an edit summary "corrected standard molar volume. It is 22.4 at 101.325 kPa".
The relevant sentence, as I have restored it, reads
The relevant text at the linked page reads
Standard conditions for gases: ... and pressure of 105 pascals. The previous standard absolute pressure of 1 atm (equivalent to 1.01325 × 105 Pa) was changed to 100 kPa in 1982. IUPAC recommends that the former pressure should be discontinued.
By my arithmetic, 22.4 × 101.325 ÷ 100 = 22.6968.
It is more or less routine that this correction appears here. Chjoaygame ( talk) 05:42, 14 December 2015 (UTC)
The expression for computing the entropy of ideal gases given in the text as
is a little problematic. The entropy is a function of temperature and two extensive variables :
therefore, tho compute the change in the entropy during a transformation, we must write
Somehow, the third term (the variation in due to variation in ) has been omitted, but the correct expression for is obtained when is varied in the section on chemical potential. This need some clarification/cleaning. — Preceding unsigned comment added by BahramH ( talk • contribs) 05:43, 22 August 2016 (UTC)
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"The molecules of the gas are indistinguishable, small, hard spheres"
vs.
"the Equipartition Theorem predicts that the constant for a monatomic gas is $\hat c_v = \frac 32$ while for a diatomic gas it is $\hat c_v = \frac 52$ if vibrations are neglected"
So do gases that are non-monoatomic count as ideal or not? The ideal gas law sure applies to them. I would consider them ideal gases as well. In consequence, the microscopic model of an ideal gas should not speak of "hard spheres" is insufficient. Maybe it is more inclusive to speak of "hard collisions" between molecules of whatever shape. Although, what about vibrational degrees of freedom? — Preceding unsigned comment added by Benzh ( talk • contribs) 14:55, 12 December 2019 (UTC)
An editor has asked for a discussion to address the redirect Behaviour of gases. Please participate in the redirect discussion if you wish to do so. Utopes ( talk / cont) 19:00, 31 March 2020 (UTC)
Hi Tuntable. At Ideal gas you wrote “An ideal gas does not cool on expansion or heat on compression. …” See your diff.
This is incorrect. The situation is correctly explained at Joule–Thomson effect# The Joule–Thomson (Kelvin) coefficient (last paragraph) where it says “For an ideal gas, is always equal to zero: ideal gases neither warm nor cool upon being expanded at constant enthalpy.” (The bolded words are most significant. The bolding has been added by me.)
When a gas is compressed in a reversible adiabatic process its temperature and internal energy both rise. The increase in internal energy is equal to the work done on the gas during the compression process. This is true of ideal gases, and also real gases such as air. Identically, when a gas is expanded in a reversible adiabatic process its temperature and internal energy both fall. The fall in internal energy is equal to the work done on the surroundings by the gas. This is true of ideal gases, and also real gases. When a gas is compressed or expanded in a reversible adiabatic process, entropy is constant throughout the process but enthalpy changes. I will comment on the constant-enthalpy process in the next paragraph.
The situation which sees an ideal gas undergo an isothermal change is the throttling process or Joule-Thomson expansion. This is a change which is definitely not reversible; it is an almost-explosive decompression and it can be shown to take place with no change in enthalpy (but an increase in entropy). When an ideal gas is throttled to a lower pressure the temperature of the gas remains unchanged; unlike the situation with real gases where the temperature falls a little (except for hydrogen, helium and neon whose temperatures increase upon throttling.) The throttling process cannot be used to increase the pressure of any gas – that would involve a decrease in entropy and so would violate the second law of thermodynamics.
I’m happy to discuss further. Dolphin ( t) 08:27, 30 June 2020
why is there a non-peer-reviewed working paper randomly inserted in this Wikipedia page? Seems wildly inappropriate. 108.48.50.164 ( talk) 20:56, 11 July 2023 (UTC)
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The entropy change for an ADIABATIC process between two well defined equilibrium states IS ZERO!! (the one written there is valid just because Cp-Cv=R for an ideal gas, but zero is what should appear in the table!). If the process is irreversible and adiabatic, all we can say is that entropy increases (whatever the system) - and the same two states connected through the reversible adiabat (isoentropic) cannot be connected through an irreversible, adiabatic process. —Preceding unsigned comment added by 18.82.6.138 ( talk) 19:13, 25 September 2008 (UTC)
The b term is an excluded volume term. It's there because gas molecules have a certain size, and two cannot coexist at the same place. Van Der Waals gas is very important because its theory is also the base for the description of liquid mixes User:ThorinMuglindir ( 81.57.151.89) 11:53, 23 October 2005 (UTC)
"Generally, deviation from an ideal gas tends to decrease with lower temperature and higher density (i.e., higher pressure)[...] The ideal gas model tends to fail at lower temperatures or higher pressures[..]"
Maybe i am misunderstanding but if the deviation from the model decreases with temperature then it shouldn't mean that it will fail, actually the opposite.
Ptsneves (
talk) 18:24, 8 November 2010 (UTC)
(First and Foremost)
With this in mind, it was rather foolish not to have even mentioned Charles's Law and Boyle's Law in this article, and so, I have added the following. One needs to realize that many people have heard of these laws in high school chemistry and general sciences courses - even if they never went to college, or perhaps majored in the humanities or the social sciences there.
"Among other things, an ideal gas would follow Charles's Law and Boyle's Law exactly at all conceivable temperatures and pressures, and an ideal gas would be impossible to liquify at any temperature or pressure." 98.67.106.251 ( talk) 19:22, 4 May 2009 (UTC)
for ideal gas, pv/rt=1. for most of the real gas, (a)p<500atm, pv/rt<1;(b)p greater 500atm, pv/rt greater 1. for (a), it's intermolecular attraction factor; can some one explain the factor of (b) molecular volumn factor? I don't understand....
"Could please someone clarify whether a noble gas such as helium which I understand is normally monoatomic could be considered as an Ideal gas. As written in this article, an Ideal Gas is defined as molecules... L.L.
Perfect Gas vs. Ideal Gas
Perfect Gas should NOT redirect to this page and the first statement in this article is incorrect. Perfect gases and Ideal gases are NOT the same thing! Katanada ( talk) 21:19, 11 January 2008 (UTC)
THEY ARE NOT THE SAME!!
-- yes they (usually) are (see below). —Preceding unsigned comment added by 146.6.143.48 ( talk) 03:38, 20 February 2008 (UTC)
I suspect they are sometimes used interchangeably on some courses. However, a perfect gas is an ideal gas that has constant specific heat capacities. A semi-perfect gas is one which has specific heat capacites that are functions of temperature only. Specific heat capacities are in fact dependent on both pressure and temperature for real gases. The Perfect Gas page should be re-made. See http://www.roymech.co.uk/Related/Thermos/Thermos_fundamentals.html for example.
In many courses (especially for engineers) they are held to be very different things.
Tannoreth ( talk) 12:14, 19 November 2008 (UTC)
A perfect gas is one with specific heats that are independent of the temperature T.
The “perfect gas approximation” has nothing directly to do with whether a gas is ideal or not (though because non-ideal gas behavior can appear at very low temperatures, cp may be considerably different at those temperatures than at, say, 300K).
As long as the number of DOF’s of the molecules does not change with temperature T, then the specific heats Cv and Cp will be constant and thus the gas will be “perfect”. A perfect gas is simply one that has constant specific heats.
Katanada ( talk) 21:10, 11 January 2008 (UTC)
An ideal gas is one in which the separation between molecules is sufficiently large (i.e., the density is sufficiently low) that intermolecular (van der Waals) forces are negligible, so that the gas satisfies the ideal gas law. A gas ceases to be “ideal” when the density becomes very large (e.g., at very high pressures and/or low temperatures), so that the intermolecular forces become large enough to produce substantial departures from the ideal gas law. The gas then instead follows non-ideal behavior of the form PV = ZRT where Z is a “ Compressibility factor”
Katanada ( talk) 21:10, 11 January 2008 (UTC)
Why is there no article real gas? -- Lode 16:44, 19 Jul 2004 (UTC)
Well done, PAR. Just a few suggestions.
Shouldn't there be a list of the assumptions made when dealing with ideal gases?
I.e. no intermolecular attractions etc...
The widipedia entry "gas" seems to list properties that I think are Ideal gas properties not real gas properties, I think that this needs clarification between the two.
Hi PAR. If n is the amount of gas measured in mol, and the gas constant R is measured in J·K−1·mol−1, then nR is the amount of gas measured in J·K−1. Bo Jacoby 17:35, 8 December 2006 (UTC)
If some fixed amount of gas has the pressure P, measured in pascal, and volume V, measured in cubic meter, and temperature T, measured in kelvin, then the product PV is measured in joule, and the expression PV/T, measured in joule per kelvin, happens to be asymptotically constant for sufficiently low pressure and correspondingly large volume. This constant value is an expression for the amount of gas, because it is proportional to the volume at constant pressure and temperature. Chemists, however, divide the mass (kg) of some substance by the molecular weight (kg/mol) to get the amount of substance measured in mol. The gas constant R is simply the conversion factor between these two units of measurement: mol and J·K−1. If the mol was never invented, then molecular weight would be measured in kg·J−1·K, and the gas constant would disappear from all the formulas. A third unit of measurement is the molecule. The conversion factor between molecule and J·K−1 is the boltzmann constant, and the conversion factor between mol and molecule is avogadro's number. Please don't introduce a fourth unit of measurement, mol·m·s−1. Bo Jacoby 06:09, 9 December 2006 (UTC)
Yes, nc could be used if no better measure existed, but n is a little better, and nR is much better. The mass of the gas is unnecessary and should be erased from the theory by Occam's razor. So the mol is unnecessary, (and so is the speed of light in this context). The simplest formula for an amount of gas measured from P,V and T is PV/T, and the SI unit for this quantity is J·K−1. You may multiply by constants to get other units, but that only complicates matters. (The standard cubic foot of gas refers to non-SI units of volume, pressure and temperature). The ideal gas law says that amount does relate to energy, because the amount of an ideal gas is PV/T, which is energy divided by absolute temperature. Bo Jacoby 23:34, 9 December 2006 (UTC)
OK. It's a pity, though, because the WP article is not helpful for understanding. Even you did not understand the meaning of the gas constant. Why not leave it to people who do understand? Bo Jacoby 23:47, 9 December 2006 (UTC)
Your own writing says that Nk=PV/T where N is the number of molecules and k is measured in J·K−1·molecule−1, such that Nk is measured in J·K−1. What more do you want? I am merely clarifying what you wrote without quite understanding. Bo Jacoby 23:55, 12 December 2006 (UTC)
I changed the word 'particles' into 'molecules' because these are the free particles of a gas. We are not talking about the number of quarks and electrons of the gas, but about the number of molecules. User Sadi Carnot reverted my edit. Please explain here on the talk page. Also you claim that "N is not the number of molecules, the Nk = nR edit is false and OR; + other grammatical and factual errors." I disagree. Please discuss first and edit later. Bo Jacoby 16:49, 14 December 2006 (UTC)
As user Sadi Carnot does not answer, I am going to revert his deletion. Bo Jacoby 13:11, 15 December 2006 (UTC)
Why can't these people explain their point of view? I hope we agree to create high quality WP articles. That Nk=nR follows from the original statement of the gas law. Agreed? That the gas particles are called molecules follow from the article on molecule: "The concept of a single-atom or monatomic molecule, as found in noble gases, is used almost exclusively in the kinetic theory of gases, where the fundamental gas particles are conventionally termed "molecules" regardless of their composition". Agreed? Bo Jacoby 13:27, 15 December 2006 (UTC)
Hi PAR. Earlier, physicists were accustomed to use many units of measurements and to convert by means of conversion factors, but now the units are standardized and the conversion factors rarely occur in physical equations any more, and so a modern physicist doesn't recognize a conversion factor when he sees one. You yourself did not recognize R as a conversion factor in PV=nRT, even if it is the only constant amongst four variables. Traditionally there are many units of measurements for pressure, (pa, bar, at, atm, mmHg, psi) and for volume, (liters, cubic feet, barrels, gallons, pint, ounces ...) and absolute temperature ( kelvin, rankine, ...), but there are only two units of amount of substance around, the mol and the molecule. So the gas law is written PV/T=nR=Nk, converting the number of moles or the number of molecules into the appropriate unit. The modern approach is to chose units such that the conversion factors disappear. Then the gas law is PV/T=n, where n is the amount of gas measured in the appropriate unit. This is easily understood by an old physicist, but not for a young one, and so the WP article must explain rather than take it for granted. As by now neither the reader nor the author understood the ideal gas law! Bo Jacoby 15:14, 15 December 2006 (UTC)
I probably have the same standard references as you do. I understand it. You don't. Bo Jacoby 16:21, 15 December 2006 (UTC)
The article does not mention the difference between a thermally perfect and a calorically perfect gas.
The difference is important when dealing with large temperature ranges. In such cases one can assume the gas to be just thermally perfect but calorically imperfect. This allows the use of equation of state for ideal gas while also accounting for variation in the heat capacities and with temperature (which can be significant).
I am not sure where to include this in the article. So I am mentioning it here. - Myth ( Talk) 08:12, 25 February 2007 (UTC)
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Google search result for this reference.Found this statement :Gases are most ideal at high temperatures and low densities.
This is WRONG since gases at higher temperatures will have MORE collisions hence the "ideal gas" property is lost (as you may be aware molecules have inelastic collisions hence kinetic energy is lost). Also at lower temperatures the distance between particles is greater, hence there is more interaction between molecules. Therefore, a gas which is at extremely high/low temperatures (in comparison to standard labratory conditions) will not obey the ideal gas laws fully.
The low density may also be checked.
I have removed this nonsense. Thanks —Preceding unsigned comment added by 121.222.226.157 ( talk) 08:54, 23 October 2007 (UTC)
Isn't it wise to specify that p is the absolute pressure? Not only here, but virtually anywhere pressure is mentioned. Even if it were clarified somewhere else, which I don't know, this lack of information is unpractical to the average user. I'm asking because I'm new to WP and I don't want to mess around with articles! Podi74 ( talk) 12:19, 19 December 2007 (UTC)
By introducing the undefined concept of 'moderate temperature' an editor has made the text nonsensical. Bo Jacoby ( talk) 09:31, 20 February 2008 (UTC).
Perhaps RIPE would be a better acronym, rather than PRIE? PRIE is just a collection of letters, RIPE actually spells something, so it is easier to remember, especially for students. Just a thought. —Preceding unsigned comment added by 151.200.36.227 ( talk) 04:15, 2 May 2008 (UTC)
Shouldn't the units be given for each of the quantities in the equation. The formula only works with that gas constant if SI units are used. Plenty of engineers (and some Americans?) still do not use SI. So if we are going to give a value for the gas constant instead of just a link, then the units have to be defined too. Yobmod ( talk) 14:56, 10 February 2009 (UTC)
"...the classical thermodynamic ideal gas is based on classical thermodynamics..."? —Preceding unsigned comment added by Paranoidhuman ( talk • contribs) 02:45, 12 August 2009 (UTC)
I'm not sure I see the benefit of having two separate articles. Its not that they are redundant (although there is overlap) but they seem to be intrinsically talking about the same thing, and it is just confusing to have two places to look for one topic. An ideal gas is by definition anything which behaves according to the ideal gas law. So I think there is a case to merge them. This would make it much easier to find what one wanted w/o searching through both of them.
If there is not support for this, then I think we will need to more clearly define the scope of the two articles. Personally, if I needed to know something about an ideal gas it wouldn't be 100% clear which one I should look in first. Thanks for your thoughts! David Hollman ( Talk) 15:41, 9 September 2010 (UTC)
This is nonsense:
Constants do not depend on temperature. 3/2 does not depend on temperature. The meaning is that is constant for the simplified model of ideal gases but vary for real gases. So it should state that
Bo Jacoby ( talk) 09:07, 12 September 2010 (UTC).
Yup -- you're right :) ... Someone should change that if it hasn't been changed already. Cv is by no means a constant. Also, its not always equal to those numbers either... You can say that it is "generally" equal or something that is less restrictive... but then we'd have to have the quantum argument :( Katanada ( talk) 01:45, 11 November 2010 (UTC)
The last sentence in this section seems spurious or at least sits at the wrong place: "Ideal gasses are not found in the real world. So they are different from real gasses. There are basic assumptions made in the kinetic theory of gasses".
Second: in this sentence gasses is written with double S. http://www.collinsdictionary.com/dictionary/english/gas says both forms are allowed, but the other occurrences on this page have a single S. -- 46.115.56.114 ( talk) 00:07, 23 August 2012 (UTC) Marco Pagliero Berlin
Spherical particles are not necessary, only that the interparticle distance be large. What is needed that rotational energy is negligible. Spherical particles will have a rotational energy. If they are small enough, it will be negligible. I will change this soon. PAR ( talk) 16:24, 14 September 2012 (UTC)
Classical thermodynamic ideal gas description contradicts the definition of an ideal gas. The definition states "composed of a set of non-interacting point particles". Classical thermodynamic ideal gas description talks about spherical particles that collide. This sounds wrong and should be removed or explained properly. — Preceding unsigned comment added by 2401:FA00:0:3:BE30:5BFF:FEE1:EBF4 ( talk) 04:53, 30 October 2013 (UTC)
I'm still trying to understand the heat capacity section. If you compare to the article on Heat capacity, there's no point where the variables with bars over them are introduced. I can't see such an introduction in this article.
Now, there is a meaningful difference between the heat capacity and the specific heat capacity. But this is generally designated with upper case versus lower case. It's not clear at all what the bars above the variables mean. I think some description should be added. But I don't know what it means, so could anyone help me? - Theanphibian ( talk • contribs) 19:39, 21 March 2014 (UTC)
I have a very nice ideal gas simulation (in Java) available here: http://www.ics.uci.edu/~wayne/Gas . Does anybody I'm totally new to editing Wikipedia articles and don't know if it would have a place or if so where to put it. If anybody thinks there is a place for such a simulation please contact me at whayes@uci.edu (Wayne Hayes, UC Irvine) Waynehayes ( talk) 14:40, 15 April 2014 (UTC)
It should include that particles can collide, but only elastically; otherwise since they do not "interact" (and their volume is zero) it seems that they do not collide. — Preceding unsigned comment added by 84.120.147.21 ( talk) 07:45, 29 August 2014 (UTC)
Is it really necessary to have two equations of state in the "Classical thermodynamic ideal gas" section? Isn't the whole point of an equation of state that it completely describes the system? Any other equation of state would be redundant (although perhaps more convenient). In this specific case, the fact that the internal energy of an ideal gas is a function of temperature only comes straight out of the total differential of U(T,V) with the use of a few Maxwell relations and the first equation of state (PV = nRT). Consequently, the second equation of state isn't necessary to satisfy the property of being a function of temperature only. Also, is it really necessary to include all of the dimensionless quantities in this article? The article should, first and foremost, be intended for the lay audience, not specialists. Does anyone else think that dimensionless quantities are a bit to over-the-top for such a basic sections of the article? JCMPC ( talk) 00:10, 19 October 2014 (UTC)
I have undone a faulty good-faith edit that has an edit summary "corrected standard molar volume. It is 22.4 at 101.325 kPa".
The relevant sentence, as I have restored it, reads
The relevant text at the linked page reads
Standard conditions for gases: ... and pressure of 105 pascals. The previous standard absolute pressure of 1 atm (equivalent to 1.01325 × 105 Pa) was changed to 100 kPa in 1982. IUPAC recommends that the former pressure should be discontinued.
By my arithmetic, 22.4 × 101.325 ÷ 100 = 22.6968.
It is more or less routine that this correction appears here. Chjoaygame ( talk) 05:42, 14 December 2015 (UTC)
The expression for computing the entropy of ideal gases given in the text as
is a little problematic. The entropy is a function of temperature and two extensive variables :
therefore, tho compute the change in the entropy during a transformation, we must write
Somehow, the third term (the variation in due to variation in ) has been omitted, but the correct expression for is obtained when is varied in the section on chemical potential. This need some clarification/cleaning. — Preceding unsigned comment added by BahramH ( talk • contribs) 05:43, 22 August 2016 (UTC)
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"The molecules of the gas are indistinguishable, small, hard spheres"
vs.
"the Equipartition Theorem predicts that the constant for a monatomic gas is $\hat c_v = \frac 32$ while for a diatomic gas it is $\hat c_v = \frac 52$ if vibrations are neglected"
So do gases that are non-monoatomic count as ideal or not? The ideal gas law sure applies to them. I would consider them ideal gases as well. In consequence, the microscopic model of an ideal gas should not speak of "hard spheres" is insufficient. Maybe it is more inclusive to speak of "hard collisions" between molecules of whatever shape. Although, what about vibrational degrees of freedom? — Preceding unsigned comment added by Benzh ( talk • contribs) 14:55, 12 December 2019 (UTC)
An editor has asked for a discussion to address the redirect Behaviour of gases. Please participate in the redirect discussion if you wish to do so. Utopes ( talk / cont) 19:00, 31 March 2020 (UTC)
Hi Tuntable. At Ideal gas you wrote “An ideal gas does not cool on expansion or heat on compression. …” See your diff.
This is incorrect. The situation is correctly explained at Joule–Thomson effect# The Joule–Thomson (Kelvin) coefficient (last paragraph) where it says “For an ideal gas, is always equal to zero: ideal gases neither warm nor cool upon being expanded at constant enthalpy.” (The bolded words are most significant. The bolding has been added by me.)
When a gas is compressed in a reversible adiabatic process its temperature and internal energy both rise. The increase in internal energy is equal to the work done on the gas during the compression process. This is true of ideal gases, and also real gases such as air. Identically, when a gas is expanded in a reversible adiabatic process its temperature and internal energy both fall. The fall in internal energy is equal to the work done on the surroundings by the gas. This is true of ideal gases, and also real gases. When a gas is compressed or expanded in a reversible adiabatic process, entropy is constant throughout the process but enthalpy changes. I will comment on the constant-enthalpy process in the next paragraph.
The situation which sees an ideal gas undergo an isothermal change is the throttling process or Joule-Thomson expansion. This is a change which is definitely not reversible; it is an almost-explosive decompression and it can be shown to take place with no change in enthalpy (but an increase in entropy). When an ideal gas is throttled to a lower pressure the temperature of the gas remains unchanged; unlike the situation with real gases where the temperature falls a little (except for hydrogen, helium and neon whose temperatures increase upon throttling.) The throttling process cannot be used to increase the pressure of any gas – that would involve a decrease in entropy and so would violate the second law of thermodynamics.
I’m happy to discuss further. Dolphin ( t) 08:27, 30 June 2020
why is there a non-peer-reviewed working paper randomly inserted in this Wikipedia page? Seems wildly inappropriate. 108.48.50.164 ( talk) 20:56, 11 July 2023 (UTC)