Talk:Van der Waals equation

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Is there any way we can find out the value of b theoretically if the value of radius of nucleus is known

and if there is any way of caluculating a plz do write

Yes, there is, but you will need heavy-duty quantum physics and a lot of computational power to get reasonably good answers. By the way, the atomic radius is considerably bigger than the size of the atomic nucleus. -- Karada 08:20, 1 Sep 2003 (UTC)
The van der Waals equation is really only an approximation valid at the macroscopi levels... not sure why you would want to bother using it at the subatomic level. yosofun 00:25, 7 Mar 2004 (PST)

Contents

[edit] Changed notation

I changed the notation for the formulas to the form more familiar to me of lower case p for pressure and lower case for per-particle properties. Hence v for volume per particle (specific volume), and V for total volume; N for total number of moles (or particles) rather than n which is used for the number density of molecules (number of particles per unit volume: n = 1/v = N/V, if N is the total number of particles)(or n = N x Na / V if N is the total number of moles, and where Na is Avogadro's number).

  • Tom Lougheed 2005-09-27

[edit] The equations at the top need fixing

Those equations need cleaning - first and foremost: same variables should not have different meanings. Fresheneesz 08:21, 21 November 2005 (UTC)

Looking at it quickly, the only problem I see is lowercase p and uppercase P both being used for pressure. Did you see others? PAR 13:58, 21 November 2005 (UTC)
I think I fixed it abakharev 23:19, 21 November 2005 (UTC)
Does anyone mind if we have the convention N=number of particles in the system, V=volume of the system, and v=V/N = volume per particle, without introducing the concept of the mole and Avogadro's number? I will do the changing. PAR 03:57, 22 November 2005 (UTC)
There won't be R (gas constant) then, just NkT, if it is OK with you, guys, it is OK with me abakharev 04:50, 22 November 2005 (UTC)

[edit] Error in vC

Should not kTc=8a/27b read NkTc=8a/27b ? Jhaagsma 09:25, 26 February 2007 (UTC)
No, I think the problem is that the equation for vC is wrong. I fixed it. Please check and see if you agree. Thanks for noticing the inconsistency. PAR 17:03, 26 February 2007 (UTC)
Are you sure the new equation is correct? If vC is supposed to be the critical specific volume, dividing b by Avogadro's number gives a really tiny answer for it. Sorry, I've probably misunderstood the meaning of vC. 84.12.252.210 15:40, 18 July 2007 (UTC)
v_C = 3b'\,. Note the prime on b! This indicates a molecular (not a molar) quantity. If you divide b by N, nothing happens to vC.--P.wormer 15:53, 18 July 2007 (UTC)

[edit] Change title of article to 'Van der Waals equation of state'

'Van der Waals equation of state' is a more accurate description of this article and so I think it should be moved. Wiki me 15:47, 18 March 2007 (UTC)

[edit] 1st equation?

The article states: v is the volume per particle of the fluid in the 1st equation;. Questions: (i) is the volume per particle v not the same as the total volume V? Both are the volume of the vessel, I would think. (V-b is the volume available to a particle). (ii) What is the 1st equation? (iii) Does anybody object if I start working on this article? —The preceding unsigned comment was added by P.wormer (talk • contribs) 15:51, 9 May 2007 (UTC).

Sorry for forgetting to sign previous comment. I reworked the article. Extended the derivation (actually gave two derivations), and checked the notations. These didn't match well, I hope I have the notation correct now and matching. I kept \,a' and \,b' as molecular variables (as they were in the original article). The corresponding molar variables are \,a and \,b.--P.wormer 11:53, 15 May 2007 (UTC)

[edit] changes by 218.223.87.142

I don't like to get into edit wars, but the changes by this anonymous are so strange that I felt compelled to revert to my own last version. The anon introduced the following sentence in parentheses:

(The number of "edge" molecules is proportional to n/V and the number of interior molecules is proportional to n/V also. The number of pairs of interacting molecules is thus proportional to n2/V2 so that the forces attracting the edge molecules to the interior are proportional to n2/V2. These forces give an additional contribution to the pressure on the gas proportional to n2/V2. from http://www.chem.arizona.edu/~salzmanr/480a/480ants/EQNOFST/states.html)

Edge molecules are not yet defined. What is n2? What is V2? Why a link to a website in the middle of a derivation? I will accept it if somebody improves/shortens the phrasing of this proof, but this is not it.--P.wormer 07:41, 16 May 2007 (UTC)

And two more things: (i) I took great care to follow IUPAC in distinguishing n, N, and NA and also v, V, and Vm. This went unnoticed by 218.223.87.142. (ii) The argument about the layer molecules is very subtle and took the genius of Laplace to discover it. It is not a matter of simple (N/V)2 pairs. The number of pairs of interacting molecules per volume is N2/2V. --P.wormer 09:45, 16 May 2007 (UTC)

Good that it's removed. It's a direct quote from a copyrighted page. Yes, it's attributed, but text written from the ground up "organically" on Wikipedia is many times preferable to cut and paste from copyrighted material. Invoking copyright issues often makes resolving edit wars easy. --HappyCamper 03:34, 23 May 2007 (UTC)

[edit] reversal

I reverted the change of 09:12, 8 June 2007 by 212.192.238.23. This anonynomous removed about half of the article without any explanation. Such a drastic edit needs some motivation! Although risking an edit war, I decided to go one step back (to the version of 18:47, 26 May 2007 by JAnDbot).--P.wormer 07:53, 8 June 2007 (UTC)

[edit] I make my apologies

212.192.238.23 08:14, 9 June 2007 (UTC)

I' sorry for unintentional deletions. I would like only to fix the second equation, which has inconsistensy in the definitios (not all quantaties were in per-mole based units). No deletions were assumed.

[edit] Fluid?

Why is the whole article referring to fluids when the van der Waals equation is used to calculate the pressure of gases? Maybe I am just not aware that one can use this equation for liquids as well, although the fact that R (ideal gas constant) is used and that the equation is a modification of the ideal gas equation makes me doubt that there is any application to liquids. If I am correct then perhaps using just gas instead of fluid would be a good change, since fluid can easily be interpreted as liquid as well as gas. Aurimas 09:46, 29 August 2007 (UTC)

It appears I was incorrect. These equations do apply to liquids. My apologies, please ignore the above comment. Aurimas 20:22, 10 September 2007 (UTC)

[edit] Internal Energy - monoatomic or general

Is the derivation for the internal energy (under other thermodynamic parameters) correct for a diatomic gas? I would think that the 3/2 comes from the specific heat capacity at constant volume and makes the formula applicable for monoatomic gasses only. Would it not be correct to change this to c_V? I think so, but I'm not sure I've done the math correctly. —Preceding unsigned comment added by 85.81.85.32 (talk) 11:01, 21 September 2007 (UTC)