Talk:Mean free path

Does anyone know if the table under 'Mean free path in kinetic theory' is for air at sea level or at least something more specific than 'some typical values for different pressures'?

Mean Free Path Table
This table is riddled with errors. Mean Free Path gives 93 nm for STP.

When the pressure varies from 300 to 1 mBar the MFP cannot vary by 10:1

Trojancowboy (talk) 03:02, 20 November 2009 (UTC)

I can see the errors as well as this table (Vacuum range) is not in accordance with http://en.wikipedia.org/wiki/Vacuum. —Preceding unsigned comment added by 192.38.64.200 (talk) 11:42, 23 September 2010 (UTC)

There are at least 2 different ways of calculating the mean free path. The one listed on the hyperphysics site uses the diameter of the molecule for its calculations. This is not a well defined value, so many people use a calculation that uses the fluid viscosity. This gives:

λair(298K, 1 atm) = 0.0651 μm

From: Seinfeld, John H. ; Pandis, Spyros N. "Atmospheric Chemistry and Physics - From Air Pollution to Climate Change" © 2006 John Wiley & Sons page 399.

—Preceding unsigned comment added by 137.82.115.193 (talk) 18:20, 18 October 2010 (UTC)

The above value from Seinfeld and Pandis is a good estimate, but Jennings (1988) provides a more robust value of 67.8-69.0 over the range 0-100% RH at RTP. See table 5. zaiken 17:55, 20 November 2010 (UTC) —Preceding unsigned comment added by Zaiken (talk • contribs) Sorry about the unsigned stamp, tried to edit my post. The range is 67.8-68.0nm and should be reported as 68nm. zaiken 17:58, 20 November 2010 (UTC) —Preceding unsigned comment added by Zaiken (talk • contribs)

The table is not "riddled" with errors. Mean free path is a statistic and is not directly measured. In reality, collisions are random events. The billiard-ball model of kinetic theory is a very useful approximation. Quibbling over the precision of an MFP estimate is pointless, and 68nm is in the ballpark, i.e. good enough. If I use Loschmidt's number (2.68E25/m^3) for n, and pi*D^2, with D = 1 Angstrom, I estimate about 84nm for the mean free path of an air molecule. This is close enough.

John (talk) 19:06, 6 September 2019 (UTC)

Not "Riddled With Errors", Just Approximations
The Hyperphysics site has a little Java calculator, but you can do this for yourself. Take O2, with a 3.6E-10 m diameter. Assume P = 1 mbar, or about 134 Pa. Insert these values into:

l = k*T/(1.4*3.14*d2*P)

At 298 K, you get about 5E-5 m. This is in the range on the table. Granted, the table's ranges are a bit loose, but they're not absurd. In fact, since the values depend strongly on the particle diameter (a squared term) providing this large a range is probably more honest than not. I suspect the creator was trying to accommodate just this kind of uncertainty. 72.196.237.60 (talk) 04:17, 20 November 2009 (UTC)

Section "Mean free path in nuclear physics"
Since the various independent particle models require nucleons to orbit within the nucleus, there should be a section about this here. How about:

Independent particle models in nuclear physics require the undisturbed orbiting of nucleons within the nucleus before they interact with other nucleons. Blatt and Weisskopf,in their 1952 textbook "Theoretical Nuclear Physics" (p.778) wrote "The effective mean free path of a nucleon in nuclear matter must be somewhat larger than the nuclear dimensions in order to allow the use of the independent particle model. This requirement seems to be in contradiction to the assumptions made in the theory... We are facing here one of the fundamental problems of nuclear structure physics which has yet to be solved." (quoted by Norman D. Cook in "Models of the Atomic Nucleus With Interactive Software" Ed.2 (2010) Springer, in Chapter 5 "The Mean Free Path of Nucleons in Nuclei".

Any other suggestions? Thanks. --TraceyR (talk) 22:07, 10 January 2011 (UTC)

In kinetic theory
The quantity under the square root appears to be "upside down". The value given on p. 17 here is dimensionally consistent.

--Smack (talk) 17:26, 20 January 2016 (UTC)

Multiple pressure units
Do we really need four columns in the table with pressure in different units? I would think that at least one of molecules per m3 and molecules per cm3 could go. Having both is excessive. Also, the new addition of mmHg is to four decimal places. That is excessively over-precise. The rest of the table seems to be to just an order of magnitude - unless a density of 10^15/m^3 really does have a mean free path of exactly 1 km. SpinningSpark 22:02, 27 January 2016 (UTC)


 * I would eliminate the molecules/cm^3. The units in the first column are odd: hPa or mBar. Why not use either Pa or Bar? Bar is also close to atomospheres: 1 bar. The columns should be in a unit without a metric prefix; not a metric-prefixed unit. Consistent with that the column heading should be "Torr (mmHg)" rather than "mmHg (Torr)". The table does not have microns (too many pressure units yet not enough?), but mean free path context is often high vacuum where microns are used. See http://www.engineeringtoolbox.com/vacuum-converter-d_460.html Glrx (talk) 18:00, 30 January 2016 (UTC)
 * Millibars is the pressure unit commonly used on weather forecasts, so is going to be the unit most familiar to the greatest number of readers. SpinningSpark 20:06, 30 January 2016 (UTC)

Remove maintenance template on Derivation section
I have added a reference to a physics textbook in the Derivation section of the article. This is bog-standard physics. I intend to remove the maintenance template after a period of a few days if there are no objections. If there are objections, I can find more references.

John (talk) 16:59, 7 September 2019 (UTC)
 * You may take the template out. I have not confirmed that the derivation is in the book but I hope you have. --MaoGo (talk) 11:18, 8 September 2019 (UTC)


 * I own a copy of the book, and I'm a physicist. Consider it confirmed.


 * John (talk) 02:45, 17 September 2019 (UTC)
 * For those who don't own a copy of the textbook (which is not referenced at the equation level btw) can we have the step missing in :$$ \frac{I(x)-I(x+dx)}{I_0} = \frac{1}{\ell} e^{-x/\ell} dx.$$ ? Catvector (talk) 10:16, 9 May 2022 (UTC)

definition not accurate
Current definition does not seem accurate to me: ...mean free path is an average distance over which a moving particle substantially changes its direction.. I'm not a native English speaker but this definition doesn't seem to capture the fact that the distance is traveled before the change happens, it almost sounds as if the change happens at the same time the distance is traveled.

Sygz (talk) 10:52, 6 October 2021 (UTC)
 * I am not a native English speaker, either, but to me, the sentence sounds correct. Yes, the change is not necessarily a result of a single event – there could be several "weak" collisions on the way, each deflecting the particle a little bit. Furthermore, it's a mean path, i.e., after averaging over some parameters (velocity, etc). Evgeny (talk) 11:40, 6 October 2021 (UTC)

Potential factor of 2 in collision frequency / mfp
Hi all, I just read a different phenomenological derivation whereby the collision volume is multiplied by half the number density - N/(2V), the idea being that two molecules make one collision. This seems right to me, and if so, then the collision frequency and mean free path have to be adjusted accordingly. I didn't edit it because most sources don't apply this concept. But I think its correct - anyone have something to add? Thanks, ChemCatMan (talk) 01:39, 6 February 2024 (UTC)