Talk:Mean free path

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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[edit]

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 —Preceding unsigned comment added by (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 (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 (talkcontribs) 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 (talkcontribs)

Not "Riddled With Errors", Just Approximations[edit]

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. (talk) 04:17, 20 November 2009 (UTC)

Section "Mean free path in nuclear physics"[edit]

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[edit]

The quantity under the square root appears to be "upside down". The value given on p. 17 here [1] is dimensionally consistent.

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

Multiple pressure units[edit]

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 (0.99 atm). 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 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)