Talk:Mean free path
Physics Start‑class Mid‑importance | ||||||||||
|
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)
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)