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N 2
z   n    vr2  (1.12.1)
t
The last formula was obtained under the supposition that only one
molecule moves. If we take into account the motion of all molecules and
define the relative molecules velocity we'll obtain mean frequency of

molecules collisions in gas per unit time

2
z   n  2    vr2  (1.12.2)

Certainly, real molecule isn't a geometrical sphere, therefore, in
molecular physics instead r2 substitute effective molecular diameter
 as minimum distance on which the centres of two molecules

are approach each other drawn at a collision therefore
2
z   n  2  v  (1.12.3)

For the most gases under normal conditions

9
 z  10  10 10 1 .
s

The distance a given molecule travels between the collisions differs
considerably from collision to collision (Fig. 2.1.1), but its average value

is again well defined. The average length of the path l over a large
number of collisions is called the mean free path.

So the mean distance that a molecule passes per time t equals to
 v t  and z collisions occur for this time, hence,

v  1
l   (1.12.4)
z  2 n     2

When the mean free path becomes comparable with the linear size of a

vessel it is said that the technical vacuum takes place. In this case the
molecule does not collide with other molecules when moving from one
wall of the vessel to another. The physical vacuum means a complete

absence of molecules.

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