The mean distance which a particle moves between two successive
collisions with other particles of the medium. Mean free path is inversely
proportional to the number of particles per cm3 (n), and the
collision → cross section (σ). In the case of a gas with
molecules having a diameter of d, the cross section
is equal to the area of a circle of radius d, i.e.
σ = πd2, and the mean free path is given by:
l = 1/(nσ). Taking into account the relative velocity
distribution of the colliding molecules, l = 1/(√2 . nσ).
For a gas at one atmosphere pressure and room temperature, the average distance
between molecules is roughly 3.5 × 10-7 cm, that is some 35 times the diameter of
a molecule. Taking the gas density n = 2.4 × 1014 molecules
cm-3, and a typical diameter d = 2 × 10-8 cm for
a molecule, the mean free path is 3.3 × 10-5 cm. This means that the average
distance between collisions is about 95 times the average distance between molecules.
Etymology (EN): → mean; → free;
→ path.
Etymology (PE): Puyeš, verbal noun of puyidan “to run, trot; wander,” from
Mid.Pers. pôy-, pwd- “to run;” cf. Gk. speudein “to hasten;”
Lith. spudinti; âzâd, → free; miyângin,
→ mean.
