Note:  Do not rely on this information. It is very old.

Free Path

Free Path, in molecular Physics. The kinetic theory of gases regards each molecule of gas as being endowed with a definite speed, and as rushing about in an irregular course dependent on the number of collisions it has with other similar molecules. It travels in straight lines, which alter their direction when oblique impact occurs. If the mass of gas be at a definite temperature, the speeds of the constituent molecules will all approximate closely to a definite average; and though each is behaving differently, the length of straight-line path between successive collisions will also approximate to a definite average length. In dealing with the mass of gas as a whole the value of these averages is required, rather than the individual values for a given molecule. They are called respectively the velocity of mean square, and the mean free path, for the given gas.