MEAN FREE PATH

average distance an object will move between collisions. The actual distance a particle, such as a molecule in a gas, will move before a collision, called free path, cannot generally be given because its calculation would require knowledge of the path of every particle in the region. The probability (dP) that a molecule will move a distance between two points (x and x + dx) without collision is proportional to an exponential factor; that is, dP = e-x/mdx, in which e is the base of natural logarithms. The constant m is the mean free path and is the average (mean) distance traveled by a molecule between collisions. The mean free path of an oxygen gas molecule under a pressure of 1 atmosphere at 0 C is about 6 10-6 cm (2 10-6 inch).