Heat and Thermodynamics

4.6 Finite Volume of a Molecule, Mean Free Path

In connection with Boltzmann's explanation of Maxwell's law of distribution of velocities, it was tacitly assumed that the molecules are not point masses. They possess finite, though small, volumes. As a result, the molecules cannot move indefinitely in a rectilinear path in an uninterrupted manner. They suffer collisions with other molecules lying in their path; and at each collision, their velocity and direction of motion are changed. If we observe a molecule for some time, its path would be found to change discontinuously in a zigzag fashion as shown in Fig. 4.14; otherwise, if the molecules were allowed to move indefinitely in a rectilinear path, a mass of gas contained in an open vessel would ...