To understand the kinetics of gas, let us consider a volume V of nitrogen gas in a cube of side L as shown in Fig. 3.19. Gasses are composed of a very large number of very small particles. They are very small compared to the distance between particles. Particles are always moving rapidly in a straight line. Particles exert no forces except during collisions.

The distributions of their velocities obey Maxwell's distribution. Their r.m.s. velocity v_r.m.s is given by where k = Boltzmann's constant, T = temperature of the gas (K) and m = mass of the molecule. For example hydrogen molecule at room temperature has v_r.m.s of 1700m/sec.

The volume swept by one molecule between collisions can be expressed as . How far does a molecule travel between collisions?