ABSTRACT
The perfect gas is an idealization of reality. As we have seen, this idealization gives rather accurate results for equilibrium properties of most gases, but there are some properties of these gases on which it can shed no light. In particular, if we are to understand transport processes such as di&usion and conduction in real gases, collisions must be taken into account (the photon gas is an exception; photons in a vacuum do not interact with each other). A gas which is ideal in its thermodynamic properties, but in which the molecules undergo collisions, is called quasi-ideal. We conhne our attention to elastic collisions; that is, encounters between two particles in which the total kinetic energy is conserved but the direction of motion of one or both particles is altered. Such a process is called scattering.
