TABLE OF CONTENTS All science is full of statements where you put your best face on your ignorance, where you say: we know awfully little about this, but more or less irrespective of the stuff we don t know about, we can make certain useful deductions.
Hermann Bondi
We are now in a position to approach the problem that we have been postponing for so long: how to deal with a many-particle system in which the individual particles cannot be distinguished from each other. The simplest and most useful example of such a system is a perfect gas, of which the ideal gas is a special case. Note the distinction, the need for which will become clear later in this chapter. [1] A perfect gas is a gas in which the energy of any one molecule is independent of the presence of the others; that is, interactions between the molecules have a negligible effect on their energy states. An ideal gas is a perfect gas whose density is sufficiently low that quantum effects can be neglected. Because this distinction is not always observed in the literature, some authors prefer to call a perfect gas a noninteracting gas. However, this terminology is somewhat misleading. Except at extremely low density, gas molecules collide frequently (see Chapter 16) and hence interact; if they did not the gas could not reach internal equilibrium. Thus even in a perfect gas there are intermolecular interactions, but they are...
