The Classical Ideal Gas

This chapter provides an introduction to an important model system in classical statistical mechanics. The classical ideal gas is defined, and it is explained why it is useful as an example in an introduction to the concept of entropy. What distinguishes an ‘ideal’ gas from a ‘real’ gas is the absence of interactions between the particles. Although an ideal gas might seem to be an unrealistic model, its properties are experimentally accessible by studying real gases at low densities. Since even the molecules in the air you are breathing are separated by an average distance of about ten times their diameter, nearly ideal gases are easy to find.