ABSTRACT
This chapter develops one of the earliest and most successful theories of matter: the so-called ideal gas theory. This theory envisages a gas as being a collection of molecules whose average kinetic energy is so large that the potential energy of interaction between the molecules is unable to hold them together. The simplest model of a real gas is known as the ideal or perfect model of a gas. An elastic collision with the wall imparts momentum to the wall. After a collision, molecules will head off into the box, bounce off the other wall, and eventually return to the same wall for another collision. Colloquially, a degree of freedom of a molecule is a ‘thing it can do’ or a ‘way it can possess energy’. Technically this corresponds to an independent ‘squared term’ in the expression for the energy of a molecule.
