ABSTRACT
This chapter considers ideal systems composed of noninteracting constituents. It discusses the ideal gas of structureless molecules, the most salient feature of which is the translational kinetic energy of its point particles. The chapter aims to calculate the heat capacities for the rotational and vibrational degrees of freedom of the ideal diatomic gas. The partition function for the ideal quantum gas thus factorizes, but the factors don't pertain to individual particles—they refer to individual energy levels. The chapter discusses the thermodynamic properties of ideal quantum gases. Coulomb interactions between electrons would seemingly invalidate the assumptions of an ideal gas. The chapter explains an astrophysical application of degeneracy pressure. It also considers paramagnets—the “ideal gas” of magnetism—systems of magnetic moments that don't interact with each other, but interact with an external magnetic field. As the system is compressed (increasing the density), particles condense into the ground state, leaving the pressure unchanged.
