Helium as a Classical Fluid

Abstract

Of all the cryogenic fluids, helium exhibits behavior that most nearly approximates that of an ideal fluid. This fact is caused mostly by the weak intermolecular potential that helium enjoys. It further manifests itself in the fact that helium has the lowest critical point of all fluids, T _c = 5.2 K, p _c = 0.226 MPa. As a result of this near ideality, much of the behavior of gaseous and liquid helium above the superfluid transition can be treated in terms of classical models. This is not to say that quantum effects do not contribute to the behavior. Rather, certain features of helium in this temperature and pressure range are controlled by a combination of physical phenomena which can be qualitatively if not quantitatively described in terms of classical models. Conversely, certain characteristics of helium, most notably that of the liquid state below the superfluid transition and also the solid state, have properties which are so determined by quantum mechanics that classical physics cannot be used in a meaningful way to interpret their behavior. The quantum aspects of helium are discussed separately in Chapter 4.