Lecture 106: Liquid and Solid Dielectrics

In this video, I explore dielectric constants in liquids and solids. I begin by illustrating the concept of a molecular electric field, using the example of a dielectric material placed between the plates of a parallel plate capacitor. Here, the macroscopic electric field is the sum of the free charge field and the bound charge field generated in response to it. I then isolate a single dipole within the dielectric material inside an imaginary sphere. In response to the macroscopic field, the dipoles within the sphere (excluding the central dipole) induce a polarization on the surface of the sphere, known as the Lorenz field. The central dipole experiences the combined effect of the macroscopic field, the Lorenz field, and the electric field generated by the dipoles within the sphere.

In liquids and highly symmetric crystal lattices, the electric field from the dipoles within the sphere cancels out, thus contributing nothing to the molecular field. Using this molecular field, I derive the Clausius-Mossotti relation, which connects the dielectric constant with atomic polarizability.

Next, I derive an expression for estimating the atomic radius of simple monoatomic atoms. I then introduce a classical model for ionic solids, such as those in ferroelectric materials, which have a permanent dipole moment. I explain how such materials can exhibit extremely large dielectric constants when lattice symmetry is broken below the Curie temperature. This exploration provides a comprehensive look into how molecular structure and field interactions influence dielectric behavior in different states of matter.

Problem set for this lecture is here: