Electromagnetism: Maxwell Equations, Wave Propagation and Emission

Chapter 4

Dielectrics

Insulators or dielectrics are mediums that contain no free charges. If a dielectric is placed in an electric field, it becomes polarized. We may consider the electronic polarization as due to the displacement of the electrons within the atoms and molecules and the orientational polarization due to the alignment of the polar molecules more or less in the direction of the electric field. The polarization of dielectrics explains some of their properties, particularly the propagation properties of electromagnetic waves (reflection, refraction, dispersion, etc.). Usually, the polarization disappears if the external field is removed, but some materials, called electrets, retain their polarization. These materials, (generally organic polymers, waxes, etc.) are the electrical analogs of permanent magnets. They are currently used in electrostatic microphones for modern phones. The purpose of this chapter is to study the polarization of dielectrics, the effects of dielectrics on the electric field, the field equations, and the energy.

4.1. Effects of dielectric on capacitors

In 1837, Faraday observed that, if a capacitor is maintained under a constant potential (by keeping it connected to a battery) and is filled with a dielectric, its charge is multiplied by a factor ε_r, which is a characteristic quantity of the dielectric called relative electric permittivity. Thus, the capacitance is multiplied by ε_r. On the other hand, if an empty capacitor is charged under a potential ...