Читать книгу Introduction To Modern Planar Transmission Lines - Anand K. Verma - Страница 147

The dielectric materials support electric polarization of bound charges and electric displacement current through it. At the micro‐level, the electric polarization creates dipole moments that appear as the permittivity of the dielectric material at the macro‐level. The permittivity is frequency‐dependent and lossy. So, permittivity is a complex quantity showing the lossy nature of dielectric materials. This is discussed in chapter 6. The relative permittivity of natural dielectric material is always positive and more than unity. However, engineered artificial dielectrics can have relative permittivity 0 < ε_r < 1. Such materials are known as the epsilon near zero (ENZ) materials. Under certain conditions, it can also acquire negative permittivity, creating the epsilon negative (ENG) medium [B.16]. It is discussed in section (5.5) of chapter 5.

The electric displacement current flowing through a dielectric medium does not involve the flow of current through free charges. It is associated with time‐dependent electric fields. The electric displacement current density is a vector quantity. It expressed as follows:

(4.1.10)