is noted above that the charge and current create the electric and magnetic flux fields, described by the flux densities where ε0 and μ0 are the permittivity and permeability of the free space. The permittivity of any medium is its ability to store electric energy, such as a capacitor. Therefore, it is identified as the capacitance of the free space. Any dielectric material medium can store more electric energy through the mechanism of electric polarization. The electric dipole is created during the process of polarization and the total induced charge is shown as electric flux density
Likewise, the free space has also an ability to store magnetic energy. It is expressed as its permeability. Magnetic material is magnetized by the process of magnetization under the influence of an external magnetic field. Thus, magnetic material stores more magnetic energy as compared to the free space. The ability of a magnetic material to store magnetic energy is expressed through its relative permeability. The permeability of the medium is μ = μ0μr. The permittivity ε0 and permeability μ0 of the free space are the primary physical constants ε0 = 8.854 × 10−12 F/m, μ0 = 4π × 10−7 H/m. Again, for the isotropic magnetic medium, the relative permeability μr is a scalar quantity, and for an anisotropic medium, it is a tensor quantity.
It is interesting to note that the velocity of EM‐wave and the characteristic (intrinsic) impedance of the free space are given in terms of these primary constants,
(4.1.8)
A material medium with a finite conductivity dissipates energy in the form of heat. The finite conductivity of a medium is due to the presence of free charge carriers. The free space is considered as a lossless medium because it has no free charge carrier. The conduction current flows through a medium under the influence of an external electric field. The conduction current density (
where σ is the conductivity of a conducting medium. The conductivity (σ) of the isotropic conducting medium is a scalar and for an anisotropic conducting medium it is a tensor. As free space has no conductivity, there is nothing like the relative conductivity of a medium. However, sometimes the conductivity of a medium is expressed in terms of the conductivity of copper.
In summary, the electrical properties of a material are described by the relative permittivity (εr), relative permeability (μr), and conductivity (σ). A material can have all three properties at a time, or it can have one predominant property at a time. Assuming the case of one predominant property at a time, all materials are classified into three basic categories.
4.1.3 Category of Materials
Dielectric Materials
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 < εr < 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)
Magnetic Materials
The magnetic materials support magnetization, i.e. magnetic polarization by the magnetic field. It also supports magnetic displacement current due to the time‐dependent magnetic field:
(4.1.11)
A diamagnetic material has permeability μ ≤ μ0 and a paramagnetic material has permeability μ > μ0. The diamagnetic behavior is caused by induced magnetic dipole moments, creating a magnetic field that opposes the externally applied magnetic field. However, the paramagnetic property comes into existence due to the alignment of already randomly existing magnetic dipole moments in the material. The conducting ferromagnetic materials have much larger permeability due to domain formation. However, the engineered artificial magnetic materials can have relative permeability less than unity, 0 < μr < 1. These are known as the mu near‐zero (MNZ) materials [B.16]. Again, the engineered materials can also have mu negative, giving the mu negative (MNG) materials. It is discussed in section (5.5) of chapter 5.
Conductors
It supports the flow of the free charge carrier, i.e. electrons.
