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Behaviour of Dielectric Materials in an Electric Field

A dielectric, or an insulator, is a nonconducting material. The main difference between a conductor and a dielectric is the amount of available free electrons. In case of conductors, the valence band is partially filled with electrons and the forbidden energy gap between the conduction and valence band is very small. Electrons can easily be liberated from the conductor by addition of a very small amount of energy.

Properties of Dielectrics

The various properties of dielectric materials are as follows.
  1. Dielectrics do not have any free charges, but all charges are bound and associated with the nearest atoms.
  2. When an external electric field is applied, there is some displacement of the bound charges, thus creating small electric dipoles within the dielectric. This phenomenon is known as polarisation.
  3. The electric field inside and outside a dielectric gets modified due to the presence of dipoles.
  4. Dielectrics can store energy.

Dielectric Polarisation

In this section, we introduce the concept of polarisation in dielectric media. Dielectric polarisation may be defined as a dynamical response of a system to an externally applied electric field.
  1. Macroscopic Polarisation
    To understand the macroscopic effect of an electric field on a dielectric, we consider an atom of the dielectric consisting of
    • A negative charge -Q (electron cloud), and
    • A positive charge +Q (nucleus)
When an electric field Description: 32073.png is applied,
  • Positive charge is displaced from its equilibrium position in the direction of Description: 21508.png by the force, Description: 25792.png
  • Negative charge is displaced to the opposite direction by the force Description: 34334.png
Polarisation of a nonpolar dielectric

Thus, an electric dipole is created and the dielectric is said to be polarised.
Dipole moment is Description: 29940.png
By superposition, the distorted charge distribution is equivalent to original distribution plus dipole.
In this way, in the presence of an electric field, all dipoles within a dielectric are aligned with the direction of the field.
The induced dipole field opposes the applied field. Thus, the entire dielectric is polarised.
If there are N dipoles in a volume v, the total dipole moment is,
Description: 59977.png
  1. Microscopic Polarisation
    The type of polarisation on a microscopic scale is determined by the material. Dipoles, which are responsible for the polarisation, are of two types:
    • Induced dipoles are those materials which exhibit polarisation only in the presence of an external field.
    • Permanent dipoles are those materials (ferroelectric) which exhibit permanent polarisation.
The connection between macroscopic and microscopic polarisation is given through polarisation vector Description: 59967.png as defined below.
Polarisation of a polar dielectric
  1. Polarisation Vector Description: 59960.png
    It is the dipole moment per unit volume of the dielectric,
Description: 59951.png

Electric Displacement, Susceptibility, Permittivity, Dielectric Constant and Dielectric Strength

Electric Susceptibility (χe)
The electric susceptibility (χe) of a dielectric material is a measure of how easily it is polarised inresponse to an electric field.
Permittivity (ε)
Permittivity is a quantity that describes how an electric field affects and/or is affected by an insulating medium.
It is determined by the ability of a material to polarise in response to the field, and thereby reduce the total electric field inside the material. Thus, permittivity relates to the ability of a material to transmit (or permit) an electric field.
Dielectric Constant or Relative Permittivity (εr)
The dielectric constant is the ratio of the permittivity of a material to the permittivity of free space. It isa measure of the extent to which it concentrates electric flux lines.
It is the ratio of permittivity of the dielectric to that of the free space.
Dielectric Strength
The maximum electric field that a dielectric can withstand without breakdown is known as dielectric strength of the material.

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