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Dielectrics are insulating (non-conducting) materials which transmit electric effect without conducting.
Fig. 5

Types of dielectrics

Polar dielectrics A polar molecule has permanent electric dipole moment 63639.png in the absence of electric field also. But a polar dielectric has net dipole moment zero in the absence of electric field because polar molecules are randomly oriented as shown in Fig. 6.
Fig. 6
In the presence of electric field polar molecules tend to line up in the direction of electric field, and the substance has finite dipole moment, e.g., water, alcohol, CO2, NH3, HCl, etc., are made of polar atoms/molecules.
Non-polar dielectric In non-polar molecules, each molecule has zero dipole moment in its normal state.
When electric field is applied, molecules becomes induced electric dipole, e.g., N2, O2, benzene, methane etc., are made of non-polar atoms/molecules. In general, any non-conducting, material can be called as a dielectric but broadly non-conducting material having non polar molecules referred to as dielectric.
Polarization of a dielectric slab It is the process of inducing equal and opposite charges on the two faces of the dielectric on the application of electric field (Fig. 7).
Fig. 7
  • Electric field between the plates in the presence of dielectric medium is E’ = EEi, where E = main field, and E’ = induced field.
  • Dielectric constant of dielectric medium is defined as:
Dielectric breakdown and dielectric strength If a very high electric field is created in a dielectric,. The dielectric then behaves like a conductor. This phenomenon is known as dielectric breakdown.
The maximum value of electric field (or potential gradient) that a dielectric material can tolerate without it’s electric breakdown is called it’s dielectric strength.
S.I. unit of dielectric strength of a material is V/m, but practical unit is kV/mm.

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