Basic Knowledge of Polarisation of Dielectrics

Physics

definition

Polarisation

When an electric field is applied there is small displacement of bound charges creating small  electric dipoles within the dielectric. This phenomenon is called Polarization.
The polarisation of a region is defined as the dipole moment per unit volume.
For linear isotropic dielectrics, it is given by 

where is the dielectric susceptibility of the dielectric medium

definition

Electric susceptibility

Polarisation in a dielectric is given by:
where
Electric susceptibility of the medium
External electric field
Electric susceptibility is related to the molecular properties of the substance.
It is also given by:

where dielectric constant of the medium

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Dielectrics-Polar and Non-polar Dielectric Molecules

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Important Questions

Two identical charged spheres are suspended by strings of equal lengths. The strings make an angle of with each other. When suspended in a liquid of density 0.8 , the angle remains the same. lf density of the material of the sphere is 1.6 , the dielectric constant of the liquid is:

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Define dielectric constant of a medium. What is its . unit?

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Dielectric constant for a metal is:

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What are polar and non polar dielectric ,give one example each other ?Define di-electric constants.

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Define the relative permittivity of a medium.

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The ratio of the forces between two small spheres with constant charges, in air and in a medium of dielectric constant , is

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The dielectric constant of an insulator can be

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small drops of mercury each of radius and change coalesce to from a big drop. The ratio of the surface charge density of each small drop with that of big drop is:

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Answer carefully:

(a) Two large conducting spheres carrying charges and are brought close to each other. Is the magnitude of electrostatic force between them exactly given by , where is the distance between their centres?

(b) If Coulomb's law involved 1/ dependence (instead of 1/),would Gauss' law be still true ?

(c) A small test charge is released at rest at a point in an electrostatic field configuration. Will it travel along the field line passing through that point?

(d) What is the work done by the field of a nucleus in a complete circular orbit of the electron? What if the orbit is elliptical?

(e) We know that electric field is discontinuous across the surface of a charged conductor. Is electric potential also discontinuous there?

(f) What meaning would you give to the capacitance of a single conductor?

(g) Guess a possible reason why water has a much greater dielectric constant (= 80) than say, mica (= 6).

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A plane electromagnetic wave in a non magnetic dielectric medium is given by with distance being in meter and time in seconds. The dielectric constant of the medium is:

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