# Dielectrics and Polarisation

Chapter-2|Electrostatic Potential and Capacitance| NCERT 12th Physics:

Dielectrics and Polarisation:

In a dielectric under the effect of an external field, a net dipole moment is induced in the dielectric due to molecular dipole moment, a net charge appears on the surface of the dielectric.

These induced charges produce a field opposing the external field. Induced field is lesser in magnitude than the external field. So field inside the dielectric gets reduced.

Dielectric Constant (K):

The ratio of the strength of the applied electric field to the strength of the reduced value of the electric field on placing the dielectric between the plates of a capacitor is called the dielectric constant of the dielectric medium.

Polarisation (P):

The induced dipole moment developed per unit volume in a dielectric slab on placing it in an electric field is called polarization.

If P is induced dipole moment required by a atom of the dielectric and N is the number of atoms per unit volume, then polarization is given by

P = Np

The induced dipole  moment acquired by atom is found to be directly proportional to the reduced value of electric field and is given by

P =α ε₀E

Where α is called constant of proportionality and is called atomic polarisability.

Electric susceptibility (χ):

The polarization density of a dielectric slab is directly proportional to the reduced value of the electric field and it is given by

Dielectric Strength:

The maximum electric field that a dielectric can with stand without breakdown is called its dielectric strength.

For air it is about 3×10⁶ V/m.

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