With the help of neat diagrams, explain how the nonpolar dielectric material is polarised in external electric field of increasing intensity. Define polarisation in dielectrics.

Explanation

In case of non-polar dielectries the centres of both positive as well as negative charges coincide. Dipole moment of each molecule in non-polar system is zero. All those molecules which belong to this category are symmetric in nature. Examples of non-polar dielectrics are : methane, benzene etc.

`(a)` Non-polar molecules of dielectric `(E=0)`
If such a material is subjected to an external electric filed, the positive and negative charges in non-polar molecules are displaced in opposite direction. This displacement is continued till external force on constituent charges is balanced by restoring force due to internal molecular field.
The non-polar molecules thus acquire induced dipole moment. The dielectric is said to be polarised in external field as shown in the figure below. The induced dipole moments of different molecules add up and give rise to net dipole moment.

`(b)` Non-polar molecules of dielectric `(Ene0)` Polarisation of dielctric material :
Polarisation may be defined as the amount of induced surface charge per unit area or the surface density of polarisation charges appearing at right angles to applied external electric field.
Thus, Polarisation , `P=(q_(p))/(A)=sigma_(p)`
where `q_(p)` is the polarisation charges, `sigma_(p)` is the charge density of polarisation charges and `A` is the area of cross-section of dielectric.