Dielectrics and Polarisation

 Dielectrics

Dielectrics are insulating materials that transmit electric effects without actually conducting electricity.

Dielectrics are of two types

There are two types of dielectrics

(a)    Non-Polar Dielectrics

(b)    Polar Dielectrics

Non-Polar Dielectrics

The molecules in which the center of positive charge coincides with the center of negative charge are called non-polar molecules. Such molecules are symmetric in shape.

Examples - `O_2,  CO_2 , CH_4` etc.

Non-Polar Dielectrics

Polar Dielectrics

If the centers of positive and negative charges do not coincide because of the asymmetric shape of the molecules, then these are called polar molecules.

Examples – `HCl, H_2O\ , NH_3` etc.

Polar Dielectrics

Dielectric Polarization

When a non-polar dielectric is held in an external electric field E, the center of positive charge (protons) in each molecule is pulled in the direction of E and the center of negative charge (electrons) is pulled in a direction opposite to E.

Non-polar Molecules

 

Therefore the two centers of positive and negative charges in the molecule are separated. The molecule gets distorted and is said to be polarised.

When no external field is applied to a dielectric with polar molecules then the different permanent dipoles are oriented randomly due to thermal agitation, so the total dipole moment is zero.

 

Polar Molecule

    When an external electric field is applied, the individual dipole moments tend to align with the field. When summed over all the molecules, there appears same net dipole moment in the direction of the external field, hence the dielectric is polarised.

Electric Susceptibility (`\chi`)

It is found that the electric polarization p is directly proportional to the effective electric field E in a polarized dielectric.

` p\propto\ E_{out}` 

`p=\ \ \chi\ E_{out}`

Where `\chi` is called the electric susceptibility of the dielectric.

 

Note –

• The electric susceptibility describes the electrical behavior of a dielectric.
• For vacuum `\chi\ =\ 0`
• Effective electric field in a polarized dielectric is `E\ =\ E_{out}\ -\ E_{\text{in}}`
• A dielectric medium is used to reduce the electric field.

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