Dielectrics and Polarisation

A dielectric is a non-conducting substance that has few free charge carriers. When a dielectric is placed in an external electric field, charges do not flow freely through it as they do in a conductor. Instead, the molecules of the dielectric respond by developing or aligning dipole moments, which reduces the effective electric field inside the material.

Important Point

In a conductor, the electric field inside becomes zero in electrostatic equilibrium, but in a dielectric, the field is only reduced, not completely cancelled.

A dielectric is an insulating material in which free charge carriers are absent or negligible.

An electric dipole consists of two equal and opposite charges separated by a small distance.

A polar molecule is a molecule that has a permanent dipole moment even in the absence of an external electric field.

Examples: HCl, H2O.

A non-polar molecule is a molecule whose centres of positive and negative charge coincide, so its net dipole moment is zero in the normal state.

Examples: O2, H2.

Polarisation is the electric dipole moment per unit volume of a dielectric.

Electric susceptibility \[\chi_e\] is a property of a dielectric that measures how easily it gets polarised in an external electric field.

Quick Interpretation

A conductor behaves like a material with mobile charges, whereas a dielectric behaves like a material with bound charges that can only shift slightly. This small internal charge separation creates dipoles and leads to polarisation.

Non-polar Molecules

In a non-polar molecule, the centres of positive and negative charges coincide, so the molecule has no permanent dipole moment in the normal state. When an external electric field is applied, the charge centres shift slightly in opposite directions, producing an induced dipole moment.

Polar Molecules

In a polar molecule, the centres of positive and negative charges do not coincide, so the molecule already possesses a permanent dipole moment. In the absence of an external field, these dipoles are randomly oriented due to thermal motion, making the net dipole moment of the sample nearly zero. When an external field is applied, the dipoles tend to align with the field.

Comparison Table

Memory Aid

• Non-polar molecules: no dipole at first, dipole appears after the field is applied.
• Polar molecules: dipole already exists, the field mainly aligns it.

When a dielectric is placed in an external electric field, its molecules develop a net dipole effect. This may happen in two ways:

1. Induced polarisation in non-polar molecules: the field slightly separates positive and negative charge centres.
2. Orientation polarisation in polar molecules: permanent dipoles rotate and align partially with the field.

Flow of the Process

External electric field applied

→ molecular charges shift, or dipoles align
→ net dipole moment develops in the dielectric
→ dielectric becomes polarised
→ induced charges appear on opposite surfaces
→ internal field opposes external field
→ resultant field inside decreases.

Simple Analogy

A polar molecule in an electric field behaves somewhat like a tiny compass needle in a magnetic field: it tends to turn and align in a preferred direction.

For a linear isotropic dielectric, polarisation is directly proportional to the electric field:

Consider a rectangular dielectric slab placed in a uniform electric field. When the slab gets polarised, positive and negative bound charges appear on the two opposite faces normal to the field direction. These induced surface charges are represented by positive and negative surface charge densities.

Key Result

A uniformly polarised dielectric has:

• no net volume charge density in the interior.
• induced surface charges on opposite faces.
• an internal electric field due to these bound charges that opposes the external field.

Final Effect

The resultant electric field inside the dielectric becomes less than the applied external field.

• The dielectric in a capacitor increases capacitance because it reduces the effective electric field between the plates.
• Water is a polar substance because its molecules have permanent dipole moments.
• Many insulating materials used in electrical devices behave as dielectrics because they do not allow free movement of charge but do respond to electric fields.