Dielectrics

A dielectric is a non-conducting material that, when placed in an external electric field, does not allow free flow of charge but instead undergoes polarisation — a slight rearrangement of internal charge that creates an induced electric field opposing the external one.

A dielectric is a non-conducting substance in which, on the application of an external electric field, induced charges appear that oppose (but do not cancel) the applied field.

A molecule in which the centres of positive and negative charge do not coincide, giving it a permanent dipole moment even without an external field (e.g., H₂O, HCl, NH₃).

A molecule in which the centres of positive and negative charge coincide, so the net dipole moment is zero in the absence of an external field (e.g., N₂, O₂, CO₂).

A dielectric made of polar molecules; in the absence of a field, molecular dipoles are randomly oriented, giving zero net dipole moment overall.

A dielectric made of non-polar molecules; an external field induces a temporary dipole moment in each molecule, aligned with the field.

1. An external electric field E_0​ is applied across the dielectric.
2. Polar dielectrics: existing molecular dipoles rotate and align with E₀; Non-polar dielectrics: field induces new dipoles.
3. Aligned dipoles create surface-bound induced charges on the dielectric faces.
4. These induced charges generate an internal field E_in opposing E₀​.
5. The net field inside the dielectric reduces to E = E₀ − E_in​.

Dielectric constant (relative permittivity):

K = \[\frac {ε}{ε_0}\]

where ε is the permittivity of the medium and ε0 is the permittivity of free space.

It can also be expressed as:

Dielectric vs Conductor vs Insulator

Polar vs Non-Polar Dielectric

Think of polar molecules as compass needles scattered randomly on a table — with no magnetic field, they point every direction (net effect: zero). Apply a magnetic field, and they all align — this is exactly how polar dielectrics behave in an electric field.

Non-polar molecules are like balanced see-saws — perfectly level with no tilt, until an external push (the field) tilts them slightly, creating a temporary imbalance (induced dipole).

• Dielectrics polarise but do not conduct; induced charges oppose (not cancel) the external field.
• Polar dielectrics: pre-existing dipoles align with the field.
• Non-polar dielectrics: dipoles are induced by the field.
• Dielectric constant K = ε/ε₀ = E₀/E; always K > 1.
• Used in capacitors to reduce field and boost capacitance.