Electric Polarisation of Dielectrics

A dielectric is an insulating material that does not conduct electricity but can be polarised by an external electric field. Unlike conductors, dielectrics have no free charge carriers — their response to a field comes from the alignment of existing molecular dipoles.

Think of a dielectric like a box of tiny compass needles (molecular dipoles). Normally, they point in random directions. When you bring a magnet (external field) near, all needles rotate to align with it — this is exactly how molecules align inside a dielectric under an electric field.

Electric Polarisation is the phenomenon in which an external electric field induces or aligns molecular dipoles within a dielectric, producing equal and opposite bound surface charges on its faces.

Dielectric Constant (K) is the ratio of the applied field to the net field inside the dielectric, indicating how much the material reduces the electric field.

Dielectric Strength is the maximum electric field a dielectric can withstand without electrical breakdown (i.e., before it starts conducting).

K = \[\frac {E_0}{E}\]

K = \[\frac {\sigma}{\sigma-\sigma_i}\]

where σ = surface charge density on plates, σ_i​ = induced (bound) charge density.

E_ds​ = \[\frac {V_{max}}{d}\]​​

SI unit: V/m

• The induced charges are called bound charges (not free charges) — they cannot move through the material.
• Polarisation opposes the applied field partially, but never cancels it completely inside a dielectric.

When a dielectric slab is placed between charged parallel plates:

• The dipoles align with the applied field E₀.
• Torque acts on each dipole, given by:
  τ = pE₀ sin⁡ θ

where p = dipole moment, E₀​ = applied field, θ = angle between dipole and field.

• Induced bound charges create an internal opposing field E′.
• The net field inside the dielectric is reduced but never zero:
  E = E0 − E′

• Capacitors — dielectrics increase capacitance and charge storage.
• Piezoelectric crystals — generate voltage under mechanical stress.
• Electrets — permanently polarised dielectrics (analogous to permanent magnets).
• LCDs — rely on controlled polarisation of liquid crystal dielectrics.