What is the primary difference between a conductor and a dielectric in an electric field?

The primary difference lies in the mobility of charges. In a conductor, free electrons can move freely throughout the material. When an electric field is applied, these free electrons redistribute themselves almost instantaneously to cancel out the internal electric field, making the net field inside a conductor zero. In contrast, a dielectric material does not have free charge carriers. Instead, its constituent molecules undergo polarisation, where charges are only slightly displaced or dipoles align. This creates an internal opposing field, but it only *reduces* the net electric field inside, it does not completely cancel it out.

How does the dielectric constant (K) relate to the electric field inside a dielectric?

The dielectric constant $K$ quantifies the extent to which a dielectric material reduces the electric field within itself. If an external electric field $E_0$ is applied, the net electric field $E$ inside the dielectric material becomes $E = E_0/K$. This means that the electric field inside the dielectric is $K$ times weaker than the field would be in a vacuum under the same conditions. A higher dielectric constant indicates a greater reduction in the electric field and thus a greater ability to store electrical energy in a capacitor.

Can a dielectric material be polarised permanently?

While the polarisation discussed in the context of an external electric field is typically temporary (it disappears when the field is removed), some materials can exhibit permanent polarisation. These are called **electrets**. Electrets are the electrical analogues of permanent magnets; they possess a persistent electric dipole moment. This can be achieved by heating certain dielectric materials above a critical temperature, applying a strong electric field, and then cooling them down in the presence of the field. The aligned dipoles then become 'frozen' in place.

What is dielectric strength, and why is it important?

Dielectric strength is the maximum electric field intensity that a dielectric material can withstand without undergoing electrical breakdown. Beyond this critical field, the material loses its insulating properties and starts conducting electricity, often accompanied by a spark or arc. It's crucial for capacitor design and insulation applications, as it determines the maximum voltage a device can safely operate at. For example, air has a dielectric strength of about $3 \times 10^6 \text{ V/m}$.

How does polarisation affect the energy stored in a capacitor?

When a dielectric is introduced into a capacitor, its capacitance increases by a factor of $K$. The energy stored in a capacitor is given by $U = \frac{1}{2}CV^2 = \frac{Q^2}{2C}$. If the capacitor is connected to a battery (constant voltage $V$), then $U = \frac{1}{2}(KC_0)V^2 = K U_0$, meaning the stored energy increases. If the capacitor is disconnected from the battery (constant charge $Q$), then $U = \frac{Q^2}{2(KC_0)} = \frac{U_0}{K}$, meaning the stored energy decreases. The change in energy depends on whether the capacitor is isolated or connected to a source.

← ALL TOPICS

Physics

NEXT TOPIC →

Capacitor and Capacitance

Polarisation

Physics

NEET UG

Version 1Updated 22 Mar 2026

Explore This Topic

Definition Deep Explanation Core Principles NEET Importance Problem Strategy Practice Problems MCQ Practice Quick Revision

Polarisation, in the context of dielectrics, refers to the phenomenon where the constituent molecules of a dielectric material, when subjected to an external electric field, either develop induced electric dipole moments (in non-polar molecules) or align their pre-existing permanent electric dipole moments (in polar molecules) with the direction of the external field. This collective alignment or …

Quick Summary

Polarisation is the phenomenon where dielectric materials, when placed in an external electric field, develop or align electric dipole moments. This occurs in two ways: non-polar molecules (like $O_2$ ) develop induced dipoles due to charge separation, while polar molecules (like $H_2O$ ) align their pre-existing permanent dipoles.

This collective alignment creates an internal electric field within the dielectric that opposes the external field, thereby reducing the net electric field inside the material. The extent of this reduction is quantified by the dielectric constant $K$ , where the net field $E = E_0/K$ .

The polarisation vector $\vec{P}$ represents the net dipole moment per unit volume. The electric susceptibility $\chi_e$ describes how easily a material polarises, and it's related to $K$ by $K = 1 + \chi_e$ .

This property is fundamental to increasing the capacitance of capacitors and providing electrical insulation.

Vyyuha

Your 6-Month Blueprint, Updated Nightly

AI analyses your progress every night. Wake up to a smarter plan. Every. Single.…

Key Concepts

Polarisation Vector (

\vec{P}

)

The polarisation vector $\vec{P}$ is a macroscopic quantity that describes the average electrical response of…

Electric Susceptibility (

\chi_e

)

Electric susceptibility, $\chi_e$ , is a dimensionless material property that quantifies how susceptible a…

Dielectric Constant (K or

\epsilon_r

)

The dielectric constant, $K$ (also known as relative permittivity $\epsilon_r$ ), is a crucial dimensionless…

Dielectric — Insulator that polarises in E-field.

Polarisation (P) — Net dipole moment per unit volume. Unit:

C/m^2

Polar Molecules — Permanent dipoles (e.g.,

H_2O

). Align in E-field.

Non-polar Molecules — Induced dipoles (e.g.,

CO_2

). Form in E-field.

Electric Susceptibility ($\chi_e$) — Material's ease of polarisation.

\vec{P} = \epsilon_0 \chi_e \vec{E}

Dielectric Constant (K) — Factor of E-field reduction.

K = 1 + \chi_e

. For vacuum,

K=1

Effect on E-field —

E = E_0/K

Effect on Capacitance —

C = K C_0

Constant V (Battery connected) —

V

constant,

E

constant,

C \uparrow K

Q \uparrow K

U \uparrow K

Constant Q (Battery disconnected) —

Q

constant,

C \uparrow K

V \downarrow K

E \downarrow K

U \downarrow K

Polarization Decreases Electric Field, Increases Capacitance.

Polar Molecules Align, Non-polar Molecules Induce.

K = 1 + Chi-E (K is 1 plus susceptibility).

Constant Voltage: Charge, Capacitance, U (Energy) Increase. Constant Q (Charge): Voltage, Electric Field, U (Energy) Decrease.

← ALL TOPICS

Physics

NEXT TOPIC →

Capacitor and Capacitance