Physics · Electrostatic Potential And Capacitance · NEET
Every dielectric is an insulator, but we use the word 'dielectric' when we care about what happens inside it in an electric field. An insulator simply does not conduct current. A dielectric is that same insulator viewed by how its molecules polarise (form tiny dipoles) when a field is applied. So the word highlights the useful behaviour, not a different material.
The bound charges move a tiny amount. In each atom or molecule, the positive nucleus is pulled one way and the negative electrons the other way by the external field E0. They do not leave the atom, they only get slightly displaced. This small displacement creates an induced dipole in every molecule. Add up billions of these and the material is polarised.
When the molecules polarise, positive bound charge collects on one face and negative bound charge on the opposite face of the slab. These bound charges set up their own field Ep that points opposite to the external field E0. So the net field inside becomes E = E0 - Ep, which is smaller than E0. This is exactly why putting a dielectric in a capacitor raises its capacitance.
Polarisation means the tiny dipoles inside the material get lined up along the field. Formally, polarisation P is the dipole moment per unit volume of the dielectric. Before the field, dipoles point randomly (or do not exist), so P = 0. After the field, they line up, so P is not zero and points along E0.
No. Bound charges stay attached to their molecules. They only appear on the surfaces because inside the slab the +end of one dipole cancels the -end of the next dipole, so only the two outer faces are left uncancelled. That is why we call them bound charges, unlike the free charges on a metal plate.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A dielectric is a non-conducting material whose molecules form induced or aligned dipoles in an electric field, reducing the net field inside it.
Free charges (in conductors) can move freely across the whole material. Bound charges (in dielectrics) are tied to molecules and can only shift a tiny distance, appearing on the dielectric surfaces.
Polarisation P is the net dipole moment per unit volume. For a linear dielectric, P = chi_e * epsilon0 * E, where chi_e is the electric susceptibility of the material.
Polarisation creates bound charges that weaken the internal field, so the same charge sits at a lower voltage. Since C = Q/V, capacitance increases by the factor K (dielectric constant).
Common examples are mica, glass, paper, and pure water. Air and vacuum are treated as the reference with dielectric constant K nearly equal to 1.
