A dielectric is an insulating material whose charges cannot move freely through the material but can shift slightly within atoms or molecules when an electric field is applied. This response is called polarization.

Polarization

Positive and negative charge within the material separate by small amounts, creating electric dipoles or aligning existing ones.

The bound charges produced by this polarization create their own electric field, which partially opposes the applied field inside the material.

Effect on a capacitor

Placing a linear dielectric between capacitor plates increases the capacitance. If the material has relative permittivity $\varepsilon_r$, an ideal fully filled capacitor has

$$C=\varepsilon_r C_0,$$

where $C_0$ is the capacitance without the dielectric.

For a parallel-plate geometry,

$$C=\varepsilon\frac{A}{d},$$

with

$$\varepsilon=\varepsilon_r\varepsilon_0.$$

Free and bound charge

Charges deliberately placed on conductors are often called free charges, while charges associated with dielectric polarization are bound charges.

The distinction helps separate the externally supplied charge from the material's internal response.

Real dielectrics also have limits: sufficiently strong fields can cause dielectric breakdown, after which the material no longer behaves as an ideal insulator.