Explain the difference in the behaviour of a conductor and a dielectric in the presence of an external electric field.

(N/A) In a conductor, there are free charge carriers.
When a conductor is placed in an external electric field $E_{0}$, the free charge carriers move and redistribute themselves such that the electric field due to the induced charges $(E_{in})$ opposes the external field within the conductor. This process continues until the two fields cancel each other, resulting in a net electrostatic field of zero inside the conductor.
$\therefore E_{0} + E_{in} = 0$
In a dielectric, the free movement of charges is not possible. However, the external field induces a dipole moment by stretching or reorienting the molecules of the dielectric.
The collective effect of all the molecular dipole moments results in net charges on the surface of the dielectric, which produce an internal field that opposes the external field. Consequently, the net electric field inside the dielectric is reduced but not zero.
$\therefore E_{0} + E_{in} \neq 0$
The extent of this reduction depends on the nature of the dielectric material.
A dielectric is a substance that does not allow charges to pass through it but allows electric forces to act through it. It is essentially an insulator that can be polarized by the limited displacement of charges.