Question

What is meant by dielectric polarisation? Why does the electric field inside a dielectric decrease when it is placed in an external field?

Answer 1

     Dielectric materials are non conductors generally. An example is glass. Normally dielectric materials have no charge on them. When they are placed in an external electrostatic field then there is a disturbance to the electrons and protons in the atoms.

   The electrons going around nucleus in a circular orbit tilt away from the direction of the magnetic field. That circular orbit become elliptic, with the center not at the nucleus. This is due to the force of external electrostatic field on the electrons.

   Thus the atom becomes a small dipole, one end being the nucleus and the other end being the center of electron orbit. Same thing happens to all atoms (molecules).

   Thus the dielectric material gets polarised (charged partially).