Electrostatic Shielding
Dielectrics
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The Electrostatic potential is constant throughout the volume of the conductor and is equal to its value on surface.
Since, conductor has no tangential component; no work is done in moving charge within conductor and on its surface. Hence the potential is constant.
Electrostatic Shielding
Electric field inside a cavity in a conductor is always zero. Even if the conductor is charged or charges are induced on a neutral conductor by an external field, all charges reside only on the outer surface of the conductor. Hence, the any cavity of any shape and size is always shielded from outer electric influence. This is called electrostatic shielding.
Dielectrics
Dielectrics are non-conducting substances having negligible number of charge carriers. In presence of an external electric field, dipole moments are induced in dielectrics by stretching and re-orienting the molecules of the dielectric. The collective dipole moment is the net charge on the surface of dielectric which opposes and reduces( does not cancel) the external field.