Question

why when we place an electric dipole at the centre of a hollow metallic sphere electric field inside the sphere is not zero? why does shifting of electrons do not take place to cancel the electric field as due to produced by dipole?

Answer 1

The electric field inside the conducting hollow metallic sphere is zero when there is no charge placed inside the sphere. The shielding effect means if a charge is placed outside the sphere, the electric field due to that charge will be zero, because on the metallic surface the charge will induced in such a manner that it cancel the effect of outer charge inside the shell.
But, if a charge is placed inside the sphere the electric field lines will be same as always and exist inside the conductor cavity.
It is possible to have a zero electric field due to a dipole, and we know that there is no point where the electric field due to a dipole is zero (not at large distance). The field due to this dipole will only be zero inside the metal of the sphere.