In a hollow conductor and in a spherical shell the charges reside on its surface and there is no electric field inside it.Please tell me the reason behind it.
In all conductors, charges reside on the surface.
The reason for this is that conductors have free electrons, that is, the electrons are loosely attached to the nucleus of the atoms in the conductors.
When placed in external electric field, the electrons migrate to one side of the conductor and an electric field is created inside the conductor itself due to separation of positive and negative charges, as is evident from the pic. In case of conductors, this electric field is equal to that of the external electric field and hence neutralises the external field.
So the net field inside the conductor is zero.