How many electrons be placed on a spherical conductor of radius 10cm so that they produce an electric field of `0.036 NC^-1` just above the surface of the conductor?

A hollow spherical conductor of radius 2m carries a charge of 1nC . The electric potential at the centre of the spherical conductor is

A charge of +1C is placed at the centre of a spherical shell of radius 10cm. What will be the work done in moving a charge of +1muC on its surface through a distance of 5cm?