A solid copper ball of radius r_i and carrying charge -q is placed inside a hollowed silver sphere of radius r_o and charge +q. Insulators are used to hold the copper sphere in place inside the silver sphere. Use E(r) = -∂V/∂r = (-∂/∂r)(kq/r) = kq/r² and the expression for electric potential between the two spheres (take electric potential to be zero at infinity) to derive an expression for the electric field magnitude between the two spheres. Express E(r) you derived using V_io, the potential of the copper ball relative to the silver shell.