(a) (i) The electric field in a region is given by $\vec{E} = 40x\hat{i}\text{ N/C}$. Find the amount of work done in taking a unit positive charge from a point $(0, 3\text{m})$ to the point $(5\text{m}, 0)$. (ii) A charge $Q$ is distributed over two concentric hollow spheres of radii $r$ and $R$ ($> r$) such that their surface charge densities are equal. Find : (I) the electric field, and (II) the potential at their common centre.