(a)[2]
Using electric field lines, explain why the charge on an isolated spherical conductor can be regarded as a point charge at its centre for points outside the sphere.
(b(i))[3]
Two isolated protons in a vacuum are separated by a distance $x$. Calculate the ratio $\dfrac{\text{electric force between the two protons}}{\text{gravitational force between the two protons}}$.
(b(ii))[1]
Using your answer in (i), suggest why gravitational forces are ignored when finding the force between charged particles.