(a)[1]
Define electric field strength in words.
(b(i))[2]
An isolated metal sphere is intended for storing charge at high potential. Treat the stored charge as though it were a point charge at the centre of the sphere. The sphere has a radius of $25\,\text{cm}$. Electrical breakdown (a spark) in the air around the sphere takes place when the electric field strength at the sphere’s surface rises above $1.8 \times 10^4\,\text{V cm}^{-1}$. Show that the largest charge that may be stored on the sphere is $12.5\,\mu\text{C}$.
(b(ii))[2]
Calculate the sphere’s potential for this maximum charge.