(a)[1]
Define electric field strength in terms of the force per unit positive charge on a stationary charge.
(b)
A lone metal sphere is intended for storing charge at high potential. The charge on it may be treated as a point charge at the sphere’s centre. The sphere’s radius is $25\,\text{cm}$. Electrical breakdown (a spark) happens in the air around the sphere when the electric field strength at its surface rises above $1.8 \times 10^4\,\text{V cm}^{-1}$.
(b(i))[2]
Show that the greatest charge the sphere can store is $12.5\,\mu\text{C}$.
(b(ii))[2]
Calculate the sphere’s potential for this maximum charge.