(a)[2]
Suggest why the electric field strength is zero for $x<4.0\,\text{cm}$.
(b)[3]
A point charge of $+8.5 \times 10^{-9}\,\text{C}$ travels from the position where $x = 7.0\,\text{cm}$ to the position where $x = 5.0\,\text{cm}$. Use Fig. 5.1 to estimate the change in electric potential energy for this point charge.