Define what electric potential at a point means.
Two point charges A and B are separated by $12.0\,\text{cm}$ in a vacuum, as shown in Fig. 5.1. A has charge $+2.0 \times 10^{-9}\,\text{C}$. A point P lies on the line joining A and B, and its distance from A is $x$. Fig. 5.2 shows how the electric potential $V$ at P varies with distance $x$.
Using Fig. 5.2, determine the charge on B.
Use Fig. 5.2 to find the change in electric potential as point P moves from the position where $x = 9.0\,\text{cm}$ to the position where $x = 3.0\,\text{cm}$.
An $\alpha$-particle travels along the line joining point charges A and B in Fig. 5.1. It moves from the position where $x = 9.0\,\text{cm}$ and just reaches the position where $x = 3.0\,\text{cm}$. Use your answer in (b)(ii) to calculate the speed $v$ of the $\alpha$-particle at the position where $x = 9.0\,\text{cm}$.