Define electric potential at a point in terms of the work done per unit charge.
As shown in Fig. 5.1, two point charges A and B are separated by $12.0\,\text{cm}$ in a vacuum. Charge A is $+2.0 \times 10^{-9}\,\text{C}$. Point P lies on the straight 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 $x$.
Use Fig. 5.2 to determine the charge on B.
Use Fig. 5.2 to find the change in electric potential as point P moves from $x = 9.0\,\text{cm}$ to $x = 3.0\,\text{cm}$.
In Fig. 5.1, an $\alpha$-particle travels along the line joining point charges A and B. It goes from the position where $x = 9.0\,\text{cm}$ and just reaches the position where $x = 3.0\,\text{cm}$. Use your answer to (b)(ii) to calculate the speed $v$ of the $\alpha$-particle at the position where $x = 9.0\,\text{cm}$.
the charge carried by B.
the electric potential change when point P moves from the position where $x = 9.0\,\text{cm}$ to the position where $x = 3.0\,\text{cm}$.