A point mass with charge is placed in a vacuum. A proton moves directly towards the mass, as shown in Fig. 4.1. When the distance between the mass and the proton is $r$, the electric potential energy of the system is $U_p$. Fig. 4.2 shows how the potential energy $U_p$ varies with $r$.
(a(i))[2]
Use Fig. 4.2 to state, with explanation, whether the mass is positively or negatively charged.
(a(ii))[2]
At a point on the graph in Fig. 4.2, the gradient is $G$. Show that the electric field strength $E$ at this point due to the charged point mass is given by $Eq = G$, where $q$ is the charge at this point.
(b)[4]
Use the expression in (a)(ii) together with Fig. 4.2 to determine the electric field strength at a distance of $4.0\,\text{cm}$ from the charged point mass.
Worked solution & mark scheme
This 8-mark question has a full step-by-step worked solution and mark scheme. One marking point: “As $r$ gets smaller, the energy drops / work is done” …