Define the electric potential at a point.
Two isolated charged metal spheres $X$ and $Y$ are positioned close to each other in a vacuum. The centres of the spheres are $1.2\,\text{m}$ apart, as shown in Fig. 5.1. Point $P$ lies on the line joining the centres of spheres $X$ and $Y$ and is at a variable distance $x$ from the centre of $X$. Fig. 5.2 shows how the total electric potential $V$ due to the two spheres varies with $x$. State three conclusions that can be inferred about the spheres from Fig. 5.2. The conclusions may be qualitative or quantitative.
A proton is kept at rest on the line joining the centres of the spheres in (b) at the position where $x = 0.60\,\text{m}$. The proton is then released. Describe and explain, without calculation, the motion of the proton afterwards.