There are two small solid metal spheres A and B, each with the same radius, in a vacuum. Their centres are separated by 15\,\text{cm}. Sphere A carries a charge of +3.0\,\text{pC} and sphere B carries a charge of +12\,\text{pC}. Fig. 5.1 shows the layout. Point P is on the straight line joining the sphere centres and is 5.0\,\text{cm} from the centre of sphere A.
(a)[2]
Suggest why the electric field strength in each sphere is zero.
(b)[3]
Show that the electric field strength at point P is zero. Explain your working.
(c)[2]
Calculate the electric potential at P.
(d)[3]
A silver-107 nucleus $^{107}_{47}\text{Ag}$ has speed v when it is far from point P. Use your answer in (c) to calculate the least value of speed v needed for the nucleus to reach point P.
Worked solution & mark scheme
This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: “If an electric field were present, charges would move; since no movement is observed, the field is zero” …