State one similarity and one difference between the force fields produced by an isolated point charge and by an isolated point mass.
An isolated solid metal sphere A of radius $R$ carries charge $+Q$, as shown in Fig. 5.1. Point $P$ is $2R$ from the surface of the sphere. Find an expression involving $R$ and $Q$ for the electric field strength $E$ at point $P$.
A second identical solid metal sphere B is now brought near sphere A. The centres of the spheres are separated by a distance $6R$, as shown in Fig. 5.2. Point $P$ is halfway between spheres A and B. Sphere B has charge $-Q$. Explain why the magnitude of the electric field strength at $P$ is the sum of the magnitudes of the field strengths due to each sphere.
Explain why the electric field strength at point $P$ due to the charged metal spheres is not, in practice, equal to $2E$, where $E$ is the electric field strength determined in (b).