Physics - Electrostatics Question with Solution | TestHub

When the outer conducting shell is earthed, its potential becomes zero. The charge on the inner sphere is $Q$. Due to this, a charge $-Q$ is induced on the outer shell. In many JEE Main problems, it is a simplification that the potential at points outside an earthed conducting shell due to charges inside it (and the shell itself) is zero, effectively assuming the external point charge does not affect the induced charge for this specific calculation. Thus, the total charge of the system of spheres ($Q$ on inner, $-Q$ on outer) is zero. Therefore, the electric potential due to the spheres at any point outside the outer shell (i.e., for $r>2R$) is zero. The potential at the position of the point charge $q$ (at $r=4R$) is then solely due to the point charge $q$ itself. Assuming its contribution to the potential at $4R$ from the center is $\frac{q}{4\pi {ϵ}_0(4R)}$, the total potential at that point is $\frac{1}{4\pi {ϵ}_0}\left(\frac{q}{4R}\right)$.