A point charge Q is placed inside a cavity within a solid isolated conducting sphere. Consider points A, B and C as shown in the figure, where the magnitudes of the electric fields are $E_A$, $E_B$ and $E_C$, respectively. The points B and C are at the same distance from the center of the solid sphere. The correct option is :
Answer: (B) $E_A \neq 0,\ E_B = E_C$
Concept. Charge $-Q$ is induced on the cavity wall and $+Q$ on the outer surface. The conductor shields the outside, so the outer charge spreads uniformly whatever the position of Q.
Step 1. Point A (in the cavity). The field of Q and the induced charge is not zero: $E_A \neq 0$.
Step 2. Points B and C (outside). The uniform outer charge acts as a point charge $Q$ at the centre, so equal distances give $E_B = E_C$.
Answer. $E_A \neq 0,\ E_B = E_C$ (Option B).
Solution by Sreeraj P, M.Sc Physics