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  4. Two concentric conducting thin spherical shells A and B having radii rA and rB(rB>rA) are charged to QA and -QB( |QB|> |QA|). The electrical field along a line. (passing through the center) is :

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Q. Two concentric conducting thin spherical shells $A$ and $B$ having radii $r_{A}$ and $r_{B}\left(r_{B}>r_{A}\right)$ are charged to $Q_{A}$ and $-Q_{B}\left(\left |Q_{B}\right|>\left |Q_{A}\right|\right)$. The electrical field along a line. (passing through the center) is :

AIIMSAIIMS 2005

Solution:

As charge on shell $B$ is negative so, potential inside $B$ is constant and negative. Also due to positive charge, potential inside $A$ is positive and constant and greater than that of $B$. Therefore, as $r$ increases potential decreases and as $r_{A}$ is crossed, potential becomes negative. At the surface of shell $B$, it is more negative. This variation is represented in graph (c).
This problem is considered as wrong, because question asked is for field and graphs are given by potential.