If on the concentric hollow spheres of radii $r$ and $R( > r)$ the charge $Q$ is distributed such that their surface densities are same then the potential at their common centre is
$\frac{{Q({R^2} + {r^2})}}{{4\pi {\varepsilon _0}(R + r)}}$
$\frac{{QR}}{{R + r}}$
Zero
$\frac{{Q(R + r)}}{{4\pi {\varepsilon _0}({R^2} + {r^2})}}$
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Consider a solid insulating sphere of radius $R$ with charge density varying as $\rho = \rho _0r^2$ ($\rho _0$ is a constant and $r$ is measure from centre). Consider two points $A$ and $B$ at distance $x$ and $y$ respectively $(x < R, y > R)$ from the centre. If magnitudes of electric fields at points $A$ and $B$ are equal, then