Question

Figure shows three concentric thin spherical shells A, B and C of radii a, b and c respectively. The shells A and C are given charge q and -q respectively and the shell B is earthed. The charge appearing on the outer surface of C is

• A. $\frac{b}{c}q$
• B. $q(1-\frac{b}{c})$
• C. $-\frac{b}{c}q$
• D. $-q(1-\frac{b}{c})$

Answer 1

Answer: (D)

Potential of sphere B is zero.

$K\left[\frac{q}{b}-\frac{q}{b}+\frac{q^{\prime }}{b}-\frac{q^{\prime }}{c}+\frac{q^{\prime }-q}{c}\right]=0$
$q^{\prime }=+\frac{b}{c}q$ Charge on outer surface (q′ − q) = −q $-q(1-\frac{b}{c})$