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# Figure shows two conducting thin concentric shells of radii r and 3r . The outer shell carries charge q . Inner shell is neutral . Find the charge that will flow from inner shell to outer after the switch S is closed .

$(a)\;\large\frac{q}{3}\qquad(b)\;-\large\frac{q}{3}\qquad(c)\;zero\qquad(d)\;\large\frac{q}{2}$

Explanation :
Let $\;q^{|}\;$ be the charge on inner shell after switch is closed
$V_{inner}=V_{outer}$
$\large\frac{k\;q^{|}}{r}+\large\frac{k\;(q-q^{|})}{3r}=\large\frac{k\;q^{|}}{3r}+\large\frac{k\;(q-q^{|})}{3r}$
$q^{|}=0\;.$