Two metallic spheres of radii $1\,\, cm$ and $3\,\, cm$ are given, charges of $-1$$ \times {10^{ - 2}}$ $C$ and $5$$ \times {10^{ - 2}}\,C$ respectively. If these are connected by a conducting wire, the final charge on the bigger sphere is
AIPMT 2012, Medium
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When the given metallic spheres are connected by a conducting wire, charge will flow till both the spheres acquire a common potential which is given by Common potential,

$V =\frac{q_{1}+q_{2}}{C_{1}+C_{2}}=\frac{-1 \times 10^{-2}+5 \times 10^{-2}}{4 \pi \varepsilon_{0} R_{1}+4 \pi \varepsilon_{0} R_{2}}$

$=\frac{4 \times 10^{-2}}{4 \pi \varepsilon_{0}\left(1 \times 10^{-2}+3 \times 10^{-2}\right)}$

$=\frac{4 \times 10^{-2}}{4 \pi \varepsilon_{0} \times 4 \times 10^{-2}}.........(i)$

$\therefore $ Final charge on the bigger sphere 

$=4 \pi \varepsilon_{0} \times 3 \times 10^{-2} \times \frac{4 \times 10^{-2}}{4 \pi \varepsilon_{0} \times 4 \times 10^{-2}}(\mathrm{Using}(\mathrm{i}))$

$=3 \times 10^{-2} \mathrm\,{C}$

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