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Q. Two identical small conducting sphere having charges $Q_1$ and $Q_2$ with ($Q_2 >>> Q_1$). The spheres are at ‘d ’ distance apart. The force they exert on each other is $F_1$. The spheres are made to touch each other and then separated by distance $d$. The force they exert on each other is $F_2$ . Then $\frac {F_1}{F_2} \,is$

Solution:

Before touching
$F_1 = \frac {1}{4\pi \epsilon_o} \frac{Q_1Q_2}{d^2}$
After touching charge will divide equally.
$Q{′}_1$ = $Q{′}_1$ = $\frac {Q_1+Q_2}{2}\simeq \frac{Q_2}{2}$
$F_2 = \frac {1}{4\pi \epsilon_o} \frac {Q^2_2}{4d^2}$
$\frac{F_{1}}{F_{2}}=\frac{\frac{1}{4\pi\varepsilon_{o}}\frac{Q_{1}Q_{2}}{d^{2}}}{\frac{1}{4\pi\varepsilon_{o}}\frac{Q^{2}_{2}}{4d^{2}}} = \frac{4Q_{1}}{Q_{2}}$