Question

Electrostatic force between two small spheres at separation '$r$' is '$F$'. Now both spheres are touched and again placed at separation '$r$', then electrostatic force between them becomes $\frac{9 F }{8}$. Then ratio of their initial charges is :-

• A. $\frac{3}{4}$
• B. $2: 1$
• C. $\frac{3}{5}$
• D. $\frac{7}{2}$

Answer 1

Answer: (B)

$F =\frac{ KQ _{1} Q _{2}}{ r ^{2}}$
After touching $\frac{9 F }{8}=\frac{ K \left( Q _{1}+ Q _{2}\right)^{2}}{4 r ^{2}}$
$\Rightarrow \frac{9}{8}=\frac{\left(Q_{1}+Q_{2}\right)^{2}}{4 Q_{1} Q_{2}} $
$\Rightarrow \frac{Q_{1}}{Q_{2}}=\frac{1}{2}$