Question

Two small identical spheres having charges $+ \, 10\text{μC}$ and $- \, 90\text{μC}$ attract each other with a force of $F$ newton. If they are kept in contact and then separated by the same distance, the new force between them is -

• A. $\frac{F}{6}$
• B. $16F$
• C. $\frac{16 F}{9}$
• D. $9F$

Answer 1

Answer: (C)

$F=\frac{k \left(10\right) \left(90\right)}{r^{2}} \, \ldots \ldots ..\left(i\right)$
charge on each when contact is made
$q_{1}′=q_{2}′=\frac{10 + \left(- 90\right)}{2}=-40\mu C$
$\therefore $ New force $F'=\frac{k \left(40\right) \left(40\right)}{r^{2}}$
$\therefore \, \frac{\textit{F} \, ′}{F}=\frac{40 \times 40}{10 \times 90}=\frac{16}{9}$
$\therefore F^{′}=\frac{16}{9}F$