Question

Two point charges of $+3\mu C$ and $+4\mu C$ repel each other with a force of $10N$. If each is given an additional charge of $-6\mu C$, the new force is:

• A. $2N$
• B. $4N$
• C. $5N$
• D. $7.5N$

Answer 1

Answer: (D)

$7.5N$

Here, $q_1=+3\mu C,q_2=+4\mu C,F=10N$

$q_1^{\prime }=+3-6=-3\mu C,q_2^{\prime }=+4-6=-2\mu C$

$\therefore \frac{F^{\prime }}{F}=\frac{\left(q_1^{\prime }\right)\left(q_2^{\prime }\right)}{q_1{q}_2}=\frac{(-3)\times (-2)}{2\times 4}=\frac{6}{8}=\frac{3}{4}$
$\therefore F^{\prime }=\frac{3}{4}\times F=\frac{3}{4}\times 10=7.5N$