Question

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

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

Answer 1

Answer: (D)

Here, $q_1 = + 3 \mu C$, $q_2 = + 4 \mu C$, $F = 10 \,N$
$q_1' = +3 -6 = -3 \mu C$, $q_2' = + 4 - 6 = -2 \mu C$,
$\therefore \frac{F'}{F}=\frac{\left(q'_{1}\right)\left(q'_{2}\right)}{q_{1}\,q_{2}}=\frac{\left(-3\right)\times\left(-2\right)}{2\times4}=\frac{6}{8}=\frac{3}{4}$
$\therefore F'=\frac{3}{4}\times F=\frac{3}{4}\times10=7.5\,N$