Question

Three charges 2q, Q, q are placed in a straight line of length l at points distance, 0, $\frac{1}{2}$ and l respectively. What should be ‘Q’ in order to make the net force on ‘q’ is zero.

• A. $\frac{-q}{2}$
• B. +q
• C. 2q
• D. -4q

Answer 1

The correct option is A $\frac{-q}{2}$

$\underset{E_{13}}{\to }+\underset{E_{23}}{\to }=0F_{13}=-F_{23}$
$\frac{1}{4\pi {ϵ}_0}\frac{2qq}{l^2}=-\frac{1}{2\pi {ϵ}_0}\frac{Qq}{{\left(\frac{1}{2}\right)}^2}$
$2q^2=-4Qq$
$Q=-\frac{q}{2}$