Question

A charge $Q$ is placed at the centre of the line joining two point charges $+q$ and $+q$ as shown in figure, The system is in equilibrium. The ratio of charges $Q$ and $q$ is

• A. $4$
• B. $1/4$
• C. $-4$
• D. $-1/4$

Answer 1

The correct option is D $-1/4$

For the system to be in equilibrium, net force on $a=0$ or $\frac{Qq}{4\pi {\epsilon }_0{x}^2}+\frac{qq}{4\pi {\epsilon }_0{\left(2x\right)}^2}=0$
or $Q=\frac{-q}{4}$