Thank you for reporting, we will resolve it shortly
Q.
Three point charges $4q,\, Q$ and $q$ are placed in a straight line of length $L$ at points $0,\, L/2$ and
$L$ respectively. The net force on charge $q$ is zero. The value of $Q$ is
$\because$ Net force on charge, $q=0$
$\Rightarrow f_{1}+f_{2}=0$...(i)
Here, $f_{1}$ is the force on charge $q$ due to charge $4 q$
$f_{2}$ is the force on charge $q$ due to charge $Q$
$\therefore $ From eq. (i),
$\Rightarrow \frac{K(4 q) q}{L^{2}}+\frac{K Q q}{(L / 2)^{2}}-0$
$\Rightarrow 4 q^{2}+4 Q q=0$
$\Rightarrow Q=-q$
