A charge Q is placed at the centre of the line joining two charges q and q. The system of three charges to be in equilibrium, what should be the value of Q?

Explanation

Here Q is in equilibrium as it is at the centre and net force on it is zero
For equilibrium of q, net force on it must be zero
`(1)/(4pi in_(0)) (Qq)/( (r//2)^(2)) +(1)/(4pi in_(0)) (q^(2))/(r^(2))=0`
where r is seperation between q and q
`rArr Q=(-q)/(4)`