Question

If a charge q is placed at the centre of the line joining two equal charges Q such that the system is in equilibrium then the value of q is

Answer 1

Let's see the force on one QF=kQ24a2+kqQa2F=kQ24a2+kqQa2For the system to be in equilibrium F =0kQ24a2+kqQa2=0kQ24a2+kqQa2=0q=−Q4.