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

  1. Q/4
  2. -Q/4
  3. Q/2
  4. -Q/2


F = q1q2/4πεr2 

For equilibrium charges, the net force should be zero.

Select a position of charge q on the line joining Q1 and Q2 and use formula of force and equate the sum of forces to zero to find the value of q.

The correct option is B.