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
∵ Net force on charge, q=0 ⇒f1+f2=0...(i)
Here, f1 is the force on charge q due to charge 4q f2 is the force on charge q due to charge Q ∴ From eq. (i), ⇒L2K(4q)q+(L/2)2KQq−0 ⇒4q2+4Qq=0 ⇒Q=−q