5. Let B be the midpoint of AC. Two point charges Q are placed at A and C. What should be the value of charge placed at B so that the system remains at equilibrium? a) –Q/2 b) –Q/4 c) +Q/2 d) +Q/4
Answer: b Explanation:  Whatever the value of q be, it will undergo equal force on both sides, hence q will be steady. Now, for stability of Q, net force on Q must be zero. 14πϵoQ2d2+14πϵoQ.q(d2)2 = 0 ⇒ Q2d2=−Qqd24 ⇒ Q = -49 ⇒ q = –Q4
