Three charges ' +3q ', ' Q ' and ' +q ' are placed in a straight line of length ' l ' at points at distances 0, l/2 and l respectively. The value of Q in order to have the net force on +q to be zero, Q = xq. The value of x is (a) 1/4 (b) -3/4 (c) -3 (d) 4

Answer: (b). Force on +q (at l) from +3q (distance l): (3kq²)/(l²), away from +3q. Force on +q from Q (distance l/2): (kQq)/((l/2)²) = (4kQq)/(l²), same line. Net zero: 3q + 4Q = 0 ⇒ Q = -3/4q. So x = -3/4, answer (b).

Three charges ' +3q ', ' Q ' and ' +q ' are placed in a straight line… | MHT CET 2025 (23 Apr, Shift 2)

Q23MHT CET 2025 · 23 APR · SHIFT 2Medium

Electric Charge & Coulomb's Law

Three charges ' $+3q$ ', ' $Q$ ' and ' $+q$ ' are placed in a straight line of length ' $l$ ' at points at distances $0,\ \frac{l}{2}$ and $l$ respectively. The value of Q in order to have the net force on +q to be zero, $Q = xq$ . The value of $x$ is

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Answer: (b)

Force on $+q$ (at $l$ ) from $+3q$ (distance $l$ ): $\dfrac{3kq^2}{l^2}$ , away from $+3q$ .
Force on $+q$ from $Q$ (distance $l/2$ ): $\dfrac{kQq}{(l/2)^2} = \dfrac{4kQq}{l^2}$ , same line.
Net zero: $3q + 4Q = 0 \Rightarrow Q = -\dfrac{3}{4}q$ .
So $x = -\dfrac{3}{4}$ , answer (b).

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