Electric Field & Field Lines question from MHT CET 2026 (18 Apr, Shift 1)

Answer: (a)

1. Opposite vertices are at equal distance $r$: $A$ and $D$ both have $+q$, $C$ and $F$ both have $-q$, so their fields at $O$ cancel.
2. Field of the five charges = field of $-q$ at $B$: $\dfrac{kq}{r^2}$, directed from $O$ towards $B$.
3. Field of $+Q$ at $E$: $\dfrac{kQ}{r^2}$, directed away from $E$, i.e. also towards $B$.
4. $\dfrac{kq}{r^2} = 3\dfrac{kQ}{r^2} \Rightarrow Q = \dfrac{q}{3}$
5. So the answer is (a) $\dfrac{q}{3}$.