Three charges, each of +4 μC are placed at the corners B, C and D of a square ABCD of side 1 m. The electric field at the center O of the square is

Here we shall use a logical trick. If a charge of 4μC had also been placed at A thenE→A+E→B+E→C+E→D=0→⇒E→B+E→C+E→D=−E→A⇒E→B+E→C+E→D=k4×10−6(1/2)2⇒E→B+E→C+E→D=9×1094×10−6(2)⇒E→B+E→C+E→D=7.2×104 NC−1towards A