Four point charges `q_(A) = 2 muC, q_(B)=-5muC, q_(C)=2mu C and q_D=-5 muC` are located at the corners of a square ABCD of side 10cm. What is the force on a charge of `1muC` placed at the centre of the square?

Explanation

Forces on the charge `1 muC` at O due to the charges `-5muC and -5muC` are equal and opposite. So they cancel each other. Similarly the forces due to `+2muC and +2 muC" on "1muC` at O, also cancel each other.
So the force on a charge of `1 muC` placed at the centre of the square is zero.