There are two equally charged identical metal spheres A and B repel each other with a force 3×10-5 N. Another identical uncharged sphere C is touched with A and then placed at the mid-point between A and B. Net force on C is

If same charges on spheres A and Bare q, then force,             F=14πε0q2r2=3×10−5NCharge on A and C after touching,              qA′=qC=q2Net force on C,F=FA+FB∴  F=14πε0(q/2)(q/2)(r/2)2−14πε0(q/2)×q(r/2)2=14πε0rq2r2−214πε0q2r2=−14πε0⋅qr2=3×10−5N