Three identical spheres, each having a charge q and radius R, are kept in such a way that each touches the other two. The magnitude of the electric force on any sphere due to the other two is

For external points, a charged sphere behaves as if the whole of its charge is concentrated at its center.Force on A due to B,FAB=14πε0q2(2R)2=14πε0q24R2 along BA→And force on A due to C,FAC=14πε0q2(2R)2=14πε0q24R2 along CA→Now as angle between BA and CA is 60o and FAB=FAC=F∴ FA=F2+F2+2FFcos⁡60=3F =14πε034qR2