The positive point charges, each equal to +q, are placed at corners A, B and C of a square of side a. The magnitude of the force experienced by a point charge +Q placed at corner D is
qQ4πϵ0a2(√2+12)
Refer to Fig. 20.41 Forces FA,FB and FC exerted on charge +Q by charges at A, B and C respectively are shown.
FA=FC=14πϵ0.qQa2andFB=14πϵ0.qQ2a2
F' is the resultant of FA and FC.
F' by using the parallelogram of vector addition is:
F' = √2qQ4πϵ0a2
The net force on Q is F"=F′+FB. It is to see that the correct choice is (c).