Two point charges `q` and `-q` are separated by the distance `l`, both being located at a distance `l//2` from the infinite conducting plane. Find :
(a) the modulus of the electric force acting on each charge,
(b) the magnitude of the electric field strength vector at the midpoint between these charges.

Explanation

(a) Using the concept of electrical image, it is clear that the magnitude of the force acting on each charge,
`|vec(F)| = sqrt(2) (q^(2))/(4pi epsilon_(0) l^(2)) - (q^(2))/(4pi epsilon_(0) (sqrt(2l))^(2))`
`= (q^(2))/(8pi epsilon_(0) l^(2)) (2 sqrt(2) - 1)`
(b) Also from the figure, magnitude of electrical field strength at `P`
`E = 2 (1- (1)/(5 sqrt(5))) (q)/(pi epsilon_(0) l^(2))`