Question

Two similar spheres having `+q` and `-q` and `+4q` charges are kept at a certain distance. The force acts between the two is `F`. If in the middle of two spheres, another similar sphere having `+q` charge is kept,then it experience a force in magnitude and direction as
A. Zero having no direction
B. `8F` towards `+q` charge
C. `8F` towards `-q` charge
D. `4F` towards `+q` charge

Answer 1

Correct Answer - c
Initially, force between `A` and `C F= k(q^(2))/(r^(2))`

When a similar sphere `B` having charge `+q` is kept at the mid line joining `A` and `C`, then
Net force on `B` is `F_("net")= F_(A)+F_(C )= k(q^(2))/((r//2)^(2))+(kq^(2))/((r//2)^(2))= 8(kq^(2))/(r^(2))= 8F`
(Direction is shown in figure)