Question

Three charges `4q, Q` and `q` are in a straight line in the position of `0. l//2` and `l` respectively. The resultant force on `q` will be zero, if `Q=`
A. `-q`
B. `-2q`
C. `-q/2`
D. `4q`

Answer 1

Correct Answer - a
The force between `4q` and q , `F_(1)= 1/(4piepsilon_(0))*(4qxxq)/(l^(2))`
The force between `Q` and `q`, `F_(2)= 1/(4piepsilon_(0))*(Qxxq)/((l//2)^(2))`
We want `F_(1)+F_(2)= 2` or `(4q^(2))/(l^(2))= (4Qq)/(l^(2))impliesQ= -q`