Two fixed charges `-2Q` and `+Q` are located at points `(-3a,0)` and `(+3a,0)` respectively. Then which of the following statement is correct?

Options

A. a
B. 4a
C. 3a
D. 7a

Answer

B

Explanation

Let (x, y) be a point where potential is zero
`implies (-2q)/(4 pi e_0 sqrt((x+3a)^2y^2)+ q/(4 pi e_0 sqrt((x-3a)^2+y^2)=0`
`implies 4{x^2+9a^2_ 6ax +y^2}=x^2+9a^2+6ax+y^2 implies 3x^2+3y^2-3ax+27a^2=0`
`implies x^2+y^2-10ax+9a^2=0 implies radius =sqrt((5a)^2-9a^2)=4a`