Question

Two point charges $q$ and $-4q$ are placed at points $A$ and $B$ respectively, at a distance $d$ apart as shown in the figure. The null point (the point where electric field is zero) is :

• A. $\frac{d}{3}$ to the right of $A$
• B. $\frac{d}{3}$ to the left of $B$
• C. $d$ to the left of $A$
• D. $d$ to the right of $B$

Answer 1

The correct option is A $\frac{d}{3}$ to the right of $A$

Let the third charge is placed at distance$$x$$ which is the null point.

So by using Coulombs law we get

$\frac{Kq}{x^2}=\frac{K4q}{{\left(d-x\right)}^2}$

By solving the above equation we get

$d-x=2x$

Hence the distance of the third charge is$x=\frac{d}{3}$

The magnitude can be determined by the equilibrium condition. The charge on the position$B$will exert a force towards itself and the charge on position$A$will exert force away from it. Hence the third charge should be placed in between these two charges.