Question

Four point charges $-Q,-q,2q$ and $2Q$ are placed, one at each corner of the square. The relation between $Q$ and $q,$ for which the potential at the centre of the square is zero is

• A. $Q=q$
• B. $Q=\frac{1}{q}$
• C. $Q=-q$
• D. $Q=\frac{-1}{q}$

Answer 1

The correct option is C $Q=-q$

Since it is a square, distance of each charge from the centre is same.
Let us say this distance is $d$.
So, potential at centre,
$V=\frac{K(-Q)}{d}+\frac{K(-q)}{d}+\frac{K\left(2q\right)}{d}+\frac{K\left(2Q\right)}{d}$
$\Rightarrow \frac{KQ}{d}+\frac{Kq}{d}=0$

$\therefore Q=-q$