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
[NEET-2012 (Pre)]

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

Answer 1

The correct option is C $Q=-q$


Given, $V_P=0$

$\Rightarrow \frac{K(-Q)}{l}+\frac{K(-q)}{l}+\frac{K\left(2q\right)}{l}+\frac{K\left(2Q\right)}{l}=0$

$\Rightarrow \frac{K}{l}(-Q-q+2q+2Q)=0$

$\Rightarrow q+Q=0$

$\therefore Q=-q$

Hence, option $\left(C\right)$ is correct.