Question

Two identical charges of value $Q$ each are placed at $\left(\right.-a, \, 0\left.\right)$ and $\left(\right.a, \, 0\left.\right)$ . The coordinates of the points where the net electric field is zero and maximum are respectively -

• A. $\left(0 , \, 0\right), \, \left(0 , \, 0\right)$
• B. $\left(0 , \frac{a}{\sqrt{2}}\right), \, \left(0 , \, 0\right) \, $
• C. $\left(0 , \, 0\right), \, \left(0 , \frac{a}{\sqrt{2}}\right) \, $
• D. $\left(\frac{a}{\sqrt{2}} , \, 0\right), \, \left(0 , \frac{a}{\sqrt{2}}\right)$

Answer 1

Answer: (C)

Net electric field will be zero at origin.
At any co-ordinate $\left(0 , \, y\right)$ $E_{N e t}=2E\frac{c o s 2 \theta }{2} \, $
$=\frac{k Q y}{\left(a^{2} + y^{2}\right)^{\frac{3}{2}}}$
For maximum electric field $\frac{d E}{d y}=0$
Solving $y=\pm\frac{a}{\sqrt{2}}$