Question

Assertion (A) If a point charge $q$ is placed in front of an infinite grounded conducting plane surface, the point charge will experience a force.
Reason (R) This force is due to the induced charge on the conducting surface which is at zero potential.

• A. Both assertion and reason are true and reason is the correct explanation of assertion
• B. Both assertion and reason are true but reason is not the correct explanation of assertion
• C. Assertion is true but reason is false
• D. Both assertion and reason are false

Answer 1

Answer: (A)

This is the concept of electric image.
If we are asked to find the force between an infinite earthed conductor and a point charge $q$ placed at perpendicular distance from the earth conductor (see figure), then we proceed as follows.
Firstly, the conductor being earth implies $v=0$.
So, we redraw the situation in which we replace the conductor and introduce an image charge $-q$ as shown.

The force between the two charges (object chargeq and image charge $-q)$ is the electrostatic force between the infinite grounded conductor and $q$.
So, $F=\frac{1}{4\pi {\epsilon }_0}\frac{q^2}{(2I{)}^2}$
$\Rightarrow F=\frac{1}{4\pi {\epsilon }_0}\frac{q^2}{4I^2}$ (attractive in nature)