Question

Three identical large plates are placed parallel to each other at a very small separation as shown in the figure. The central plate is given a charge $Q$. What amount of charge will flow to earth when the key is closed.(The other two plates are initially neutral)

Answer 1

Since the distance between the plates are very small so upon earthing the third plate, the charge on central plate will shift to its right face as shown in figure.


Thus, $-Q$ charge will be induced on the induced on the left face of the earthed plate.
So, the final charge on this plate, $q_f=-Q$

Since initial charge on the earthed plate is zero..i.e.,$q_i=0$

The charge distribution on the plates will be as shown .So, charge flow to the earth,
$q=(q_i-q_f)$
$q=0-(-Q)$
$\therefore q=Q$

Thus, $+Q$ charge will flow to the earth.

Hence, option (c) is the correct answer.