Figure shows three large metallic plates with chrges `-Q, 3Q` and `Q` respectively. Determine the final charges on all the surfaces.

Explanation

We assume that charge on surface 2 is x. Following conservation of charge, we see that surfaces 1 has charge `(-Q-x)`. The electric field inside the metal plate is zero. So, field at P is zero.
Resultant field at P
`E_(P)=0`
`implies (-Q-x)/(2Aepsi_(0))=(x+3Q+Q)/(2Aepsi_(0))` or `-Q-x=x+4Q`
`:. x=(-5Q)/2`

Note : We see that charges on the facing surfaces of the plates are of equal magnitude and opposite sign. This can be in general proved by Gauss theorem also. Remember this, it is important result. Thus the final charge distribution on all the surface is as shown in figure :