Question

Two large, thin metal plates are parallel and close to each other. On their inner faces, the plates have surface charge densities of opposite signs and of magnitude $17.0 × 10^{−22} Cm^{-2}$. What is E: 
(a) in the outer region of the first plate, 
(b) in the outer region of the second plate, and (c) between the plates?

Answer 1

The situation is represented in the following figure.
                                       
A and B are two parallel plates close to each other. Outer region of plate A is labelled as $I$, outer region of plate B is labelled as $III$, and the region between the plates, A and B, is labelled as $II$.
Charge density of plate A, $σ =\, 17.0 × 10^{−22} Cm^{-2}$
Charge density of plate B, $σ = −17.0 × 10^{−22} Cm^{-2}$
In the regions, $I$ and $III$, electric field E is zero. This is because charge is not enclosed by the respective plates.
Electric field E in region $II$ is given by the relation,
                                                               $E = \frac{σ }{ ε_0}$
Where, 
$ε_0$ = Permittivity of free space = $8.854 × 10^{-12} N^{-1}C^2m^{-2}$
                                                             $E = \frac{17.0 × 10^{-22} }{ 8.854 × 10^{-12}}$
$= 1.92 × 10^{-10} NC^{-1}$
Therefore, electric field between the plates is $1.92 × 10^{-10} NC^{-1}$.