Calculate Surface Charge Densities for Parallel Plates

[tigigi]

2 infinite parallel plates separated by a distance s are at the potentials zero and V0.
find the surface charge densities on the plates.

I got the answer that it first calculate E and then as

1. x=o, Ex = σ / ε, then could know σ
2. x=s, Ex = - σ / ε ---> here's what I don't know. why there's a minus sign here ?

Thank you so much ~

 

[tigigi]

There's another question in EM.

the original question is like below :

consider a spherically symmetric charge distribution ρ = ρ(r). By dividing the charge distribution into spherical shells, find the potential and the eletrica field strength in terms of ρ(r)

I got the answer that it first consider it as charge only on a shell with radius r', get E and V in terms of σ, then infinitely many this kind of shells build a sphere with charge density ρ(r), and then it shows that :

σ =[ 4π (r')^2 dr' ρ(r) ] / 4π (r')^2 = ρ(r') dr' = σ(r')

I don't get this. how come would have this equation ?
it seems σ(r') should be : total charge / surface area, and shouldn't total charge be
(4/3) π (r')^3 ρ(r) ?

Thank you.
σ =