Electricity Formula: Difference between E=σ/2ε0 and E=σ/ε0

[Astronomy]

Hi
I want to know what is the diffrences between E=σ/2ε0 & E=σ/ε0
of course I know we use the first for a very huge page but what about the second formula?
I saw in some solving of question when we have 2 pages infront of each others we use both of them...
I want to understand the deep meaning and usage...
pls tell me simply beacuse my profession english is not good!
thank u

 

[dauto]

The first formula is used for a thin sheet where the field lines split half in each direction. The second formula is used for the surface of a metal where all the field lines are in one side of the surface (outward), and there is no field in the other side (inward).