Electric Field Strenght on a point due to surface charge from an infinte sheet

[Failbot]

Homework Statement

A large, thin, non-conducting sheet has a surface charge density of 0.3540 \muC/m^{2}. Determine the magnitude of the electric field at a distance of 6.40 cm from the sheet.

Homework Equations

Electric Field of Charged Disc
E=\frac{\sigma}{2e\ =\ 1.602176462(63)\ \times\ 10^{-19}\ C}(1-\frac{z}{\sqrt{z^{2}+R^{2}}})

Infinite sheet
E=\frac{\sigma}{2e}

The Attempt at a Solution

I have no idea what to do here. I assume I use the infinite sheet equation but how would I use the 6.4 cm to find the field strength there? I feel like maybe I have to use the equation for field strength E=\frac{kQ}{r^{2}} some how but I have no ideas.


Sorry for the sloppy post this is my first post on these boards...Help please.

 

[stevenb]

The Attempt at a Solution

I have no idea what to do here...
Infinite sheet
E=\frac{\sigma}{2e}

If you have a formula you believe is correct, why not use it? Sure, you can't substitute the distance into the formula, but that is because the formula says that the field is approximately constant for distances small compared to the expanse of the slab, so distance does not matter.

An analogous question in mechanics would be, "what is the gravitational field 1 meter off the ground of the Earth?". Then someone else might ask, "what is the gravitational field 100 meters off the ground of the Earth?". Hopefully the fact that someone threw in the exact height, even though it is not critical, would not prevent you from answering the question.