# Gauss' law which formula to use

1. Jul 21, 2011

### teknonjon

1. The problem statement, all variables and given/known data

how do I know which equation to use for electric field? E=a/2e0 or E=a/e0 when a = surface charge density.

2. Relevant equations

3. The attempt at a solution

2. Jul 21, 2011

### cupid.callin

first one is for single surface of a charged sheet ... when it is accompanied by another it changes to the second one (therefore it is twice of it)

3. Jul 23, 2011

### teknonjon

I looked in my physics book and they use the formula E=a/2e0 when there is only one nonconducting sheet. The picture they show is a cylinder which is the Gaussian surface going through the sheet with both ends outside of the sheet. When they use the formula E=a/e0 half of the Gaussian surface is within the sheet so only one end of the cylinder is sticking ouside the sheet. I don't understand how making the changing the Gaussian surface like this would result in one answer for the electric field being only half the other. Shouldn't the electric field be the same no matter what you use for the Gaussian surface?