Thick nonconducting sheets

1. Sep 3, 2007

quantum_bit

1. The problem statement, all variables and given/known data
Two very large, nonconducting plastic sheets, each 10.0 cm thick, carry uniform charge densities a,b,c,d on their surfaces. These surface charge densities have the values a= -6.00 nC, b= +5.00 nC, c= +2.00 nC, and d= +4.00 n\C.

Find the magnitude of the electric field at the point C, in the middle of the right-hand sheet.

looks like:

a----10 cm------b------12cm-----c------10cm------d
Point C is here----------------------------->

2. Relevant equations
Infinite sheet of charge field (charge)/(2*epsilon_0) N/C

3. The attempt at a solution
Well because the sheet is "Two very large, nonconducting plastic sheets," I treated them as thin infinite sheets with the distance of the thickness between them.

Adding the electric force vectors does not yield the correct answer, not sure where to go from there. I get the answer 169.491. I am not off by an order of magnitude even though it looks so. I have other problems of the same type that also give incorrect results.

I know the answer is 1.69×10^6 N/C but I am not sure how to get there.

Last edited: Sep 4, 2007
2. Sep 4, 2007

quantum_bit

Close this I figured it out, the answer I was given as "correct" was the wrong one there was another answer in the paragraph talking about a correction. This was actually noted in the paragraph I just passed over it. I also used nanoC rather than micro Columbs, the symbols looked similiar in the problem and I thought it it said n not the mu symbol.

Last edited: Sep 4, 2007
3. Jun 28, 2009

mgreene

can you explain how you did this calculation? when I use the formula for the electric field due to an infinite sheet of charge that you have entered in your first post, i get 1.129x10^5 N/C instead of 1.69x10^5

I have simply ((4x10^-6)-(2x10^-6))/(2*E_o) = 1.129x10^5

thanks

4. Jun 28, 2009

mgreene

ok i realized you must add the charges from all 4 surfaces
so 2+5-4-6= -3microcoulombs