# Homework Help: Electric Fields of horizontal sheets

1. Feb 6, 2010

### sdoug041

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

Two very large horizontal sheets are 4.25 cm apart and carry equal but opposite uniform surface charge densities of magnitude $$\sigma$$. You want to use these sheets to hold stationary in the region between them an oil droplet of mass 364 $$\mu$$g that carries an excess of five electrons. Assume that the drop is in vacuum.

2. Relevant equations

E = FE / q

Fg = mg

E = $$\sigma$$ / 2$$\epsilon$$0

$$\epsilon$$0 = 8.854 * 10 -12 C2/N*m2

3. The attempt at a solution

I calculated the Fg because I know the FE must be equal and opposite of gravity in order to be suspended.

I also calculated the net charge of the oil drop which was 5xe- (the charge of one electron) = 8.01 * 10-19 C.

I divided the FE by the net charge of the drop to find the Electric Field in N/C. I then multiplied E (electric field) by 2$$\epsilon$$0 to solve for $$\sigma$$ but this is not the correct answer apparently.

Where have I went wrong?

2. Feb 6, 2010

### kuruman

The expression E = σ/(2ε0) is the electric field due to a single plate. What is the electric field in the region between the two plates?