Electric Fields of horizontal sheets

[sdoug041]

Homework Statement

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.

Homework Equations

E = F_E / q

F_g = mg

E = $$\sigma$$ / 2$$\epsilon$$₀

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

The Attempt at a Solution

I calculated the F_g because I know the F_E 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 F_E 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?

 

[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?