How Do You Calculate the Surface Charge Density on a TV Screen?

[UF6]

Homework Statement

1. 
   You've probably noticed that TV screens and computer monitors have an annoying tendency to accumulate dust. Wanting to understand this better, you decide to model the situation as follows: you assume a small mote of dust might have an excess charge of 1 nC and a mass of 5 μg. It floats near the surface of a television screen, accelerating toward the screen at a rate of 10 mm/s^2. You would like to know the surface density of the charge on the screen.

Homework Equations

F = ma
E = F / q
σ = 2*E*εo

The Attempt at a Solution

I keep getting the problem wrong and would like someone to check over my work and tell me what I'm doing wrong.

F = ma = 5E-6kg * 0.010m/s² = 5E-8 N

E = F / q = 5E-8N / 1E-9C = 50 N/C

σ = 2*E*εo = 2 * 50N/C * 8.85E-12C²/N·m² = 8.85E-10 C/m²

 

[TSny]

Check the conversion from μg to kg.