# Damn units and surface charge density.

1. Sep 20, 2008

### frasifrasi

An electron close to a large, flat sheet of charge is repelled from the sheet with a 2.1×10^−12 {N} force.

Q. find the surface charge density on the sheet in C/m^2

What i did was use the formula F = qe with 2.1e-12 for the force and 6.1e-19 for the charge of an electron. This gives me E,which I used in the formula E = (surface charge density)/2epsilon_0.

Plugging everything in (epsilon_0 = 8.85e-12), I get 2.32e-42, but It says it is wrong.

Does anyone know why?

Thanks.

2. Sep 20, 2008

### Redbelly98

Staff Emeritus
Re: WTF? Damn units and surface charge density.

Your formulas are correct, so there's a calculation error somewhere.

2.32e-42 C/m^2 is less than 1 electron per square kilometer!

3. Sep 20, 2008

### frasifrasi

Re: WTF? Damn units and surface charge density.

I keep getting it wrong!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Can anyone verify the above is correct?

Much appreciated.

4. Sep 20, 2008

### Redbelly98

Staff Emeritus
Re: WTF? Damn units and surface charge density.

The above is not correct. It's 2.32 x (A different power of 10 then -42)

Can you show some intermediate results? What did you get for the E-field?