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Damn units and surface charge density.

  1. Sep 20, 2008 #1
    Ok, the question asks:

    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. jcsd
  3. Sep 20, 2008 #2

    Redbelly98

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    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!
     
  4. Sep 20, 2008 #3
    Re: WTF? Damn units and surface charge density.

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

    Can anyone verify the above is correct?

    Much appreciated.
     
  5. Sep 20, 2008 #4

    Redbelly98

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    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?
     
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