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Homework Help: What is the net charge

  1. Jan 8, 2008 #1
    1. The problem statement, all variables and given/known data

    A metal sphere has a charge of +7.0 µC. What is the net charge after 8.00 10^13 electrons have been placed on it?
    in µC

    2. Relevant equations

    3. The attempt at a solution

    I believe that the charge of an electron is: -1.6 x 10^-19 C
    and there are 8 electrons,

    so -1.6 x 10^19 C * 8.00 x 10 ^13 electrons right?

    then I add it to 7 uC? well i'm having trouble with the sigfigs, I got 5.72 uC I don't believe that is correct.

    please help.
  2. jcsd
  3. Jan 8, 2008 #2
    Do you have a rounding error? I am getting -5.8µC.
  4. Jan 8, 2008 #3
    ah, I see from your answer. Sometimes it just takes someone else.

    it was my sign! it has to be negative.

  5. Jan 8, 2008 #4
    also, when i get the answer, after multiplying the electrons, does +7.0 uC have sigfigs? i can't seem to figure out why my calculater won't give me anything past 1.28 x 10^33 i believe.
  6. Jan 8, 2008 #5
    Sorry I don't understand your calculator comment. 7.0µC has 2 significant figures, and it means [itex]7.0*10^{-6}[/itex]C. 7.00µC would be 3 sig figs, and 7µC would be 1 sig fig.
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