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Homework Help: Calculating voltage from multiple point charges

  1. Feb 19, 2009 #1
    1. The problem statement, all variables and given/known data
    A point charge 10 km in the air with a charge of 40 coulombs
    A point charge 5 km in the air with a charge of -40 coulombs
    A point chare 2 km in the air with a charge of 10 coulombs
    -Find the electric potential due to these charges at a point which at a height of 8 km and on the vertical line passing through the charges.

    2. Relevant equations
    kq1/r1 + kq2/r2

    3. The attempt at a solution

    So I summed them up

    v = k (40C/2000m - 40C/3000m + 10C/6000m) = 7.5E7

    My question is, is do I need to convert km to m and was I correct in subtracting the negative 40 coulomb charge from the rest of the positive charges?
  2. jcsd
  3. Feb 19, 2009 #2


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    Homework Helper

    Yes and yes. Looks good to me.
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