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Homework Help: Electric Potential Produced by Point Charge

  1. Jan 18, 2007 #1
    1. The problem statement, all variables and given/known data

    A point charge of -8 µC is at the origin. What is the electric potential at
    (a) (3.0m, 0),
    (b) (-3.0m, 0), and
    (c) (3.0m, -3.0m)?

    2. Relevant equations


    3. The attempt at a solution

    a & b) V=(8.99x10^9)(-8x10^-6)/3
    V= -23973.3

    c) V=(8.99x10^9(-8x10^-6)/6

    I understand part a and b and got the correct answer. However, for part c, the only thing I am doing different is r=6 instead of 3 because the coordinates are (3.0m, -3.0m) and I am not getting the correct answer. I don't really understand. Thank you.
  2. jcsd
  3. Jan 18, 2007 #2


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    r does not equal 6. Think about the coordinate system.
  4. Jan 18, 2007 #3
    I thought r=6 because there is 3 in the + x direction and 3 in the -x direction on either side of the origin (x=0) so I added them to get a total of 6 from -x to +x. I'm not really sure how to look at the coordinate system any other way?
  5. Jan 18, 2007 #4


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    The two numbers denote the x and y coordinates in a cartesian plane. How can you have a position with two x coordinates? Draw the x an y axes and work out what r should be for the (3, -3) point.
  6. Jan 18, 2007 #5
    ohhhh wow I'm so blind. Thank you very much for your help!
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