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Electric Potential - Conceptual question

  1. Oct 16, 2011 #1
    1. The problem statement, all variables and given/known data

    At a certain distance from a point charge, the magnitude of the electric field is 500 V/m and the electric potential is -3.00 kV. (a) What is the distance to the charge? (b) What is the magnitude of the charge?

    2. Relevant equations

    3. The attempt at a solution

    a) 6 meters
    b) -2 micro-Coulombs
    *These are correct, I mention this fact cause I don't want you to have to waste your time checking so.

    My question is: for part b, if you solve for Q, the magnitude of charge, from E instead of V (E = kq/r^2), what is the visual concept behind why the charge is negative? Because obviously if you solve from V, you will get a negative 2 uC.

    Of course the book asks for magnitude anyway, but the answer key gives the negative sign as well. I want to understand the concept behind it.
     
  2. jcsd
  3. Oct 16, 2011 #2

    Doc Al

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    Staff: Mentor

    You cannot tell the sign of Q just using E, since you were not told the direction of the field.
     
  4. Oct 16, 2011 #3
    Thank you, and just to verify - because of the potential given, it is a fact that the charge is negative?
     
  5. Oct 16, 2011 #4

    Doc Al

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    Staff: Mentor

    Yes.
     
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