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Electric Potential Difference diagram

  1. Apr 19, 2015 #1
    1. The problem statement, all variables and given/known data
    Point charges q and Q are positioned as shown. If q= +1.0 nC, Q = -2.0 nC, a = 3.0m, and b = 4.0m, what is the electric potential difference, Va - Vb? **The question with the diagram is also attached **

    2. Relevant equations
    V = ke (q/r)

    3. The attempt at a solution
    V = (8.99x10^9)[(1.0x10^-9)/4) = 2.2475 V.

    The anser is 3.6V though, so I don't understand what I am doing wrong!
     

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  3. Apr 19, 2015 #2

    Orodruin

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    You are computing the electric potential at the -2 nC charge resulting from the other. This is not what is being asked for.

    How would you compute the potentials at the points A and B?
     
  4. Apr 19, 2015 #3
    How would you suggest to go about it? Because I tried it with the -2.0nC as well and i still didn't get the right answer :/
     
  5. Apr 19, 2015 #4

    Orodruin

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    What you are trying is not what is being asked for.

    How would you compute the potential at the point A resulting from the upper charge?
     
  6. Apr 19, 2015 #5
    what do you mean by the upper charge?
     
  7. Apr 19, 2015 #6

    Orodruin

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    The upper charge in your image is the 1 nC charge.
     
  8. Apr 19, 2015 #7
    The potential at that point would be V=ke(q/r), where q is the 1 nC charge and r is a?
     
  9. Apr 20, 2015 #8

    Orodruin

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    Yes. So what would be the potential at that same point from the other charge?

    In the same fashion, what would be the potential at B from both charges?
     
  10. Apr 20, 2015 #9
    So the potential at A is V= (8.99e9)(1e-9)/3 = 2.9967 V. The potential at B is V= (8.99e9)(-2e-9)/4 = -4.495 V?
     
  11. Apr 20, 2015 #10

    Orodruin

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    No, you need to compute the potential at both points using both charges and the correct distances.
     
  12. Apr 20, 2015 #11
    I'm sorry but I am quite confused right now and I really don't understand how to go about this question! Do you mind explaining conceptually why i have to find the potential at each point for both charges?
     
  13. Apr 20, 2015 #12

    Orodruin

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    Because both charges contribute to the potential at both points and you are interested in the potential difference between the points. The situation is not that of a single point charge.
     
  14. Apr 20, 2015 #13
    Oh that's what I was forgetting, thank you! It makes so much more sense now! :)
     
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