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Eletric Potential

  1. Feb 12, 2013 #1
    1. The problem statement, all variables and given/known data
    A positive charge +q1 is located to the left of a negative charge −q2. On a line passing through the two charges, there are two places where the total potential is zero. The first place is between the charges and is 3.35 cm to the left of the negative charge. The second place is 7.65 cm to the right of the negative charge.
    (a) What is the distance between the charges?
    (b) Find |q1|/|q2|, the ratio of the magnitudes of the charges

    2. Relevant equations
    V=kq/r


    3. The attempt at a solution
    I don't know where to start. I know that I need to solve for the distance between +q1 and the spot 3.35 cm to the left of the negative charge to be able to solve for the distance between them. I'm just not sure how to use the charge to the right of -q2.
     
  2. jcsd
  3. Feb 12, 2013 #2

    gneill

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    Staff: Mentor

    Start by drawing a diagram of the setup. Label the distance between the charges as d.

    What equations can you write to describe the given conditions?
     
  4. Feb 12, 2013 #3
    See that's the thing, i'm still iffy on the relationshiphs. My best educated guess would be:

    Since V @ .0335m is equal to zero. That would mean the net electric field at that point is also 0 because E=-ΔV/Δs.

    Or am I going in a completely wrong direction.
     
  5. Feb 12, 2013 #4

    gneill

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    Staff: Mentor

    You can work with the electric field or the potential. The potential is simpler as it is a scalar value that depends only on the charge and the distance from it. Since the problem statement specifically mentions potential, that would suggest that potential is a likely way to proceed :smile:

    What's the expression for the electric potential at a distance r from a charge q?
     
  6. Feb 12, 2013 #5
    V=kq/r
     
  7. Feb 12, 2013 #6

    gneill

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    Staff: Mentor

    Yes, that's correct. So write expressions for the potential at the two given locations assuming that the distance between the charges is d.
     
  8. Feb 12, 2013 #7
    Ok, sorry I had a spark of genius. And I've been kind of going with it. I just want to know if I'm on the right track. I set the +q1 charges equal to each other and got:

    r=.11/(V1-V2)

    Then I set the -q2 charges equal to each other and got:

    .035V1=.0765V2

    Substitute to get

    r=.11/(V1-.035V1)

    I got stuck again. I need one more equation, but I just can't seem to find it.
     
  9. Feb 12, 2013 #8

    gneill

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    Staff: Mentor

    You'll have to explain the above in detail. I don't understand what the equations are meant to represent. In particular, what are the "+q1 charges" and "-q2 charges"? The problem statement mentions only two charges, +q1 and -q2. And the potentials of interest are both zero.
     
    Last edited: Feb 12, 2013
  10. Feb 12, 2013 #9
    The problem states a "-q2" not a "+q2".
     
  11. Feb 12, 2013 #10
    Nvm I solved it. Thanks :D
     
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