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Electric potential and field

  1. Jul 15, 2013 #1
    1. The problem statement, all variables and given/known data

    A -11.5nC point charge and a +24.6nC point charge are 14.0cm apart on the x-axis.

    a) What is the electric potential at the point on the x-axis where the electric field is zero? - The answer is 1.58E2 V

    b) What is the magnitude of the electric field at the two points on the x-axis where the electric potential is zero? (Input your answers in order of increasing distance from the negative point charge.)

    I figured out a, but I wasn't able to get b. There should be two answers for b.


    2. Relevant equations

    E=(kQ)/r^2 - equation 1

    V=(kQ)/r - equation 2



    3. The attempt at a solution

    I know what to do but I wasn't able to execute it. I think you have to find the value of r in equation 2 where it equals 0 and then sub that into equation 1. I'm not sure how to do this though, any help would be appreciated!
     
  2. jcsd
  3. Jul 15, 2013 #2

    mfb

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    This is the potential for one charge only. You have two different charges.

    Right. That will work as soon as you get the right formula for your potential.

    Keep in mind that the electric field has a direction - don't just add its magnitude, pay attention to the signs.
     
  4. Jul 15, 2013 #3

    Oh, sorry, I meant:

    (kQ1)/r + (kQ2)/r - equation 2.

    and the field for the negative charge would be towards it and the field for the positive charge would be away from it. But, I still don't know what to do with that information.
     
  5. Jul 15, 2013 #4

    mfb

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    That is not right, the distance is different, too.

    Oh, and please describe your problem in the topic. "Physics question" is pointless in a physics forum. I changed it to "Electric potential and field" and moved the thread to the right subforum.
     
    Last edited: Jul 15, 2013
  6. Jul 15, 2013 #5
    What would it be then? I really can't figure it out :S

    And okay
     
  7. Jul 15, 2013 #6

    mfb

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    If you are at a position x0 and have a charge at position x1, what is the distance to the charge?
     
  8. Jul 15, 2013 #7
    it would be x0 + x1
    so, would that make the equation this:

    (kQ1)/r + (kQ2)/ ( x0 + x1 )
     
  9. Jul 16, 2013 #8

    mfb

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    It is not. Just use some values to check it: is the distance between x0=3m and x1=4m the sum of both (7m)?

    And don't forget the other r.
     
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