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Two point charges

  1. May 7, 2007 #1
    Two point charges are located on the x−axis: a charge of +7.80 nC at x = 0 and an unknown charge q at x = 0.50 m. No other charges are nearby. If the electric field is zero at the point x = 1.0 m, what is q?


    I separated the charges.
    q1 = +7.80 nC
    q2 = unknown

    I am confused about the last part of the question where the electric field is zero.

    Would I use the equation
    E= k(Q) / r^2

    (Correct answer is: -1.95 nC)
     
  2. jcsd
  3. May 7, 2007 #2

    Hootenanny

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    Yes you would use the defintion of the electric field to set up an equation of the form;

    [tex]E_1 + E_2 = 0[/tex]
     
  4. May 7, 2007 #3
    What are the electric field vectors at x=1m due to Q1 and Q2?
     
  5. May 7, 2007 #4
    I'm not getting the right answer.
    I did...

    0 = ((8.99x10^9)(7.80)) / 0.5^2) + ((8.99x10^9)(q)) / 0.5^2)

    which does not equal zero.

    I think I still need to do something else.. something about point x = 1.0m ?
     
  6. May 7, 2007 #5

    Hootenanny

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    Indeed, how far is the first charge (q1) located from the point of zero electric field?
     
  7. May 7, 2007 #6
    it would be
    1.0 meter
     
  8. May 7, 2007 #7
    Check your expession for electric feild due to q1.
     
  9. May 7, 2007 #8

    Hootenanny

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    That's correct, not 0.5m as you had previously :wink:
     
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