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Question of electricity net force

  1. Mar 29, 2016 #1
    1. The problem statement, all variables and given/known data

    Three point charges are located on the positive x-axis of a coordinate system. Charge q1 = 1.0 nC is 2.0 cm from the origin, charge q2 = -3.5 nC is 4.0 cm from the origin and charge q3 = 6.0 nC located at the origin. What is the net force ((a)magnitude and (b) direction) on charge q1 = 1.0 nCexerted by the other two charges?

    The online homework grader is marknig my answer for part a wrong. Can someone tell me whats wrong with my solution for part a? My work is in the attachment.

    2. Relevant equations

    see attachment

    3. The attempt at a solution

    see the attachment for my solution. The online homework grader is saying my answer is wrong.
     

    Attached Files:

    Last edited by a moderator: Mar 29, 2016
  2. jcsd
  3. Mar 29, 2016 #2

    gneill

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    Staff: Mentor

    Check the locations of the individual charges. q3 is at the origin according the problem description. Your image has it off to the right.
     
  4. Mar 29, 2016 #3

    berkeman

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    Staff: Mentor

    It looks like you maybe placed the charges in the wrong spots? q3 is at the origin, q1 is at 2cm and q2 is at 4cm. So there is a repulsive force to the right from q3 on q1, and an attractive force to the right from q2 on q1...

    EDIT: Rats, rats, rats, rats, rats! :cry:
     
  5. Mar 29, 2016 #4
    if the pic isnt clear enough, here is a close up.
    oh i fixed that. [tex] F_{q3q1} [/tex] should be 1.35x10^-4. The answer though is sitll wrong. I subtract the two forces because one pair is attractive while the other force is repulsive. The answer I get is 5.625x10^-5. The answer from homework grader gave me is 2.1x10^-4. This is what you would get if you add the two forces. Why would you add them?
     
  6. Mar 29, 2016 #5

    gneill

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    Staff: Mentor

    What directions do the two force vectors point? Make a sketch.
     
  7. Mar 29, 2016 #6
    oh yeah. i see the problem. thank you.
     
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