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Electric Field question. Please help

  1. Jul 18, 2010 #1

    Ush

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    1. The problem statement, all variables and given/known data

    The question, along with my attempt is in the image attached. P

    2. Relevant equations

    E = KQ/r2
    E = electric field
    Q = source charge
    r = distance

    3. The attempt at a solution

    see document attached

    ----
    thank you!
     

    Attached Files:

  2. jcsd
  3. Jul 19, 2010 #2
    Well I haven't run through the numbers, but theoretically it seems possible.
    The electric field at whatever point is what would act on an imagined point positive charge, so with that in mind:

    You have q1 pulling the charge left, and q3 pushing it also left. q2 has no horizontal affect.

    Then you've done the nice Pythagoras stuff and found that q1 pulls the charge down, but q2 and q3 push it upwards. Since q2 + q3 have a greater magnitude than q1, they will win.

    Pythagoras-ing horizontal and vertical components gives your perfectly depicted arrow, and a magnitude that agrees in sign with this hand waving argument.
    Was there something you didn't understand?
     
    Last edited: Jul 19, 2010
  4. Jul 19, 2010 #3

    Ush

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    to me, it makes sense. But to the computer marking my answer, it does not =[
     
  5. Jul 19, 2010 #4
    Hmm, do you know what the correct answer is?

    I got a different answer to yours with just the superposition equation:
    ddca45ddf0a04f30a7280a846389febf.png

    It may just be a computational mistake - when you are calculating the components, you have them on the order nC (x10^-9 in SI units). But the Coulomb constant is
    cimg336.gif
    So they should not be of this order.
     

    Attached Files:

  6. Jul 19, 2010 #5

    Ush

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    I checked over my calculations but i get the same number, I don't know what the correct answer is

    I'm not sure how you did your superposition,
    but that just looks like the equation for E, - the sum of the electric fields. Electric fields are vectors, you can't add them without breaking them into components- at least I don't know how to yet-
     
  7. Jul 19, 2010 #6

    Ush

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    pleaseeee someone help!?
     
  8. Jul 19, 2010 #7

    Doc Al

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    Staff: Mentor

    I didn't check your arithmetic, but your method looks correct.
     
  9. Jul 19, 2010 #8

    Ush

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    the computer keeps marking it wrong though =/ this seems like a simple question..
     
  10. Jul 19, 2010 #9

    Doc Al

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    Staff: Mentor

    Often the online systems are fussy about significant figures--some of them insist on 3 sig figs no matter what.
     
  11. Jul 19, 2010 #10

    Ush

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    gosh.. thanks o_o it worked!
     
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