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Homework Help: Very simple charge question, please review

  1. Jul 14, 2010 #1

    Ush

    User Avatar

    1. The problem statement, all variables and given/known data
    A -14.1 nC charge is located at position (x,y) = (2.48 cm, 1.24 cm). At what (x,y) position is the electric field -262600.0i? (Enter your x position first followed by the y position.)


    2. Relevant equations
    E = kQ/r2
    E = electric field strength
    k = 9 x 109
    r = distance to the point from the charge

    3. The attempt at a solution

    E = kQ/r2
    ∴ r = √(kQ/E)
    r = 0.02198

    original position: (0.0248m, 0.0124m)
    electric field: -262600.0i,
    unit vector 'i' indicates only x component, no y component:
    therefore y component is the same (electric field vectors go straight in toward -'ve charge)

    therefore x-component is..
    0.0248 + r
    =0.0248 + 0.02198
    = 0.04678m

    final answer:
    (0.04678m, 0.0124m)

    Originally what I did was
    0.0248 - r
    =0.0248 - 0.02198
    = 2.82E-3

    final answer:
    (2.82E-3m, 0.02198m)


    the computer rejected this.

    The computer only accepted (0.04678m, 0.0124m) as the correct answer

    I was wondering... is the other answer equally as valid or am I missing something?
     
  2. jcsd
  3. Jul 15, 2010 #2
    For the other answer, E would be +262600.0i and not -262600.0i.

    You could draw the field lines around the charge and see for yourself.
     
  4. Jul 15, 2010 #3

    Ush

    User Avatar

    thanks!
     
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