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Vectors are confusing me. I'm not sure if I'm doing it right

  1. Jul 17, 2016 #1
    1. The problem statement, all variables and given/known data
    Charge q1 = +8.36 μC is fixed at the origin and charge q2 = -4.28μC is fixed on the +x-axis, 0.371m from the origin.

    (a) Find the direction and magnitude of the electric field at a point P that has coordinates (0.466, 0.466) m.

    (b) Find the direction and magnitude of the force on a -2.00 μC charge placed at the point P. (Please state the direction as an angle measured counterclockwise from the positive x-axis.)

    2. Relevant equations
    E = Kq/r^2

    3. The attempt at a solution

    Find θ1 = atan(0.466/0.466) = 45 degrees
    θ2 = atan(0.466/0.095) = 78.48 degrees

    I figured you just add the two vectors. For some reason I dont know how to... What is confusing me is that a can't use -4.28μC, for example, as the actual value. I think I need to use E=kq/r2 Here's what the diagram looks like

    I really appreciate any help


    EDIT:

    I got the first part. It is attached
    Would the second part just be F=Eq?

     

    Attached Files:

    Last edited: Jul 17, 2016
  2. jcsd
  3. Jul 17, 2016 #2

    jedishrfu

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    You should review this as its important.

    Bascially convert the two vectors into components along the x and y axis and go from there.
     
  4. Jul 17, 2016 #3
    But what values should I be using?
     
  5. Jul 17, 2016 #4

    SammyS

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    Hello josh12399 . Welcome to PF .

    upload_2016-7-17_20-51-58.png

    The resultant you have for E1 + E2 looks to be correct providing that you used correct magnitudes for E1 and E2. (You have placed the vectors in a rather unusual position relative to point P.)

    What did you get for the magnitudes of E1 and E2 ?
     
  6. Jul 17, 2016 #5
    To be honest I have no idea what to use for the magnitude. I tried a bunch of different things and got totally different answers.

    I thought of splitting them up into EX1, EY1, EX2 and EY2, but I don't know what the actual magnitude should be. Should it be (k*q*cosθ)/r2 for the x and (k*q*sinθ)/r2 for the y?
     
  7. Jul 17, 2016 #6

    SammyS

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    Use the following:
    for the electric field (magnitude) due to each charge.
     
  8. Jul 17, 2016 #7
    For each component (x and y)? Or should I just use the pythagorean theorem to find the diagonal distance from the charge to P?
     
  9. Jul 17, 2016 #8

    SammyS

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    ... just use the Pythagorean theorem to find the diagonal distance from the charge to P
     
  10. Jul 17, 2016 #9
    E1 = 1.730*105 N/C
    E2 = -1.701*105 N/C

    Now I take the X and Y components using sine and cosine?
     
  11. Jul 17, 2016 #10

    SammyS

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    Yes.
     
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