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Homework Help: Cant get y-component of point charges on test charge

  1. Oct 3, 2007 #1
    1. The problem statement, all variables and given/known data
    Six point charges, four positive (+q) and two negative (-q), are fixed at thepositions shown (i attached a .gif file --pls tell me if this isnt visable) in the x-y plane; q = 2.1 × 10-6C and a = 0.93 m. A test charge Q = 0.6 × 10-6C is at the origin.

    (a) Find the x- and y-components of the total force on Q:




    2. Relevant equations
    i successfully calcd the x-component (-.017016N) but cant get the y component


    3. The attempt at a solution

    heres what i did

    1. labelled the charges A thru F starting with the charge on the neg y-axis (A) and the labelled in cw direction.

    2. so for y component everything either cancels or has no y-component, EXCEPT for the 2 charges on the y-axis. These are both pos charges.
    so heres the calc
    Ay = (9*10^9)(2.1*10^-6)(.6*10-6) / (.93^2) = .01311N (pos charge)
    Dy = (9*10^9)(2.1*10^-6)(.6*10-6) / (.93^2) = .01311N (also pos charge)
    so 2(.01311) = .02622N
    this is wrong!
    please help!
    thanks
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Oct 3, 2007 #2

    learningphysics

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    Homework Helper

    No. this is wrong. The two charges on the y-axis exert equal but opposite forces on Q, hence they cancel.

    Which are the two charges you need to consider?
     
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