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Net Electric field

  1. Oct 11, 2013 #1
    1. The problem statement, all variables and given/known data

    Figure 22-7a shows three particles with charges q1= +2Q,
    q2=-2Q, and q3 = -4Q, each a distance d from the origin.
    What net electric field E is produced at the origin?
    2. Relevant equations

    E=q/r^2* 1/(4∏ε0)



    3. The attempt at a solution
    So i calculated the of each particles separately.

    E1= (2Q/d^2)*1/(4∏ε0)
    E2= (-2Q/d^2)*1/(4∏ε0)
    E3=(-4Q/d^2*1)/(4∏ε0)


    now ƩFx= [-E1cos(30)-E2cos(30) -E3cons(30)]1/(4∏ε0)
    ƩFx= (-8Q/d^2)*cos(30)*1/(4∏ε0)

    ƩFy= [E1sin(30)+E2sin(30)-E3sin(30)]1/(4∏ε0)
    ƩFy= 2Q/d^2*sin(30)+2Q/d^2*sin(30) -4Qsin*(30)
    ƩFy=0

    this means that we have a net electric field on the negative x direction of -8Q/d^2 *cos(30)*(1/(4∏ε0)).

    however, my book has the same answer but in the positive x direction. they used a different method to solved this.
    Screen Shot 2013-10-11 at 11.45.31 PM.jpg

    I attached the book's answer.
    what did i do wrong?
     
  2. jcsd
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