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Homework Help: Q3 position if E = 0

  1. Feb 2, 2009 #1
    1. The problem statement, all variables and given/known data
    In free space, Q1 = 10nc at P1(0,-4,0) and Q2 = 20nC at P2(0,0,4).
    a. Find E at the origin (answered)
    E(r) = 5.617ay - 11.235az
    b. Where should a 30nC point charge be located so E=0 at the origin? (need help)

    2. Relevant equations
    E(r) = (1/4(pi)epsilon) sum ((Qm)am/(|r-rm|^2))
    formula for electric field intensity

    3. The attempt at a solution
    E = 0, Q3 = 30nC
    E = 0 = (1/4(pi)epsilon) ( (Q1)a1/(|r-r1|^2))+ ....)
    a3 = (0-x)ax + (0-y)ay + (0-z)az/ |r-r3|
    x = 0
    P3 = (0, y, z)

    5.617ay - 11.235 az = ((Q3)a3/(|r-r3|^2))

    *5.617ay - 11.235 az = Q3((0-y)ay + (0-z)az/ |r-r3|^3)
    **|r-r3| = sqrt(0^2 + (0-y)^2 + (0-z)^2)
    * and ** aren't enough to solve I'm missing one thing any help would be appreciated.
     
    Last edited: Feb 2, 2009
  2. jcsd
  3. Feb 10, 2009 #2
    (0, -5.62, +11.23) = (3x10-8+12)/(4pi8.854) (0, -y, z)\(y 2+z 2) 1.5 ; -5.62 = 270 -y/(y 2+z 2) 1.5 and 11.23 = 270 -z/ (y 2 + z 2); -0.5 = y/z, (y 2 + 4y 2)1.5=48y, y=2.07, z = -4.14, The location is (0, 2.07, -4.14). The trick is dividing y by z!
     
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