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Homework Help: What is the magnitude E of the field?

  1. Sep 15, 2012 #1
    1. The problem statement, all variables and given/known data

    Two tiny spheres of mass = 7.20 mg carry charges of equal magnitude, 72.0 nC , but opposite sign. They are tied to the same ceiling hook by light strings of length 0.530 m . When a horizontal uniform electric field E that is directed to the left is turned on, the spheres hang at rest with the angle between the strings equal to 50.0 degrees.

    What is the magnitude E of the field?

    2. Relevant equations
    Fe1 = Eq
    Fe2 = (Kq1q2)/r2
    Fg = mg

    3. The attempt at a solution

    I assumed that the state it is in is in equilibrium so I set up:

    ƩF = Fg + Fe1 + Fe2 + T = 0

    ƩTx = Tcosθ - Fe1 + Fe2 = 0

    θ being 65 degrees since I split the triangle made by the 50 degrees into 2 right triangles.

    ƩTy = Tsinθ - mg = 0

    T = (Fe1 - Fe2)/cosθ

    Ty = (Fe1 - Fe2)tanθ - mg = 0

    Fe1 = mg/tanθ + Fe2

    E = (mg/tanθ + kq1q2/r2)/q

    (I found r to be .448 using the right triangles I formed)

    I plugged in all the given values and I've got...1.17 x 10^6

    Which is wrong. Can someone please explain what I messed up on?
  2. jcsd
  3. Sep 15, 2012 #2
    Whoops I used the wrong angle during my calculations so my actual answer is 4.6 x 10^5 but it is still wrong
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