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Homework Help: Electric field of a ring and beads

  1. Jul 9, 2012 #1
    1. The problem statement, all variables and given/known data
    a plastic ring of radius R = 50.4 cm. Two small charged beads are on the ring: Bead 1 of charge +2.00 μC is fixed in place at the left side; bead 2 of charge +6.00 μC can be moved along the ring. The two beads produce a net electric field of magnitude E at the center of the ring. At what (a) positive and (b) negative value of angle θ should bead 2 be positioned such that E = 2.22 × 105 N/C?

    2. Relevant equations
    Enety by -q2sinθ/4πε0R^2
    Enetx by q1/4πε0R^2 - q2cosθ/4πε0R^2

    3. The attempt at a solution
    so i found enet of y and x by equations above then did E^2=(q1^2+q1^2-2q1q2cosθ/(4πε0R^2)^2

    which simples to

    θ=inverse cos [(q1^2+q1^2- (4πε0R^2)^2E^2)/(2q1q2)]

    basically when i take the cos its a neg number and you can't take cos of neg number so i am confused what i am doing wrong bc my professor said method was correct

    Attached Files:

  2. jcsd
  3. Jul 9, 2012 #2


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    You can take the cosine of a negative number (e.g., cos(-30o) = .5) and you can also take the inverse cosine of a negative answer (e.g., inv cos (-.5) = 120o).

    But, anyway, if I substitute the numbers into your expression for θ, I find that you should be taking the inverse cosine of a positive number and you get a positive angle for θ.

    Overall, your work looks good to me.
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