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Homework Help: ElectroStatics Question

  1. Feb 28, 2008 #1
    1. The problem statement, all variables and given/known data
    A small conducting sphere of mass M is attached to a string of length L. It is at rest in a uniform electrical field of magnitude E directed due right. The sphere has a charge of Q and the string makes a 30 degree angle with the electric field.


    a. Calculate the tension in the string if M = .002 kg, L = .45 M, Q = 10*10^-6 columbs

    b. Calculate the magnitude of the Electric field.

    c. Desrcibe what the mation of the conducting sphere would be if the string is cut

    2. Relevant equations
    E = k(q/r^2)

    3. The attempt at a solution
    a. Fg = Ft cos Theta
    (.002 *10) = ftcos30
    ft = .023 N

    b. E kq/r^2
    E = 9 * 10 ^9(10*10^-6)/.45^2 = 444444.4 N/C

    c. The coundcting sphere will move down and to the right

    I'm not if I got any of the parts can someone please help me out
  2. jcsd
  3. Feb 28, 2008 #2
    a) should be correct
    b) what you want is the magnitude of the uniform electric field, not the field produced by the point charge, might want to use newton third law to relate columb force to tension in part a
    c) if you remove the string what are the forces acting on the sphere?
  4. Feb 28, 2008 #3
    ok i got a and c. but i'm having trouble with b. i'm not getting how i should relate the 3rd law to relate the columb force to the tension
  5. Feb 28, 2008 #4
    well coulomb force can be expressed as F=qE , draw a force diagram like in part a and sum of forces equal zero...
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