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Dipoles angular velocity when aligned with electric field?

  1. Feb 8, 2012 #1
    1. The problem statement, all variables and given/known data

    An electric dipole consists of 2.0 g spheres charged to 5.0 nC (positive and negative) at the ends of a 12 cm long massless rod. The dipole rotates on a frictionless pivot at its center. The dipole is held perpendicular to a uniform electric field with field strength 1400V, then released. What is the dipole’s angular velocity at the instant it is aligned with the electric field?

    2. Relevant equations

    F=mω^2r
    F=qE

    3. The attempt at a solution

    F=qE
    F=(5*10^-9)(1400)
    F=7*10^-6N

    7*10^-6=mω^2r
    ω=√(7*10^-6/(.004*.12))
    ω=.121rev/s?

    I'm just unsure if I did this correctly, I thought it seemed to easy that way.
     
  2. jcsd
  3. Feb 8, 2012 #2

    gneill

    User Avatar

    Staff: Mentor

    The force that you calculated is only perpendicular to the rod when the rod is horizontal. Only a portion of that force will result in torque on the rod when it is at some other angle. That is to say, the torque is going to vary with the rod's angle with respect to the field. You could integrate the torque*dθ to find the work done, or,...

    You might consider an energy conservation approach. What's the difference in potential energy between the rod horizontal and the rod vertical, given that the field strength is 1400 V/m?
     
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