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Homework Help: Electic Field

  1. Jan 29, 2008 #1
    A long, thin rod (length = 4.0 m) lies along the x axis, with its midpoint at the origin. In a vacuum, a +8.0 µC point charge is fixed to one end of the rod, while a -8.0 µC point charge is fixed to the other end. Everywhere in the x, y plane there is a constant external electric field (magnitude = 5.0*10^3 N/C) that is perpendicular to the rod. With respect to the z axis, find the magnitude of the net torque applied to the rod.

    I just don't know where to start. I just need a heads-up or a guide on how to start the problem.
  2. jcsd
  3. Jan 29, 2008 #2
    So there will be a force on each of the ends from the external field, and torque is force cross distance. Just think about what would happen if you were to push on the rod with the appropriate forces that would correlate to what the electric field is doing.
  4. Jan 29, 2008 #3
    I know that there is a force, two actually acting on each end. The problem is that I don't know what force can there be. Do I use the equation F=qE?
  5. Jan 29, 2008 #4
    Yep. :)

    One force will be pointing up and one will be pointing down. Torque!
  6. Jan 30, 2008 #5
    How does a perpendicular electric field force a charge to move?
  7. Jan 30, 2008 #6
    The field is perpendicular, you might be confusing it with parallel. Picture the rod lying on the x axis, and now picture an electric field in the y direction everywhere. The problem statement is a little vague on what happens afterwards, but presumably the electric field is always perpendicular.
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