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Torque and work of dipole

  1. Sep 24, 2016 #1
    1. The problem statement, all variables and given/known data
    An electric dipole consists of +/- Q = 330nC separated by a distance of 1200nm. The dipole is initially oriented in the -i direction (along the negative x axis). The dipole is in a uniform external electric field of E=2500j N/C Determine the following:

    a) The magnitude of the torque on the dipole when it is at +165° to the +x axis
    b) The work done by the electric field to rotate the dipole to an angle of +110° to the +x-axis from its initial position

    2. Relevant equations
    |T| = |P| |E| sin θ
    W = Uf-Ui

    3. The attempt at a solution
    I get what to plug in to solve the problems, but the only issue I have is that I'm not sure what θ should be in these equations because I can't really visualize between what two vectors the angle is supposed to be between? When it says it's initially oriented in the -i direction, does that mean it has already rotated 180°? I'm confused.
     
  2. jcsd
  3. Sep 24, 2016 #2
    draw a diagram and then it will be/can be seen how your confusion stands..
     
  4. Sep 24, 2016 #3
    So for a) would theta be 165°-90°=75°?

    And for b) would final theta be 110°-90°=20° and initial theta be 180°-90°=90°
     
  5. Sep 25, 2016 #4
    what is your theta angle ?
    As you write the equation for Torque the angle theta seems to be angle between the dipole moment and electric field...
    Initially your dipole moment is along -i direction and E is in j direction so are you turning the dipole through 75 degree...check from diagram.
    post the diagram for us to see.
     
  6. Sep 25, 2016 #5

    haruspex

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    No. Oriented at -i must mean at 180 degrees to the +x axis. It is then rotated to be at 110 degrees to the +x axis.
    In part a) there is no actual rotation.
     
  7. Sep 25, 2016 #6

    haruspex

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    Yes.
     
  8. Sep 25, 2016 #7
    Okay, I did that and I got the right answer for a).

    For b), plugging 20 degrees and 90 degrees didn't seem to work. Any ideas?
     
  9. Sep 26, 2016 #8

    haruspex

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    What are the starting and finishing angles in b)?
     
  10. Sep 26, 2016 #9
    Initial is 180, final is 110.
     
  11. Sep 26, 2016 #10
    That's not the answer though...
     
  12. Sep 26, 2016 #11
    the angle is between dipole moment and E and the work done in rotating a dipole through an elementary angular displacement should be torque times the angle say differential element of theta.
     
  13. Sep 26, 2016 #12
    Oh wait, the final angle is actually 20° (165°-90°) and the initial is 90°(180°-90°), like I said previously. I found something else in my problem that was wrong, and I got the right number. My work is negative, which makes sense because it is rotating clockwise to align with the electric field, right?
     
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