1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

  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


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: Torque and work of dipole
  1. Torque and Dipoles (Replies: 2)