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Finding the Magnitude of a Dipole Moment

  1. Sep 25, 2007 #1
    [SOLVED] Finding the Magnitude of a Dipole Moment

    Hi there,
    I'm new to posting, but I've used this forum many times to help me with my homework :) I went the "take a photo of the textbook" route (there is a picture with the problem), so I hope that's acceptable.

    1. The problem statement, all variables and given/known data
    A certain electric dipole is placed in a uniform electric field E of magnitude 40 N/C. Figure 22-51 gives the magnitude (t) of the torque on the dipole versus the angle (theta) between field E and the dipole moment p. What is the magnitude of p?

    E = 40 N/C

    2. Relevant equations
    (t) = -pEsin(theta)

    3. The attempt at a solution
    The hint we were given was "When is the torque minimum, and when is it maximum?"

    I'm not sure where to start. The textbook and my lecture notes aren't helping me at all.

    I don't quite understand the given figure, or what is being counted on the y axis (I think that number on the left is multiplied by 10^-28?). I assume that somehow I am supposed to use the min/max torque, set them equal to each other, and solve for p.. but I don't know how I'm supposed to find the min/max angles in order to do this.

    edit: Apparently file attachments don't work..
    http://www.clan-dm.net/members/jen/dipolequestion.jpg [Broken]
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Sep 25, 2007 #2


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    The minimum torque will be when sin theta is equal to zero. The maximum will be when sin theta is equal to one. Yes, the y-axis is multiplied by 10-28. Since the minimum torque is zero i would suggest using the maximum torque.
  4. Sep 25, 2007 #3
    Ok, that makes sense now. For some reason max torque being at 90 degrees was completely glossed over in class. In retrospect it does seem obvious now, but when I'm learning new stuff I become confused very easily.. especially when my last calc class was over a year ago.

    Thank you very much. I'm sure I'll be back, because this semester is already killing me. :)
  5. Sep 25, 2007 #4


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    No problem. P.s. Welcome to the forums. :smile:
  6. Sep 25, 2007 #5
    Thank you! I only wish I thought of registering sooner. :)
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