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Homework Help: Needle suspended from string - dipole moment?

  1. Sep 23, 2008 #1
    1. The problem statement, all variables and given/known data

    A needle suspended from a string hangs horizontally. The electrical field at the needle's location is horizontal with magnitude 3.7 x 10^3 N/C
    and it's at an angle of 30 degrees with the needle. There is no net electrical force acting on the needle, but the string exerts a torque of 3.7 x 10^-4 Nm to hold the needle in equilibrium. What is the needle dipole moment???

    2. Relevant equations

    t = torque
    p = dipole moment
    E = electric field

    t = p x E

    3. The attempt at a solution

    The way I am trying to solve this seems very easy and I thought I better check before turning this in.

    we want the dipole moment which is p, therefore

    p = t / E -->> p = 3.7 x 10^-4 / 3.7 x 10^3

    but then I'm thinking, the angle given in the problem statement come in play somewhere right???
    Am I in the right track???
  2. jcsd
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