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Homework Help: What is the torque on loop

  1. Apr 6, 2012 #1
    1. A small current loop of area A=0.0007m2 and moment of inertia I=6.6e-07kgm2 carries a current i=0.0045A with its magnetic moment perpendicular to the Earth's magnetic field. The magnitude of the Earth's field is B=4.2e-05T.

    What is the magnitude of the net torque T on the loop?

    2. Relevant equations
    (1) τ(torque) = I([itex]\alpha[/itex]

    (2) τ = μ X B

    (3) torque = current*area*B

    3. The attempt at a solution

    I'm not sure where to begin. I would plug numbers into the third equation but it doesn't make sense of where the moment of inertia comes into play...help?

    Okay so I did use (3) to find the right answer. I'm not sure how this works though. so in my drawing i just have a square loop and the question says it is perpendicular to Earth's B field, I thought perpendicular meant they cancel out?
    Last edited: Apr 6, 2012
  2. jcsd
  3. Apr 6, 2012 #2


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    Science Advisor
    Homework Helper

    Hi Alouette! :smile:
    no, the question says that the magnetic moment (not the loop itself) is perpendicular to Earth's magnetic field

    magnetic moment, µ, is a vector, current times area (amp-turns m2), and it points perpendicular to the loop

    the H field is the magnetic moment density (amp-turns/m), which eg points along the axis of a solenoid, ie perpendicular to the loops :wink:
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