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Homework Help: Finding Torque on current loop.

  1. Nov 14, 2011 #1
    1. The problem statement, all variables and given/known data
    The current loop is in the y-z plane. The direction of the magnetic field is in the z-direction.

    A) Which way is the torque on the current loop?
    B) If the current is 4 amps and the magnetic field strength is 2.5 Tesla, what is the magnitude of the torque?

    here is the picture (which I drew): http://twitpic.com/7e0zv9 [Broken]

    2. Relevant equations

    Right Hand Rule 2

    T=IAB, T=NIABsinĪø. The torque formulas I have necessitate finding an area of the current loop. But, as you can see from the picture (which displays all info. given) there is no way to find the area.

    3. The attempt at a solution

    A) Well, the loop is parallel to the field. If I use the right hand rule 2, the direction of the torque would be into the page, would it not?

    B) I feel that I am missing something crucial. I have done a few problems similar to this one, but I was always given some number to work with to find the area of the current loop.

    Seriously, ANY help is very much appreciated. Thank you.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Nov 14, 2011 #2


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    Homework Helper

    In high school physics, we use F = I*L*B (component of B perpendicular to I).
    You could use that the calculate the force on the two sides of the loop that are perpendicular to the B field. If you knew their length. A larger loop would feel a larger force and greater torque than a smaller one. So, same conclusion, the size of the loop is necessary to find the answer.
  4. Nov 14, 2011 #3

    Thank you, Delphi. I think that perhaps the torque is zero. My reasoning is that the forces acting on the loop are not trying to rotate it. At least that is how it appears.

    What do you think of that?
  5. Nov 14, 2011 #4


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    Homework Helper

    No, it is exactly like the loop of wire in an electric motor. The forces are opposite in the top and bottom wires because the current is reversed, so you do have torque about the center of the loop.
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