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Homework Help: Need help on torques / magnetic fields

  1. Apr 3, 2005 #1
    A square coil and a rectangular coil are each made from the same length of wire. Each contains a single turn. The long sides of the rectangle are twice as long as the short sides. Find the ratio tsquare/trectangle of the maximum torques that these coils experience in the same magnetic field when they contain the same current.

    Im very confused how to do this :/
  2. jcsd
  3. Apr 4, 2005 #2
    Let L be length of the squares sides. Using the fact that the perimeter's are equal you can write the length and width of the rectangle in terms of L.

    Remember that torque = u X B where u is the magnetic dipole moment of the loop. Since there is no requirement for the magnitude or direction of B, you can just let B=1 and be perpendicular to u so that u X B = u.
  4. Apr 4, 2005 #3
    i dont know if i did this right
    it seems too easy..
    t = torque, s=square, r=rectangle

    t = NIABsino (o = theta)
    ts/tr = NIAsB/NIArB = As/Ar = L^2/2L^2 = 1/2

    is this right?
  5. Apr 4, 2005 #4
    Your right in that it's just the ratio of the areas, but you don't have the correct area for the rectangle. You know that 2 sides of the rectangle are twice as long as the other sides.

    Use a different variable to define the sides of the rectangle then use the fact that the perimeters are the same to find the sides of the reactangle in terms of L.
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