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Current carrying coil - torque on a circular one

  1. Apr 20, 2010 #1

    Could someone please either verify my proof or find the error in it - its driving me nuts!

    if Torque (T) = Fd

    but F = BIlsin theta and F changes as you move around the circle

    1. therefore to find F on the semi-circle:

    [tex]\int[/tex] between 0 and pi of BIlsin theta d theta = 2BIl ?

    2. arc length 'l' of the semi circle is l = r phi -> l = r.pi

    3. therefore F = 2BIrpi

    4. into T= Fd -> T = 2BIrpi . 2r

    5. therefore T = 4 BI. pi r squared

    6. hence T = 4 BIA where A is the area of the circle?

    please! say this is wrong, every time i look at it i wish it was becuase T = BIA for all planar shapes is just so much more simple :)

    is faraday sufficiently spinning in his grave yet ?
  2. jcsd
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