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Magnetic force and fields

  1. Jun 5, 2009 #1
    http://lh6.ggpht.com/_H4Iz7SmBrbk/SikOL1lh7BI/AAAAAAAABCY/_70wWSK_-Vw/s640/DEVAN.jp [Broken]

    given a square wire frame with a side of length L, placed in a constant extarnal magnetic field B, (B=B*[tex]\hat{k}[/tex]). a current of i flows through the frame anticlockwise(as shown by arrows in diagram)

    the frame can turn freely about the x axis (which passes through its centre)
    the frame has a linear mass density of λ

    a) what is the force applied on the frame?
    b) what is the momentum applied of the frame?
    c) what is the angular acceleration of the frame?

    ------------------------------------------------

    a) since the force applied would be ILxB,
    the force on either side parallel to the field B has no contribution since ILB*sinθ=0 (θ=0/180)
    for either side perpendicular to the field ILxB=ILB

    therefore my total force on the frame would be 0+0+ILB-ILB=0

    ----------------------------------------------------

    b) for the momentum i say that N=RxF=(L/2)*F=(L/2)*ILB=0.5L2IB

    and since i have 2 contributors to the momentum and the are both on x-,
    N=-L2IB*[tex]\hat{i}[/tex]
    ------------------------------------------------------

    c) for the angular acceleration α, i need to somehow find the moment of enetria for a frame like this, and then i can say that N=Iα
    α=N/I

    can i use the moment of a pole 1/12(mL2) and multiply it by 2 to take into account each side??? this doesnt seem right to me, what aout the other 2 sides wgich arent revolving about their centre???
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
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