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Homework Help: Current needed to keep coil levitating

  1. Mar 13, 2012 #1
    1. The problem statement, all variables and given/known data
    A horizontally oriented coil of wire of radius r=5.00cm and carrying a current I, is levitated by south pole of a vertically oriented bar magnet suspended above center of coil. If magnetic field (B) on all parts of the coil makes an angle θ=45.0 with the vertical, determine the magnitude and direction of the current needed to keep the coil floating. B=0.0100T, number of turns in the coil is N=10.0, and the total coil mass is m=10.0grams

    2. Relevant equations

    Magnetic field equation for a solenoid: B=μ(N/L)I
    Force of magnetic field: Fb=ILB
    Force of gravity: Fg=mg

    3. The attempt at a solution

    To keep the coil floating, magnitude of Fg=magnitude of Fb

    Bcosθ=μ(N/L)I since Bcosθ is the vertical magnetic field
    so mg=ILB=I(2∏R)(μNI/(Lcosθ)) where L=2∏R

    I think I'm using L wrong when finding Bcosθ. But the problem doesn't give the height of the coil. Am I approaching this wrong?

    Attached Files:

  2. jcsd
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