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Calculating angular acceleration of a wire

  1. Jan 13, 2015 #1
    1. The problem statement, all variables and given/known data
    A rigid uniform horizontal wire PQ of mass M, pivoted at P, carries a constant current I. It rotates with a constant angular speed in a uniform vertical magnetic field B. If the current were switched off, the angular acceleration of the wire in terms of B, M and I would be
    [It is not shown here, but in the diagram, PQ is the radius. P is the centre]

    ans= ( 3BI ) / ( 2M )

    2. Relevant equations
    (angular acceleration) = (torque) / (Moment of Inertia)
    (Moment of Inertia) = (MR^2) / 3 for a rod about one end
    (Torque) = Force * Radius sin(angle between them)

    3. The attempt at a solution

    M of I = ( MR^2) / 3
    Torque = Force * R
    = BIR^2sin( angle between them)

    My problem is here. Is the angle 45 degrees? If it is, shouldn't the answer be (3BI)/M*root2?

    Also, if current is switched off, the only force acting is B. Even then the rod will be rotating for some time. But current will not be induced as the field is uniform. I am terribly confused.
  2. jcsd
  3. Jan 13, 2015 #2


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    I believe it is rotating in a horizontal plane in a vertical field. All angles are right angles, no?
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