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Homework Help: Induced EMF and circular coil

  1. Jul 28, 2008 #1
    1. The problem statement, all variables and given/known data

    A circular coil, with radius of 10 cm, and 25 turns, rotates in a constant magnetic field of
    strength 2.4 T, with the axis of rotation perpendicular to the direction of the magnetic

    (a question about finding the induced voltage- 5.92 V)

    part C. If the magnetic field that the coil is in points down the page, which direction
    does the coil rotate? Answer for both cases: (1) q traveling from left to right
    and (2) q traveling from right to left in the coil. (Draw a diagram to help you.)

    2. Relevant equations

    Torque on a loop of wire in a magnetic field: τ = I · A · B · sinθ

    Magnetic flux: φ = B · A · cosθ

    Induced potential difference (emf)
    due to a changing magnetic field: V = -N · ∆φ/∆t

    3. The attempt at a solution

    I really have no idea how to start this one, since the oh-so helpful Apex (Not) Learning doesn't give me any information about this, instead restating essentially the same Faraday's Law problem throughout the 'notes'. I'm not even sure my original answer concerning the voltage is correct, since there is no area facing the B-field and therefore negligible flux.

    I've thought of using the Lorentz Force and saying that the loop rotates out of the page, but the problem seems to imply that it is (counter)clockwise in the page.
  2. jcsd
  3. Jul 30, 2008 #2

    Andrew Mason

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    Science Advisor
    Homework Helper

    This is a Lorentz force problem for a current carrying conductor in a magnetic field. You can use the right hand rule or you can work it out: The Lorentz force is a cross product of what two vectors? Take a small length of the coil 90 deg. from the axis. What is the direction of the cross product vector? Then take a small section of the coil diametrically opposite. What is the direction of the Lorentz force? That will tell you how it will rotate.

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