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Homework Help: Calculating 'Push' Force to Maintain Constant Speed

  1. Jul 2, 2008 #1
    1. The problem statement, all variables and given/known data
    A conducting rail in contact with conducting wires, oriented perpendicular to both wires, is pushed with constant speed, causing an induced current of 0.69 A. B = 0.50 T and R (Resistor) = 2.0

    Calculate the "push" force necessary to maintain the rail's constant speed.

    2. Relevant equations

    I assume I use F = q * v * b

    3. The attempt at a solution

    I don't think I am using the correct equation here because there is no charge on the conducting rail. I am thinking I need to use an equation that I am not yet familiar with. I went to the chapter in my book over induction and I can't find any equations dealing with a 'push' force.
  2. jcsd
  3. Jul 2, 2008 #2

    Doc Al

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    Staff: Mentor

    You want the magnetic force on a current-carrying wire in a magnetic field, not the force on a single charge. Read this: http://hyperphysics.phy-astr.gsu.edu/Hbase/magnetic/forwir2.html" [Broken]
    Last edited by a moderator: May 3, 2017
  4. Jul 2, 2008 #3
    Ohhh! Duhh. Lol. Thanks so much! That helped.

    So, F = (0.69A)(0.34m)(.50T) = .117 N

    And this would be considered the "push" force? Or would it be negative since the force is acting in the opposite direction?
  5. Jul 2, 2008 #4

    Doc Al

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    Staff: Mentor

    I assume they just want the magnitude of the force.

    The induced current leads to a magnetic force on the wire. If you don't push the wire with a force opposite to the magnetic force to cancel it out, the wire will slow down due to the magnetic force. (This is the point of Lenz's law: The induced current is not "free"--you must push on the wire to maintain it.)
  6. Jul 2, 2008 #5
    Thank you! That makes sense.
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