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Maximum torque on an electric motor

  1. Oct 6, 2008 #1
    1. The problem statement, all variables and given/known data
    In an electric motor, a coil with 200 turns of radius 2 cm can rotate between the poles of a magnet. The magnetic field strength is 0.2 T. When the current through the coil is 50 mA, what is the magnitude of the maximum torque that the motor can deliver?

    N = 200 turns
    R = 2e-2 m
    B = 0.2 T
    I = 50e-3 A

    τ = ?

    2. Relevant equations

    F = ILB
    B = µ0(N/L)I/sqrt (1 + (2R/L)2)

    µ0 = 1.257e-6 N*s2/C2

    3. The attempt at a solution

    I first wanted to find what the L was so I could solve for the force. So I tried that. That didn't work out too well. I then tried anything. I'm still not sure I entirely understand the concept. I cannot receive credit for the problem even if I get it right anymore, but I'd like to know how to handle this if it pops up on the exam.

    http://img48.imageshack.us/img48/1562/electricmotor1oc1.th.jpg [Broken]

    Thanks for any and all help,
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Oct 6, 2008 #2


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    Science Advisor
    Gold Member

    The torque arises ultimately from the Lorentz forces on the charges moving at velocity v within the wires

    Look in your text, or in a book like Reitz and Milford, Foundations of Electromagnetic Theory. You'll see that, for a loop or coil, the torque T becomes proportional to coil current I and area A
    [tex]I \vec{A}[/tex] is defined to be the magnetic moment.
    Last edited: Oct 6, 2008
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