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Homework Help: Electric Motor and Magnetic Field

  1. Jul 17, 2007 #1
    The rotor in a certain electric motor is a flat, rectangular coil with 90 turns of wire and dimensions 2.50 cm by 4.00 cm. The rotor rotates in a uniform magnetic field of 0.800 T. When the plane of the rotor is perpendicular to the direction of the magnetic field, it carries a current of 9.1 mA. In this orientation, the magnetic moment of the rotor is directed opposite the magnetic field. The rotor then turns through one-half revolution. This process is repeated to cause the rotor to turn steadily at 3600 rev/min.

    (a) Find the maximum torque (max) acting on the rotor.
    (b) Find the peak power output (max) of the motor.
    (c) Determine the amount of work (W) performed by the magnetic field on the rotor in every full revolution.
    (d) What is the average power (avg) of the motor?

    I cannot figure out the answer to part D.

    I know for part A that the torque would equal = (90)(9.1x10-3A)(0.025mx0.04m)(0.800T) = 6.553e-4

    Then for part B, it equals the answer to A x 2pi(60) = 0.247W

    Then part C is given by 4(9.1 x 10-3A)(90)(0.025mx0.04m) = 0.00262J

    For part D, I thought it would equal part C/(3600/60) ie you know the total work in Joules for a full revolution and now want to divide it by the number of revolutions per second (3600/60)....when I do this on my calculator, I get an answer of 4.3666e-5

    The online web program is telling me that I am off by orders of magnitude....but I cannot figure out what I am doing wrong.
  2. jcsd
  3. Jul 17, 2007 #2
    if c is the amount of work done on the rotor and the amount of work the rotor does per rev and it does 3600rev/min why is it not C * 3600/60, if that is correct then your answer is off by two orders of magnitude
  4. Oct 18, 2008 #3
    Then part C is given by 4(9.1 x 10-3A)(90)(0.025mx0.04m) = 0.00262J

    Can you explain why this part is:


    N = number of loops
    I = Current
    A = Area of Loop
    B = Magnetic Field.

  5. Oct 20, 2008 #4
    I am not sure where the 4 comes from in part C. Everything else, however, makes sense.
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