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Homework Help: Electric motor: torque problem

  1. Oct 23, 2008 #1
    1. The problem statement, all variables and given/known data
    An electric motor consumes 12.0 KJ of electrical energy in 1.00 min. If one-third of this energy goes into heat and other forms of internal energy of the motor, with the rest going to the motor output, how much torque will this engine develop if you run it at 3000 rpm?

    2. Relevant equations

    3. The attempt at a solution
    Since one third of the energy is converted to heat only 2/3 contribute to torque or 8.0 KJ

    I get P = 8KJ * 60s = 480 * 10^3 W

    converting angular speed from rpm to rad/s i get (3000 * 2 pi) / 60s = 314.2 rad/s

    since torque = power / angular speed:
    T = (480 * 10^3) / 314.2 = 1.37 *10^3

    but im not getting the right answer. any help would be greatly apreciated.
    Last edited: Oct 23, 2008
  2. jcsd
  3. Oct 23, 2008 #2


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    Homework Helper

    (I accidentally deleted my last reply.)

    I believe here is an error. A Watt is a Joule/second, so power is not the energy multiplied by the time.
  4. Oct 23, 2008 #3
    yup. thanks a lot. Just a silly mistake on my part. Thats what i get for trying to do phyics at 1 am.
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