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a 12V , 3A motor takes 120 minutes to do a certain job. a 120V, 8A motor takes 5 minutes to do the same job. Which motor uses the least energy and by how much?

  • science -

    energy=voltage*current*timeinseconds
    where energy is in Joules.

  • science -

    a)

    Power of the motor P = 3.8hp
    = (3.8hp)(746W/hp)
    = 2834.8W
    Motor is 90% efficient.
    Therefore Power absorbed by the motor is (2834.8W)/(0.9) =3150W
    But power P = V I
    then current in the motor I =(3150W)/(120V)
    = 26.25A

    (b)
    Energy delivered to the motor E = P*t
    = (3150W)(4h)(3600s/h)
    = 4.536 *107J

    (c) E = (3150W)(4h)
    =(3.15*4)kWh
    =12.6kWh
    Therefore cost to run the motor for 4h is(12.6kWh)($0.16/kWh)
    = $2.016

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