Posted by **Anonymous** on Saturday, October 20, 2007 at 5:45pm.

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 -
**bobpursley**, Saturday, October 20, 2007 at 6:59pm
energy=voltage*current*timeinseconds

where energy is in Joules.

- science -
**Anonymous**, Wednesday, February 27, 2013 at 1:51am
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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