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physics question?
(Picture)
A) p = e/t
5kw * 20 = 100kJ??
B) 12kj/20 = 600w
600:5000 = 12%??
1 Answer
- ?Lv 76 years ago
motor time of operation = 20 s
motor input power = 5000 w = 5000 J/s
motor input energy = 5000(20) = 100,000 J ANS a)
motor output power = 12,000 J/20 s = 600 J/s = 600 w
motor output energy = 600(20) = 12,000 J
motor efficiency = energy out/energy in = 12,000/100,000 = 0.12 = 12% ANS b)*
*Comment: This is one of those questions I dislike - because it teaches an incorrect practical fact:
electric motors are by themselves very efficient (80% efficient range) in transforming electrical energy into mechanical energy. It is when the motor is connected to other equipment that the COMBINATION becomes a less efficient SYSTEM. In this case the motor was identified but the other equipment it was operating was not and it leaves the impression that the motor, itself, was 12% efficient which in the real world it is not.