Power and Efficiency

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Work, Energy and Power Learning Material
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5.5Power and Efficiency

Power and Efficiency Revision Questions

1. The engine of a car produces 12000 J of energy every second. From this amount, 4500 J convert into heat, 100 J into sound energy and 200 J into light energy. What is the efficiency of the engine?

(Hint: The rest is the mechanical energy of the car)

  1. 40 %
  2. 60 %
  3. 93.75 %
  4. 72 %

Correct Answer: B

2. An electric motor shows the value 2000 W stamped on its back. Its owner turns off all the other electric appliances except the motor and finds out the motor has drawn 150 000 J electrical energy from the main source in one minute. What is the efficiency of this electric motor?

  1. 80 %
  2. 20 %
  3. 75 %
  4. 1.3 %

Correct Answer: A

3. A crane can lift a 1.2 t heavy object at h = 10 m in 20 s. Its input power is 10 kW. What is the crane's efficiency? (Take g = 10 N/kg. Also, 1 t = 1000 kg and 1 kW = 1000 W).

  1. 50 %
  2. 60 %
  3. 80 %
  4. 83 %

Correct Answer: B

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