Efficiency Knowledge-Based Practice

Questions

1. 1.

   A hydroelectric plant uses a water turbine with an efficiency of 40% coupled to an electrical generator with an efficiency of 75%. What is the overall efficiency of this combined energy conversion system?

   • A) 115%
   • B) 57.5%
   • C) 35%
   • D) 30%
2. 2.

   Machine A has an efficiency of 20%, and Machine B has an efficiency of 50%. If both machines perform the same amount of useful work, what is the ratio of the energy input of Machine A to that of Machine B?

   • A) 0.4
   • B) 1.5
   • C) 2.0
   • D) 2.5
3. 3.

   A machine performs 6000 J of useful work. If its efficiency is improved from 25% to 40%, by how much does the required total energy input decrease?

   • A) 900 J
   • B) 6000 J
   • C) 9000 J
   • D) 15 000 J
4. 4.

   A machine performs 1800 J of useful work while dissipating 600 J of energy as heat to the surroundings. What is the efficiency of the machine?

   • A) 25%
   • B) 33%
   • C) 75%
   • D) 67%
5. 5.

   A mechanical system is supplied with 80 kJ of total energy and operates with an efficiency of 65%. How much energy is dissipated as waste energy to the surroundings?

   • A) 12 kJ
   • B) 52 kJ
   • C) 48 kJ
   • D) 28 kJ
6. 6.

   An electric winch operates with an efficiency of 75% when supplied with an electrical power input of 800 W. How much time does it take for the winch to perform 3600 J of useful work?

   • A) 4.5 s
   • B) 6.0 s
   • C) 8.0 s
   • D) 18 s
7. 7.

   A crane lifts a 400 kg load vertically through a height of 15 m at constant speed in 10 s. If the crane operates with an overall efficiency of 75%, what total electrical power must be supplied to the motor? (Assume $g = 10\text{ m/s}^2$.)

   • A) 4.5 kW
   • B) 6.0 kW
   • C) 7.5 kW
   • D) 8.0 kW
8. 8.

   An electric motor with an efficiency of 70% is supplied with 2000 W of electrical power. How much useful work does the motor perform in 15 seconds?

   • A) 9,000 J
   • B) 21,000 J
   • C) 30,000 J
   • D) 42,860 J
9. 9.

   An electric motor operates with an efficiency of 60% and dissipates 400 W of power as heat to its surroundings. What is the total input power supplied to the motor?

   • A) 240 W
   • B) 667 W
   • C) 1000 W
   • D) 1600 W
10. 10.

    An electric hoist consumes a total electrical power input of 500 W while delivering 350 W of useful mechanical output power to lift a load. What is the efficiency of the hoist?

    • A) 30%
    • B) 43%
    • C) 70%
    • D) 143%

Answers and step-by-step solutions appear after you finish the test.