Power and efficiencyEdexcel International A Level Physics: Subtopic test

1. 1
   A crane's electric motor raises a 250 kg load vertically through 12 m at constant speed in 20 s. The electrical power supplied to the motor is 2.4 kW. The acceleration of free fall is 9.81 m s⁻².

   (a)

   What is the gain in gravitational potential energy of the load?

   [1 mark]

   • A29 kJ
   • B3.0 kJ
   • C2.5 kJ
   • D1.5 kJ

   (b)

   What is the useful power output of the motor?

   [1 mark]

   • A590 kW
   • B1.5 kW
   • C29 kW
   • D0.00068 W

   (c)

   Calculate the efficiency of the motor.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A car moves at constant speed along a level road against a constant resistive force of 600 N. In 30 s it travels 750 m. The fuel supplies energy to the engine at a rate of 60 kW.

   (a)

   What is the work done by the engine against the resistive force in the 30 s?

   [1 mark]

   • A18 kJ
   • B22.5 kJ
   • C1.25 kJ
   • D450 kJ

   (b)

   What is the useful power output of the engine?

   [1 mark]

   • A450 kW
   • B13.5 MW
   • C15 kW
   • D25 W

   (c)

   Calculate the efficiency of the engine and the rate at which energy is wasted.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A hydroelectric power station uses water that falls through a vertical height of 40 m before passing through turbines. Water flows through the turbines at a rate of 1500 kg every second, and the electrical power output of the generators is 480 kW. The acceleration of free fall is 9.81 m s⁻².

   (a)

   Calculate the rate at which the falling water transfers gravitational potential energy to the turbines.

   [3 marks]

   (b)

   Calculate the efficiency of the power station. Suggest two reasons why it is not 100%.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A winch with an electric motor raises a crate of mass 120 kg vertically at constant speed through 15 m in 25 s. Over this time the motor takes 28 kJ of electrical energy from its supply. The acceleration of free fall is 9.81 m s⁻².

   (a)

   Determine the efficiency of the winch motor and explain, using the principle of conservation of energy, what happens to the energy that is not usefully transferred.

   [6 marks]

   (b)

   A manager claims that a motor with twice the power output would raise the same crate in half the time and would also be twice as efficient. Evaluate the manager's claim.

   [6 marks]

   Total for question 4: 12 marks

End of questions