Power and efficiencyEdexcel International A Level Physics: Mind map

Exam questions on Power and efficiency

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.
   Calculate the efficiency of the motor.2 marks
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.
   Calculate the efficiency of the engine and the rate at which energy is wasted.2 marks
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.
   Calculate the rate at which the falling water transfers gravitational potential energy to the turbines.3 marks