Power and efficiencyEdexcel International A Level Physics: Flashcards
What these 13 flashcards ask
- Define power.
- What is the unit of power?
- A force does 450 kJ of work in 30 s. What is the power?
- Write two equations for efficiency.
- What is the unit of efficiency?
- Can efficiency be greater than 100%?
- Where does wasted energy go?
- What is the useful energy output when a motor lifts a load?
- How is the useful power of a lifting motor found?
- Why is it wrong to say energy is 'lost' in a machine?
- A machine has an input power of 60 kW and a useful output of 15 kW. What is its efficiency?
- How many watts are in a kilowatt and a megawatt?
- How can the efficiency of a motor be improved?
Exam questions on Power and efficiency
- 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
- 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
- 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
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).