Power and efficiencyEdexcel A-Level Physics: Flashcards
What these 13 flashcards ask
- What is power?
- Write two equations for power.
- What is the unit of power, in base terms?
- How do you find the energy transferred from a power and a time?
- What is efficiency in terms of energy?
- What is efficiency in terms of power?
- Can an efficiency be greater than 100%?
- What is the unit of efficiency?
- How is the wasted energy found?
- In what form is most wasted energy transferred?
- How do you find the overall efficiency of two devices working in a chain?
- A hoist raises a mass m through a height h in time t. What is the useful power output?
- A motor with a useful output of 200 W is 25% efficient. What is its input power?
Exam questions on Power and efficiency
- A motor in a building site hoist lifts a load of mass 250 kg through a height of 12 m at constant speed in 30 s. The electrical power supplied to the motor during the lift is 1.4 kW. Take g = 9.81 N kg⁻¹.Calculate the energy wasted by the motor during the lift.2 marks
- A householder compares two lamps that both give 3.0 W of light output. The filament lamp has an input power of 60 W. The LED lamp has an input power of 9.0 W.The householder replaces a filament lamp with the LED lamp and uses it for 1000 hours. Calculate the energy saved, in joules.2 marks
- A cyclist, with a bicycle, has a total mass of 85 kg and rides at a steady 4.0 m s⁻¹ up a road that rises 6.0 m for every 100 m travelled along the road. Resistive forces may be ignored. Take g = 9.81 N kg⁻¹.Calculate the useful power the cyclist must produce to raise herself and her bicycle up the road.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).