1. Power – Learning Outcomes  Define power and its unit.  Solve problems about power.  Solve problems about efficiency.  Estimate power developed by:  person running upstairs,  person repeatedly lifting weights,  Discuss efficient energy use in the home.  Recognise the power of common devices.

2. Power

3.  e.g. A light bulb uses 18 000 J in 20 minutes. What is the power of the bulb?  e.g. A heater is rated at 15 kW. How long will it take to use 60 MJ of energy?  e.g. A weight lifter raises a mass of 30 kg through a height of 0.6 m. She does this 50 times in half a minute. Find the average power she develops.  e.g. Estimate the power of someone running up one storey of stairs.

4. Efficiency

5.  e.g. An engine has an input power of 4500 W. The useful output power is 3000 W. Calculate its percentage efficiency.  e.g. A 60 kW motor in a crane lifts a mass of 4000 kg through a height of 10 m in 20 s. Calculate the efficiency of the motor if it is working at full power.  e.g. A petrol car engine is 30% efficient. Kinetic energy is produced at a rate of 105 kW. The rest of the energy input appears as heat. Calculate the rate at which heat is produced.