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.

Power Power is the rate of doing work Formula: P = power, W = work, t

= time Power is measured in watts, W

Power can also refer to the rate at which energy is converted from one form into another.

Formula: E = energy converted

Power 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.

Efficiency Energy conversion is not lossless. Energy in most conversion is lost to heat, sound, or

sometimes light. These processes are not efficient. Formula:

Efficiency 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.