It is a scalar quantity with an SI unit of watt (W). (average) power 1 watt of power is used when 1 joule of work is done per second. 1 W 1 J s1
= work time
SAMPLE PROBLEM 5E Electrical energy is often measured in kilowatt-hours; calculate the energy in joules of 1 kilowatt-hour.
SAMPLE ANSWER 5E
1 kW h 1000 watts of power used continuously for 1 hour. 1000 joules of energy used every second continuously for (60 60) seconds
1000 60 60 3 600 000 There is 3.6 MJ of energy in 1 kW h. SAMPLE PROBLEM 5F
JAMES WATT, 1736–1819 (SCOTTISH)
He invented a condenser for steam
engines, making them more efficient.The SI unit of power is named in his honour.
Calculate the average power developed by a 70 kg woman running up a set of stairs, given that each of the 15 steps is 20 cm high and it takes her 10 s to reach the top.Take acceleration due to gravity as 9.8 m s2.
SAMPLE ANSWER 5F
Using the formula for work and power, making sure all values are in their standard unit: The height reached by the woman is given by: h 15 20 cm 15 0.2 m 3 m Work gain in energy mgh 70 9.8 3 2 058 J
The average power developed by the woman running up the stairs is 205.8 W. power work
time 2 058 10 205.8 SAMPLE PROBLEM 5G
Taking acceleration due to gravity as 9.8 m s2, calculate the average power developed by a man when lifting weights if: (i) he lifts an 80 kg barbell 60 cm in half a second
(ii) he performs 10 of those lifts in 40 s
SAMPLE ANSWER 5G Using the formula for potential energy and power, making sure all values are in their standard unit:
