P1a: 10
The efficiency of a machine can be worked out using the equation:
Efficiency = useful energy output total energy input
Use this equation to answer the following questions:
1 An engine has an energy input of 100 J and gives out 27 J of useful kinetic energy. What is the efficiency of the engine?
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
2 A hair dryer has an energy input of 1500 J and gives out 1300 J of useful energy. What is the efficiency of the hair dryer?
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
3 Compare these two machines to decide which is more efficient.
i Machine A uses 1000 J of energy to do 300 J of useful work.
ii Machine B uses 500 J of energy to do 200 J of useful work.
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
4 Compare these two cranes to decide which is the more efficient. i Crane A uses 800 J of energy to do 350 J of useful work. ii Crane B uses 750 J of energy to do 300 J of useful work.
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________

P1a: 10 – continued
5 Look at these energy diagrams. Work out the efficiency of each device.
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
6 The motor for a lift in a tall building uses 12 000 W of power. The lift and its passengers has a mass of 500 kg. The lift motor takes 10 s to raise the lift and its passengers through a height of
20 m. Work out the efficiency of the lift motor.
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________

P1a: 10
Worksheet P1a: 10
1 27 ÷ 100 = 27%.
2 1300 ÷ 1500 = 87%.
3 Machine A efficiency: 300 ÷ 1000 = 30%. Machine B efficiency: 200 ÷ 500 = 40%.
4 Crane A efficiency: 350 ÷ 800 = 44%. Crane B efficiency: 300 ÷ 750 = 40%.
5 Hydrogen fuel cell efficiency: 650 ÷ 1000 = 65%. Steam-diesel hybrid efficiency:
200 ÷ 500 = 40%. Electric car efficiency: 480 ÷ 600 = 80%.
6 Total work done by lift = mass

10

height = 500

10

20 = 100 000 J. Work done each second = 100 000 ÷ 10 = 10 000 J. Efficiency = 10 000 ÷ 12 000 = 83%.