Cambridge International AS and A level Physics
Exercise 5.4 Power Power is the rate at which work is done, or the rate at which energy is transferred. Its unit is the watt, W (1 W = 1 J s −1). 1 You can apply the idea of power to any situation where energy is transferred. In these examples, divide energy by time to find power, or multiply power by time to find energy. a Calculate the power of an electric motor which transfers 180 000 J of energy in one minute. Give your answer in kW. b A car has an engine rated at 45 kW. Calculate the energy transferred by the engine in one minute. c A healthy, adult human requires about 10 MJ of energy from their food each day. Estimate their average power. 2 A lift in a shopping centre can transport 20 people to a height of 54 m in a time of 14 s. The lift compartment has a mass of 1420 kg and the average person has a mass of 60 kg. a Calculate the combined mass of the compartment and the 20 people in it. b Calculate the energy gained as the lift travels upwards. Think about the lift as it rises. Which form of energy is increasing? c Calculate the rate at which energy is transferred by the lift motor to the compartment. 3 The power rating of an electric lamp tells you the rate at which it uses electrical energy. 32
Lamp A has a power rating of 24 W, and a light output of 2.3 W Lamp B has a power rating of 100 W, and a light output of 3.2 W Which lamp is the more efficient? Show your calculation for each lamp. Note that efficiency can be calculated using values of power instead of energy.
Exam-style questions 1 a A steel ball of mass 20.0 g is placed on a smooth, curved track as shown:
h
When released, the ball rolls back and forth along the track. At its lowest point, its speed is 0.47 m s −1. Calculate its KE at this point.
[3]
b Deduce the height h above the bottom of the track from which it was released.
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