Year 9 Science · Unit 3 · Lesson 3
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Learning Goals
1. Write the equation for efficiency. Explain why no real device can have an efficiency of 100%.
Scenario: a factory conveyor belt
An electric motor in a factory takes in 500 J of electrical energy. It turns 350 J into kinetic energy that drives a conveyor belt. The rest becomes waste heat, because of friction in the motor’s moving parts.
2. Calculate the waste heat produced by the motor, and the motor’s efficiency.
Waste heat J
Efficiency %
3. An engineer redesigns the motor to reduce friction. Predict what happens to the motor’s efficiency. Explain the effect on both the useful output and the waste heat, if the input stays at 500 J.
4. An e-bike battery stores 0.5 kWh of energy when full. (1 kWh = 3.6 MJ.) (a) Convert 0.5 kWh into megajoules and into joules. (b) The e-bike’s motor is 80% efficient. How much of the stored energy becomes useful kinetic energy?
(a) MJ
(a) J
(b) MJ
5. A 1,200 kg car travels at 25 m/s. (a) Use KE = ½mv² to calculate its kinetic energy in joules and in kilojoules. (b) The driver brakes to a stop. If all of the kinetic energy becomes thermal energy in the brakes, tyres and road, how much thermal energy is produced?
(a) J
(a) kJ
(b) kJ
Data: four household devices
Table 1 shows the energy input and the useful energy output of four devices, each measured over the same short time.
| Device | Energy input (J) | Useful output (J) | Waste energy (J) | Efficiency (%) |
|---|---|---|---|---|
| LED light globe | 100 | 25 | ||
| Incandescent light globe | 100 | 5 | ||
| Microwave oven heating soup | 1,200 | 720 | ||
| E-bike motor | 1,500 | 1,200 |
Source: sample values, typical of each device.
6. Complete the last two columns of Table 1.
7. Which device wastes the most energy in joules? Is it also the device with the lowest efficiency? Explain what this shows about comparing devices.
8. Two students disagree about the conveyor-belt motor from the scenario. Maya says 70% efficiency is “good enough”. Jordan says it is “very wasteful”. Evaluate both views.