Year 9 Science · Unit 3 · Lesson 3
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Learning Goals
Read the data table
The table below shows energy input and useful output for four common household devices. Calculate the waste energy and efficiency for each device. Round efficiency to the nearest whole number.
Reminder: Waste energy = Input − Useful output | Efficiency (%) = (Useful output ÷ Input) × 100
| Device | Energy input (J) | Useful output (J) | Waste energy (J) | Efficiency (%) |
|---|---|---|---|---|
| LED light globe | 100 | 25 | ||
| Incandescent bulb | 100 | 5 | ||
| Electric kettle | 2,000 | 1,800 | ||
| Petrol car engine | 1,000 | 250 |
(a) Which device wastes the most energy in joules? Does this mean it has the lowest efficiency? Explain.
(b) Compare the LED and incandescent bulb. Both receive 100 J of input. Which is more efficient, and by how many percentage points?
(c) Predict which device you would recommend to someone trying to reduce their energy bills. Give one reason using your calculated data.
Scenario
An electric motor in a factory receives 500 J of electrical energy. It converts 350 J into kinetic energy to drive a conveyor belt. The rest of the energy becomes waste heat due to friction in the motor's components.
(a) Calculate the amount of waste heat produced by the motor. Show your working.
(b) If an engineer redesigns the motor to reduce friction, predict what would happen to the motor's efficiency. In your prediction, explain the effect on both the useful output and the waste heat.
1. A wind turbine in South Australia receives 5,000 J of kinetic energy from the wind and converts 2,000 J into electrical energy. Calculate its efficiency and identify where the remaining energy most likely goes.
2. Two students disagree. Maya says a device with 70% efficiency is "good enough." Jordan says 70% is still "very wasteful." Evaluate both views using specific numbers, if a device receives 1,000 J at 70% efficiency, how much energy is wasted?
Wrap Up
In one sentence, what was the main idea of this lesson?