Year 7 Science · Unit 3 · Lesson 18
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Learning Goals
Because… chain
Fill in the missing effects in the chain below. Each cause leads to the next step — follow the arrows.
Overall outcome — how does this compare to an electric vehicle?
Scenario
A mining company in Western Australia is replacing its equipment and must choose between two electric motors. Both motors use exactly 1000 J of electrical energy per cycle. Motor A converts 800 J into useful mechanical work and wastes 200 J as heat. Motor B converts 600 J into useful mechanical work and wastes 400 J as heat. The motors will each run for approximately 8 hours per day, 6 days per week.
(a) Calculate the efficiency of Motor A and Motor B. Show your working using the formula: Efficiency (%) = (useful energy out ÷ total energy in) × 100.
(b) Predict which motor would have a lower long-term electricity bill. Justify your prediction using the efficiency values you calculated.
(c) Motor B gets much hotter during operation. Using the lesson's ideas about waste energy, explain why excessive heat is a sign of low efficiency — and name one method to improve efficiency and reduce heat loss.
1. An LED light bulb has an efficiency of about 90%. An old incandescent bulb has an efficiency of about 10%. Both receive 100 J of electrical energy. How much useful light energy does each produce? What happens to the rest of the energy in the incandescent bulb?
2. Australia's Energy Star rating system rates appliances from 1 to 6 stars. A 6-star rated washing machine uses much less energy per wash than a 2-star machine. Using the concepts of efficiency and waste energy, explain why choosing a 6-star appliance saves money over time.
Wrap Up
In one sentence, what was the main idea of this lesson?