Checkpoint 1, Energy Conservation
In 2023, CSIRO's solar thermal test rig achieved 750 °C, applying every concept from this checkpoint in one real system.
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Lessons 1–6 covered the core physics of energy. You explored how energy cannot be created or destroyed, only transformed, and how we track those transformations through calculations, Sankey diagrams, and efficiency values, building on the ideas of energy forms, transfer, and work.
"A ball at the bottom has more total energy than at the top."
Total mechanical energy is conserved (ignoring friction). GPE converts to KE, total stays the same.
"90% efficient means it wastes 90%."
90% efficient means 90% is USEFUL, only 10% is wasted.
"Holding a box still does work because it's heavy."
Work = Force × distance. No movement = no work done.
"Wider waste arrow in a Sankey diagram means more efficiency."
Wider waste arrow = MORE wasted energy = LOWER efficiency.
- Conservation of energy
- Efficiency
- Sankey diagram
- Diagram showing energy flow, arrow width = energy amount
- Energy cannot be created or destroyed, only transformed
- Useful energy out divided by total energy in × 100%
At the start of Lessons 1–6, you were new to energy conservation. Now that you have worked through the full checkpoint, reflect on your understanding. Which concept clicked most for you? Which still feels uncertain?
Q1. 6. Draw a simple Sankey diagram for a device with 800 J input, 200 J useful output, and 600 J waste. Use a scale of 1 cm = 100 J. Label all arrows with energy values, forms, and units. Calculate and state the efficiency.
1 mark for correct arrow widths (8 cm, 2 cm, 6 cm). 1 mark for labels with values, forms and units. 1 mark for efficiency = 25%.Q2. 7. A family is choosing between two kettles. Kettle A is 2,000 W and boils 1 litre of water in 3 minutes. Kettle B is 1,000 W and boils the same amount in 6 minutes. Both are 90% efficient.
1 mark for calculating energy for Kettle A (360,000 J or 0.1 kWh). 1 mark for calculating energy for Kettle B (360,000 J or 0.1 kWh). 1 mark for explaining that both use the same energy but Kettle A is faster. 1 mark for cost calculation and recommendation with reasoning.Model answers (click to reveal)
Comprehensive Answers
▼Multiple Choice
1. B Useful = 1,000 × 0.35 = 350 MJ. Waste = 1,000 − 350 = 650 MJ.
2. C Water at height has GPE → falls and gains KE → spins turbines → generates electrical energy.
3. A Force = 30 × 10 = 300 N. Work = 300 × 2 = 600 J. The student used mass instead of force.
Short Answer Model Answers
Q6 (3 marks): Input arrow: 8 cm wide, labelled "800 J chemical energy" [0.5]. Useful output: 2 cm wide, labelled "200 J useful energy" [0.5]. Waste: 6 cm wide, labelled "600 J waste thermal energy" [0.5]. Scale stated: 1 cm = 100 J [0.5]. Efficiency = (200 ÷ 800) × 100 = 25% [1 mark].
Q7 (4 marks): (a) Kettle A: 2,000 W × 180 s = 360,000 J (0.1 kWh) [0.5]. Kettle B: 1,000 W × 360 s = 360,000 J (0.1 kWh) [0.5]. (b) Both use the same energy because they heat the same water [0.5]. Kettle A is more powerful, doing the same work in half the time [0.5]. Cost: both = 0.1 × $0.30 = $0.03 per boil [0.5]. Recommendation: Kettle A for busy households where speed matters; Kettle B for energy-conscious users on a budget (lower upfront cost) [0.5].
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