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.
Printable Worksheets
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Lessons 1–10 covered energy conservation, transfers and thermal energy. You explored energy forms, work, power, efficiency and Sankey diagrams, then connected those ideas to heat and temperature, conduction, convection, radiation, specific heat capacity and thermal expansion.
"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%
Across Lessons 1–10, you moved from energy conservation and efficiency into thermal energy and heat transfer. 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. Explain why expansion joints are built into steel bridges. In your answer, use the particle model to describe what happens to the steel when its temperature increases and explain what could happen if the bridge had no room to expand.
1 mark for particles gaining kinetic energy. 1 mark for increased average particle spacing causing expansion. 1 mark for linking expansion to bridge length/forces. 1 mark for explaining how expansion joints prevent buckling or structural damage.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. B Conduction and convection need particles, so the vacuum strongly reduces both. Thermal radiation can cross a vacuum, so shiny surfaces are used to reflect it.
3. A Force = 30 × 10 = 300 N. Work = 300 × 2 = 600 J. The student used mass instead of force.
4. D Q = mcΔT = 2.0 × 4,200 × 5.0 = 42,000 J.
5. A Power = work ÷ time = 1,200 ÷ 4 = 300 W.
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): When steel is heated, its particles gain kinetic energy and vibrate more strongly around their fixed positions [1]. Their average separation increases slightly, so the steel expands [1]. Across a long bridge, this small change produces a measurable increase in length and large stresses if movement is constrained [1]. Expansion joints provide room for this movement, preventing buckling, cracking or damage to the structure [1].
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