MYP Physics · 18 · extended response
Is recovered waste heat the same as perfect efficiency?
Climate and energy systems · Original GioPhysics question with a detailed, mark-by-mark answer guide.
- Demand
- discriminating
- Marks
- 8
- Topics
- 3
- Answer
- Complete
A combined heat-and-power plant receives 100 MJ of fuel energy. It supplies 35 MJ of electrical energy, rejects 65 MJ as thermal energy, and can deliver 40 MJ of that thermal energy to nearby buildings in winter. An advertisement calls the plant ‘100% efficient because no energy is destroyed.’
- a
Calculate Calculate the electrical efficiency and the winter useful-energy efficiency.
3 marks - b
Explain Explain why conservation of energy does not make every device 100% efficient.
2 marks - c
Evaluate Evaluate the advertisement and the plant's value across a full year.
3 marks
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a
Calculate Calculate the electrical efficiency and the winter useful-energy efficiency.
Build a chain of claim, evidence and physics reasoning. Address more than one relevant factor, identify a limitation or assumption, and finish with a conclusion that is conditional on the evidence rather than absolute.
- 1
Calculates electrical efficiency = 35 ÷ 100 = 35%.
- 2
Adds useful outputs: 35 MJ + 40 MJ = 75 MJ.
- 3
Calculates winter useful-energy efficiency = 75 ÷ 100 = 75%.
b
Explain Explain why conservation of energy does not make every device 100% efficient.
Build a chain of claim, evidence and physics reasoning. Address more than one relevant factor, identify a limitation or assumption, and finish with a conclusion that is conditional on the evidence rather than absolute.
- 1
States that total energy is conserved.
- 2
Explains that some energy becomes less useful or is transferred to unwanted surroundings, so useful output is smaller than input.
c
Evaluate Evaluate the advertisement and the plant's value across a full year.
Build a chain of claim, evidence and physics reasoning. Address more than one relevant factor, identify a limitation or assumption, and finish with a conclusion that is conditional on the evidence rather than absolute.
- 1
Rejects the 100% claim as a confusion between conservation and useful-energy efficiency.
- 2
Recognises that 75% depends on a real demand for the recovered heat, which may fall in warm seasons or at distant buildings.
- 3
Gives a balanced conclusion: heat recovery can substantially improve resource use, but performance must be reported for actual demand and system boundaries.
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