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S1 Science: Energy Forms and Conversion (Paper 3: challenge)
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S1 Science: Energy Forms and Conversion (Paper 3: challenge)
The third paper compares and works backwards: get the efficiency from the waste, and read power next to efficiency.
Direct answers For a comparison, work out each useful output first and then subtract, never compare the efficiency figures alone.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 20 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: S1 Science: Energy Forms and Conversion |
| Level | S1 Science |
02 Questions (10 items)
- FoundationWhy is the efficiency of a real machine usually below 100 per cent?
- FoundationDoes high power mean high efficiency?
- FoundationBoth the kilowatt hour and the joule measure energy, so which suits an electricity bill?
- IntermediateA machine takes in 1 200 J and wastes 300 J. What is its efficiency?
- IntermediateA machine is 40 per cent efficient and gives 240 J of useful output. What are the input and the waste?
- IntermediateA 3 000 W air conditioner runs for 4 hours at $1.2 per kWh. What is the cost?
- ChallengeA solar panel receives 5 kWh of light at 20 per cent efficiency. How much electrical energy does it produce?
- ChallengeA lift carrying 2 000 kg rises 10 m in 20 seconds, with g taken as 10 N/kg. What is the power in kW?
- ChallengeMachine A is 50 per cent efficient on 800 J while B is 30 per cent efficient on 2 000 J. Which gives more useful output, and by how much?
- Error redoA student reports an efficiency of 120 per cent. Why must this be wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Because of friction, heat and sound losses | The energy is not lost but turns into forms we cannot use, so the useful output falls short. |
| 2 | No, they are different quantities | Power is the energy converted each second while efficiency is the useful share, and a powerful machine can still be wasteful. |
| 3 | The kilowatt hour | Household appliances are rated in kilowatts and time in hours, so the kilowatt hour is the direct unit. |
| 4 | 75 per cent | The useful output is 1 200 minus 300 or 900 J, and 900 over 1 200 is 75 per cent. |
| 5 | Input 600 J and waste 360 J | 240 divided by 40 per cent is 600 J, and 600 minus 240 leaves 360 J wasted. |
| 6 | $14.40 | Three kilowatts for four hours is 12 kWh, and 12 times 1.2 gives $14.40. |
| 7 | 1 kWh | 20 per cent of 5 kWh is 1 kWh, and the other 4 kWh becomes heat. |
| 8 | 10 kW | The work is 2 000 times 10 times 10 or 200 000 J, and dividing by 20 seconds gives 10 000 W or 10 kW. |
| 9 | B gives 200 J more | A gives 400 J and B gives 600 J, a gap of 200 J, so lower efficiency does not mean less output. |
| 10 | The useful output can never exceed the total input | Conservation of energy caps the useful part at the input, so 100 per cent is the ceiling. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
The challenge paper runs three comparisons: work back from the wasted energy to the efficiency, judge power together with efficiency, and total up several appliances. Convert every unit first.Tutor Registration →
Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.