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S4 Physics heat transfer paper 3: ten multi-step energy items
S4 Physics heat transfer paper 3: ten multi-step energy items
Ten items link conduction rate, specific heat capacity and electricity cost, and dissect a flask layer by layer.
Direct answers Items 01 to 03 cover energy, specific heat and cost, 04 to 06 time, savings and insulation thickness, 07 to 09 mechanisms. Allow 25 minutes.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 25 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: S4 Physics Heat Transfer |
| Level | S4 |
02 Questions (10 items)
- FoundationA board transfers heat at 200 W for 30 minutes. Find the total energy transferred.
- FoundationHow much energy heats 1 kg of water by 20 K? (c = 4200 J/(kg K))
- FoundationA 1.5 kW heater runs 2 hours. Find the energy in kWh, and the cost at 3.6 dollars per kWh.
- IntermediateA 200 W heater warms 1 kg of water from 20 C to 40 C. How many seconds does it take?
- IntermediateDouble glazing passes 51.3 W and single glazing 4000 W. What percentage is saved?
- IntermediateInsulation of k = 0.04 W/(m K) covers 10 m2 across 20 K and must pass no more than 100 W. Find the minimum thickness.
- ChallengeFor the same 5 mm thickness, area and temperature difference, air passes what percentage of the power that glass does?
- ChallengeA metal spoon stands in hot water. How does heat reach the spoon, the water surface, and a hand held away from it?
- ChallengeFor a vacuum flask, what does each of the vacuum gap, silvered wall, stopper and case mainly block?
- Error redoA conduction rate of 200 W lasts 2 minutes, and a student answers 200 J. What is the correct energy?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 360000 J | Q = 200 x 1800 = 360000 J; the time must be in seconds. |
| 2 | 84000 J | E = mc(delta T) = 1 x 4200 x 20 = 84000 J; keep it apart from the conduction formula. |
| 3 | 3 kWh; 10.8 dollars | E = 1.5 x 2 = 3 kWh, times 3.6 gives 10.8 dollars; kW pairs with hours to give kWh. |
| 4 | 420 s (7 minutes) | The energy needed is 84000 J, so time = 84000 / 200 = 420 s, that is 7 minutes. |
| 5 | about 98.7% | The saving is (4000 – 51.3) / 4000 x 100%, about 98.7%, mostly from the air gap. |
| 6 | 0.08 m (8 cm) | L = kA(delta T) / (Q/t) = 0.04 x 10 x 20 / 100 = 0.08 m; thicker insulation passes less. |
| 7 | about 2.6%, a 97.4% reduction | The rate is proportional to k, and 0.026 / 1.0 = 2.6%, so an air gap barely conducts. |
| 8 | Conduction along the spoon, convection in the water, radiation to the distant hand | A solid spoon conducts by collisions, the water circulates by convection, and a hand without contact must receive radiation. |
| 9 | Vacuum blocks conduction and convection; silvering blocks radiation; the stopper limits both; the case protects | Match layer by layer: a vacuum has no particles so convection is impossible too, which is what earns the last marks. |
| 10 | 24000 J | Q = 200 x 120 = 24000 J; a watt is a joule per second, so skipping the time loses a factor of 120. |
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.
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