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S3 Physics: Household Electricity, Power and Cost (10 Challenge Items)
S3 Physics: Household Electricity, Power and Cost (10 Challenge Items)
Ten challenge items: total current on a board, tiered tariffs, carbon emissions, kettle efficiency, working back to power or time, and a missed unit conversion.
Focus Write the equation before substituting; efficiency and tiered bills need two steps, then check the units.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 30 |
| Difficulty mix | Challenge 10 |
| Chapter | Chapter: S3 Physics: Household Electricity, Power and Cost |
| Level | S3 |
02 Questions (10 items)
- ChallengeA extension board carries 1,500 W, 800 W and 500 W appliances at 220 V. Find the total current and judge whether a 13 A board is safe.
- ChallengeFirst 100 units cost $0.9 each and later units $1.4 each. Find the bill for 260 units.
- ChallengeUsing 36 kWh with 0.7 kg of carbon dioxide per unit, find the total emissions.
- ChallengeA 2,000 W kettle heats 1 kg of water from 20 °C to 100 °C in 3 minutes (specific heat capacity 4,200 J/(kg·°C)). Find the efficiency.
- ChallengeAn appliance uses 2.4 kWh in 4 hours. Find its power.
- ChallengeHow long must a 1,200 W appliance run to use 1.8 kWh?
- ChallengeConvert 0.75 kWh into joules.
- ChallengeA 10 A, 220 V circuit already has 1,200 W and 800 W loads. Can another 300 W be added?
- ChallengeOption A: 2,500 W for 0.8 hour. Option B: 1,000 W for 2.2 hours. Which uses less energy and by how much?
- ChallengeCorrect the work: the energy of a 1,200 W appliance used for 3 hours was given as 3,600 kWh. Find the error and the correct answer.
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Total 2,800 W; I = 2,800 ÷ 220 ≈ 12.73 A. It is below 13 A but close to the limit, so it is not safe for long full-load use. | Near the limit the cable heats up, so split the load across another socket. |
| 2 | First 100 units: $90; remaining 160 × $1.4 = $224; total $314. | Tiered tariffs must be calculated tier by tier, not with one flat rate. |
| 3 | 36 × 0.7 = 25.2 kg. | Emissions = units used × emission factor per unit. |
| 4 | Useful energy = 4,200 × 1 × 80 = 336,000 J; input = 2,000 × 180 = 360,000 J; efficiency = 93.3%. | Efficiency = useful output ÷ total input, both in the same energy unit. |
| 5 | P = E ÷ t = 2,400 Wh ÷ 4 h = 600 W. | Convert kWh to Wh first, then divide by the time. |
| 6 | t = 1,800 Wh ÷ 1,200 W = 1.5 hours. | Energy ÷ power gives time; Wh over W gives hours. |
| 7 | 0.75 × 3.6 × 10⁶ = 2,700,000 J. | A decimal number of units is still multiplied by 3.6 × 10⁶. |
| 8 | Limit 2,200 W; present 2,000 W; 2,300 W exceeds the limit, so no. | Work out the total power before adding, not just the small extra appliance. |
| 9 | A: 2.5 × 0.8 = 2 kWh; B: 1 × 2.2 = 2.2 kWh; A uses 0.2 kWh less. | Compare total energy, not power or time alone. |
| 10 | 1,200 W × 3 h = 3,600 Wh = 3.6 kWh, not 3,600 kWh. | Wh must be divided by 1,000 to give kWh; skipping it inflates the answer a thousand times. |
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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