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Science · Topic

Work, energy, power and cost of electricity

Convert time to seconds for joules and to hours for kilowatt-hours — mixing them is the commonest error.

2 original questions in the bank, worth 7 marks · above the two-per-topic floor

Drill this topic All work, energy, power and cost of electricity questions

The questions, with the examiner reasoning

Calculate4 marksBGCSE

A 2.0 kW kettle is used for 6 minutes each day for 30 days. Electricity costs P1.85 per kWh. Calculate the total cost of running the kettle for the 30 days.

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Answer: P11.10

  1. Time per day in hours: 6 ÷ 60 = 0.1 h
  2. Energy per day: 2.0 kW × 0.1 h = 0.2 kWh
  3. Energy over 30 days: 6 kWh
  4. Cost: 6 × 1.85 = P11.10

Examiner technique: Kilowatt-hours need hours, not seconds. Convert the time before you multiply, and keep the unit attached to the number all the way through.

Calculate3 marksIGCSE

A learner of mass 45 kg climbs a 4.0 m staircase in 6.0 s. Take g = 10 N/kg. Calculate the power she develops.

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Answer: 300 W

  1. Work done = mgh = 45 × 10 × 4.0 = 1800 J
  2. Power = work ÷ time = 1800 ÷ 6.0
  3. Power = 300 W

Examiner technique: Energy first, power second. Answers that go straight to a power formula without showing the 1800 J normally lose the first mark.