GCSE Physics, Unit 3, Lesson 3 of 6

Energy transferred, kilowatt-hours and cost

GCSE Physics, Mains electricity, the National Grid and static, 25 minutes

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E = Pt and E = QV, energy in kilowatt-hours, and working out what it costs to run an appliance.

What you will learn

  • Calculate energy with E = Pt and E = QV, with the time in seconds
  • Calculate energy in kWh with the power in kW and the time in hours
  • Work out the cost of using an appliance from its kWh and the price
  • Compare appliances by the energy they transfer

Two ways to find the energy

Energy transferred depends on how fast it goes (power) and for how long:

energy transferred = power × time
E = P × t

Energy in joules (J), power in watts (W), time in seconds (s). You can also count the charge: each coulomb that passes through a p.d. of 1 V transfers 1 J, so:

energy transferred = charge flow × potential difference
E = Q × V

Energy in J, charge in coulombs (C), p.d. in V. Recall both (AQA, Edexcel and OCR).

If you know the current instead of the charge, find the charge first with Q = I × t (charge = current × time, from the circuits unit), with the time in seconds. Rearranged: Q = E ÷ V, and t = Q ÷ I.

  1. A 3 A lamp runs from a 6 V battery for 10 minutes. Find the energy.
  2. t = 10 × 60 = 600 s
  3. Q = I t = 3 × 600 = 1800 C
  4. E = Q V = 1800 × 6 = 10 800 J = 10.8 kJ
  1. A 1500 W hairdryer is used for 4 minutes
  2. t = 4 × 60 = 240 s
  3. E = 1500 × 240 = 360 000 J = 360 kJ

Minutes into seconds (× 60) before you multiply, and kJ is J ÷ 1000.

The rest of this lesson and its 9 practice questions are locked.

In this lesson

  1. Two ways to find the energy
  2. Kilowatt-hours
  3. What it costs
  4. Try it: E = Pt with minutes

9 practice questions, marked as you go; about 25 minutes in all.

In this unit: Mains electricity, the National Grid and static

Direct and alternating current, the three-core cable and plug, electrical power and energy, the cost of electricity, the National Grid and transformers, and static charge and electric fields.

All of Mains electricity, the National Grid and static

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