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How is energy transferred between stores, and how is efficiency calculated?

Energy stores and transfers, the conservation of energy, wasted energy, and the calculation of efficiency as the useful output over the total input.

A focused answer to WJEC GCSE Physics topic 1.3 on energy, covering energy stores and transfers, the conservation of energy, energy wasted as heat, and how to calculate efficiency as the useful energy or power output divided by the total input.

Generated by Claude Opus 4.89 min answer

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  1. What this topic is asking
  2. Energy stores and transfers
  3. Conservation of energy
  4. Wasted energy and efficiency
  5. Showing transfers with diagrams
  6. Try this

What this topic is asking

WJEC wants you to describe energy stores and transfers, state the conservation of energy, explain why energy is wasted, and calculate efficiency. This is part of topic 1.3 Making use of energy in Unit 1 of WJEC GCSE Physics (3420).

Energy stores and transfers

Conservation of energy

Wasted energy and efficiency

A more efficient device wastes less energy, which lowers running costs and environmental impact.

Showing transfers with diagrams

Energy transfers can be shown with a Sankey diagram, an arrow whose width is drawn to scale with the amount of energy. The total energy enters from the left, and the arrow splits: the useful energy continues to the right, and the wasted energy branches off, usually downwards as heat. Because the widths are to scale, you can read off the useful and wasted amounts and calculate the efficiency directly from the diagram. A wide useful branch and a narrow wasted branch show a high-efficiency device. The conservation of energy means the total width going in always equals the total width coming out, so the useful and wasted branches must add up to the input. Drawing or reading a Sankey diagram is a common exam skill, and it makes clear why improving efficiency is really about making the wasted branch as small as possible. Reducing friction with lubrication, or reducing electrical resistance, are typical ways to shrink the wasted heat and raise efficiency.

Try this

Q1. A kettle is 80% efficient and takes in 2000 J2000\,\text{J}. Calculate the useful energy. [2 marks]

  • Cue. Useful =0.80×2000=1600 J= 0.80 \times 2000 = 1600\,\text{J}.

Q2. State the most common form to which energy is wasted. [1 mark]

  • Cue. Heat (thermal energy), dissipated to the surroundings.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC 20193 marksA lamp transfers 100 J100\,\text{J} of electrical energy each second and produces 15 J15\,\text{J} of light. Calculate its efficiency as a percentage.
Show worked answer →

A topic 1.3 efficiency calculation. Efficiency is the useful output over the total input: efficiency=useful energytotal energy=15100=0.15\text{efficiency} = \dfrac{\text{useful energy}}{\text{total energy}} = \dfrac{15}{100} = 0.15 (2 marks). As a percentage, 0.15×100=15%0.15 \times 100 = 15\% (1 mark). Markers reward the correct ratio and the conversion to a percentage. The other 85 J85\,\text{J} is wasted as heat. A common error is to use the wasted energy as the useful output.

WJEC 20224 marksExplain what is meant by conservation of energy, and why no real device is 100% efficient.
Show worked answer →

A topic 1.3 Explain question. Conservation of energy means energy cannot be created or destroyed, only transferred from one store to another or dissipated (1 mark). The total energy before and after a transfer is always the same (1 mark). No real device is 100% efficient because some energy is always transferred to less useful forms, usually as heat to the surroundings through friction or resistance (1 mark), so the useful output is always less than the total input (1 mark). Markers reward the conservation statement, the constant total, the wasted heat and the resulting efficiency below 100%.

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