OCR GCSE Physics A (Gateway)
OCR GCSE Physics A P7.2 Power and Efficiency Exam Questions
Free OCR GCSE Physics A exam questions for P7.2 Power and Efficiency, with a mark scheme for every pack. Covers power as rate of energy transfer, power ratings, efficiency, energy dissipation, reducing unwanted transfers, lubrication and thermal insulation.
P7.2 Power and Efficiency exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About OCR GCSE Physics A P7.2 Power and Efficiency
P7.2 completes the P7 Energy chapter of OCR Gateway Physics A. It defines power as the rate at which energy is transferred or work is done, and asks you to compare the power ratings of appliances. You calculate efficiency and explain ways of reducing unwanted energy transfers, for example with lubrication and thermal insulation, and how the thickness and thermal conductivity of walls affect the rate of cooling of a building. It links back to P7.1 Work done and P3.2 electrical power, and forward to P8.2 Powering Earth. Examiners often set power and efficiency calculations together and ask you to explain how energy is wasted.
What you need to know
- Power Power = energy transferred / time = work done / time. Power is measured in watts; 1 W = 1 J/s.
- Power ratings Appliances with higher power ratings transfer more energy each second. Energy transferred = power x time.
- Efficiency Efficiency = useful output energy transfer / input energy transfer, which can be given as a decimal or percentage. It can also be calculated using power.
- Dissipation In all mechanical processes some energy is dissipated, often heating the surroundings, and becomes less useful.
- Reducing waste Lubrication reduces friction; thermal insulation reduces unwanted heating of surroundings. Streamlining reduces air resistance.
- Thermal conductivity The rate of cooling of a building depends on the thickness and thermal conductivity of its walls. Thicker walls made of low conductivity materials reduce energy loss.
Common mistakes in P7.2 Power and Efficiency
- Mixing up power and energy, or giving power in joules. Power is in watts.
- Not converting minutes into seconds, or kW into W.
- Giving an efficiency over 100%. Check you divided useful output by total input.
- Saying wasted energy disappears. It is dissipated to the thermal store of the surroundings.
- Saying a high thermal conductivity material is a good insulator. Insulators have low thermal conductivity.
Try three P7.2 Power and Efficiency exam-style questions
1. A 1500 W kettle is switched on for 2.0 minutes. Calculate the energy transferred. (2 marks)
Show the mark scheme answer
Energy = power x time = 1500 x 120; = 180 000 J.
2. A motor transfers 900 J usefully from 1200 J supplied. Calculate its efficiency. (2 marks)
Show the mark scheme answer
Efficiency = useful output / input = 900 / 1200; = 0.75 or 75%.
3. Explain how lubricating a bicycle chain increases efficiency. (3 marks)
Show the mark scheme answer
Lubrication reduces friction between moving parts; so less work is done against friction and less energy is dissipated as thermal energy; so a greater proportion of the input energy is transferred usefully.
P7.2 Power and Efficiency FAQs
What does OCR GCSE Physics A (Gateway Science) P7.2 Power and Efficiency cover?
It covers power, power ratings, efficiency, energy dissipation, lubrication, insulation and thermal conductivity.
Which paper is P7.2 examined on?
Topics P5 to P8 are assessed on Paper 2 (Foundation) or Paper 4 (Higher), which can also draw on knowledge from P1 to P4 and practical skills.
What equations do I need for Power and Efficiency?
Learn power = energy transferred / time, power = work done / time and efficiency = useful output energy transfer / input energy transfer.
