p2 conservation and efficiency
4 detailed 50-minute lessons with teaching scripts, worked examples, parent guides, and assessment criteria.
Lesson Overview
Total Lessons: 4
Tier: Foundation and Higher
Duration: 50 minutes per lesson (200 minutes total)
Exam Boards: AQA, Edexcel, OCR, Eduqas, CCEA
Tier: Foundation and Higher
Duration: 50 minutes per lesson (200 minutes total)
Exam Boards: AQA, Edexcel, OCR, Eduqas, CCEA
Learning Objectives
- Explain the key ideas of p2 conservation and efficiency
- Apply p2 conservation and efficiency to exam-style questions
- Develop independent study skills
- Build cross-topic connections
Prerequisites
- Read the revision notes for p2 conservation and efficiency first (GCSE Revise summary)
- Key vocab to pre-teach: Conservation of energy, Efficiency, Useful energy
- Basic skills: reading the summary notes and answering the practice questions there
Materials & Equipment
- Exercise book, coloured pens
- Scientific calculator
- Ruler
- Printed revision notes (link below)
- Internet for videos (see Resources)
Lesson 1: Introduction: p2 conservation and efficiency
Duration: 50 minutes
Starter Activity (5 minutes)
Quick Recall
Write down everything you already know about p2 conservation and efficiency. Then check against the key terms: Conservation of energy, Efficiency, Useful energy. Use a mini-whiteboard or paper.
Main Content (35 minutes)
Parent/Teacher Guide:
Before lesson: Read the script below. Pre-teach key vocab: Conservation of energy, Efficiency, Useful energy.
If stuck: Re-read the revision notes (link above), then break the content into smaller steps.
Extension: See the Stretch & Challenge ideas in Lesson 4.
Before lesson: Read the script below. Pre-teach key vocab: Conservation of energy, Efficiency, Useful energy.
If stuck: Re-read the revision notes (link above), then break the content into smaller steps.
Extension: See the Stretch & Challenge ideas in Lesson 4.
Teaching Script (35 mins):
Mins 0-5 - Hook: "Today: p2 conservation and efficiency. By the end you will be able to answer exam questions on it unaided. It connects to the rest of Combined Science (Trilogy) because the ideas here recur across the spec."
Mins 5-20 - Direct Instruction: Work through the core ideas below one at a time; after each, ask your student to explain it back in their own words.
Mins 20-30 - Guided Practice: Model the worked example together, then let your student attempt the first practice question with guidance.
Mins 30-35 - Independent Practice: 2-3 practice questions from Lesson 3 below, with immediate feedback.
Mins 0-5 - Hook: "Today: p2 conservation and efficiency. By the end you will be able to answer exam questions on it unaided. It connects to the rest of Combined Science (Trilogy) because the ideas here recur across the spec."
Mins 5-20 - Direct Instruction: Work through the core ideas below one at a time; after each, ask your student to explain it back in their own words.
Mins 20-30 - Guided Practice: Model the worked example together, then let your student attempt the first practice question with guidance.
Mins 30-35 - Independent Practice: 2-3 practice questions from Lesson 3 below, with immediate feedback.
First Look
Start with the revision notes summary, then attempt: Foundation A device has a total energy input of 1000 J. The useful energy output is 700 J. Calculate the efficiency as a percentage.
Plenary (5 minutes)
Check Out
Your student states one thing they learned and one question they still have about p2 conservation and efficiency.
Lesson 2: Core Concepts: p2 conservation and efficiency
Duration: 50 minutes
Starter Activity (5 minutes)
Review Previous Lesson
Quick recap: write 3 key points from Lesson 1 on p2 conservation and efficiency. Check them against the notes below.
Main Content (35 minutes)
Conservation of energy: Energy can be transferred usefully, stored or dissipated, but it can never be created or destroyed.
Efficiency: A measure of how much of the total energy input to a device is transferred usefully. The greater the proportion of useful energy, the more efficient the device.
Useful energy: Energy transferred to the output store that is the intended purpose of the device.
Wasted energy: Energy that is transferred to stores other than the useful output — usually the thermal store of the surroundings.
No device is 100% efficient: — some energy is always dissipated to the thermal store of the surroundings. The only exception is an electric heater , where all the wasted energy is thermal energy, which is the useful output anyway.
Reducing wasted energy improves efficiency.: Any energy transferred to unwanted stores is wasted, so reducing these transfers makes a device more efficient.
| Term | Meaning | Example |
|---|---|---|
| Electric heater | Thermal (heat) | 100% |
| LED lamp | Light | 80–90% |
| CFL lamp | Light | 60–70% |
| Filament lamp | Light | 5–10% |
| Electric motor | Kinetic (movement) | 70–80% |
| Petrol car engine | Kinetic (movement) | 20–30% |
| Diesel car engine | Kinetic (movement) | 30–40% |
| Reduce friction | Lubricate moving parts so less energy is dissipated as heat | Oiling gears in a motor |
Practice (10 minutes)
Q: Foundation A device has a total energy input of 1000 J. The useful energy output is 700 J. Calculate the efficiency as a percentage.
Answer: Efficiency = (700 ÷ 1000) × 100% = 70%.
Plenary (5 minutes)
Explain Back
Your student teaches the key points back to you without looking. Fill any gaps immediately.
Lesson 3: Application: p2 conservation and efficiency
Duration: 50 minutes
Starter Activity (5 minutes)
Quick Recall
Recall the key terms: Conservation of energy, Efficiency, Useful energy. Define each in one sentence.
Main Content (35 minutes)
Parent/Teacher Guide: Let your student attempt each question alone first, then compare with the model answer. Award method marks for correct working even if the final answer is wrong.
Q1: Foundation A device has a total energy input of 1000 J. The useful energy output is 700 J. Calculate the efficiency as a percentage.
Answer: Efficiency = (700 ÷ 1000) × 100% = 70%.
Q2: Foundation State why no machine can be 100% efficient (except an electric heater).
Answer: Some energy is always dissipated (wasted) to the thermal store of the surroundings due to friction, air resistance, or electrical resistance. An electric heater is the exception because all the "wasted" thermal energy IS the useful output.
Q3: Foundation A motor has an input power of 400 W and an efficiency of 75%. Calculate the useful output power.
Answer: Useful power out = 0.75 × 400 = 300 W.
Q4: Higher An electric kettle is 90% efficient. It transfers 180 000 J of useful energy to the water. Calculate the total electrical energy input.
Answer: Total energy in = useful energy out ÷ efficiency = 180 000 ÷ 0.90 = 200 000 J.
Q5: Higher Explain two ways to improve the efficiency of a machine that has moving parts. Refer to energy stores in your answer.
Answer: 1) Lubricate the moving parts — this reduces friction, so less energy is dissipated to the thermal store of the surroundings. 2) Use more streamlined shapes — this reduces air resistance, so less energy is dissipated to the thermal store via drag. Both methods reduce wasted energy and increase the proportion of useful energy, improving efficiency.
Plenary (5 minutes)
Error Review
Review any questions answered incorrectly. Identify whether the error was knowledge, method, or reading the question.
Lesson 4: Exam Practice: p2 conservation and efficiency
Duration: 50 minutes
Starter Activity (5 minutes)
Command Words
Review what these command words require: state (one point), describe (say what happens), explain (say why), compare (both sides), evaluate (judgement).
Main Content (35 minutes)
Extended Answer
Extended question: Extended Answer 6 marks: Explain why no device (except an electric heater) can be 100% efficient, and describe two ways to improve the efficiency of a machine with moving parts. <div class="
No device can be 100% efficient because whenever energy is transferred, some energy is always dissipated to the thermal store of the surroundings. In machines with moving parts, friction between surfaces causes energy to be transferred to the thermal store rather than the useful output. Air resistance also dissipates energy to the thermal store. Electrical resistance in wires and components dissipates energy as heat. Sound energy from vibrations is also wasted. An electric heater is the only exception because its intended useful output IS thermal energy, so even the "wasted" heat counts as useful. Two ways to improve efficiency: (1) Lubricate the moving parts — this reduces friction, so less energy is dissipated to the thermal store of the surroundings and more energy goes to the useful output. (2) Streamline the shape — this reduces air resistance, so less energy is dissipated as heat due to drag. Mark scheme: 1 mark — some energy always dissipated; 1 mark — friction causes thermal dissipation; 1 mark — electric heater exception explained; 1 mark — lubrication reduces friction; 1 mark — streamlining reduces air resistance; 1 mark — both methods reduce wasted energy and increase us
Exam Tips: Always show your working in efficiency calculations: write the equation, substitute values, then give the answer. | Remember to multiply by 100% if the question asks for efficiency as a percentage. | If asked "why can no device be 100% efficient?", the answer is always: some energy is always dissipated to the thermal store of the surroundings . | The exception is an electric heater — be ready to explain why it can be considered 100% efficient. | When asked to improve efficiency, think about reducing friction, reducing heat loss (insulation), or reducing air resistance (streamlining). | If you know efficiency and useful energy, rearrange the equation: total energy = useful energy ÷ efficiency
Common Errors: Watch Out! 1. Wrong: Efficiency can be greater than 100% Correct: Efficiency can never exceed 100% — you cannot get more useful energy out than the total energy you put in (conservation of energy) 2. Wrong: Wasted energy disappears Correct: Wasted energy is transferred to the thermal store of the surroundings — it still exists but is no longer useful 3. Wrong: All devices have similar efficiencies Correct: Efficiencies vary hugely — an electric heater is 100% efficient, a filament lamp is only about 5–10% efficient
AO3 - Reasoning & Interpretation: Analysis and Evaluation A student tests two lamps. Lamp X (LED) has a total input power of 10 W and useful light output power of 8.5 W. Lamp Y (filament) has a total input power of 60 W and useful light output power of 5.4 W. Both lamps produce a similar brightness of light. (a) Calculate the efficiency of each lamp as a percentage. (b) Calculate how much power is wasted by each lamp. (c) A householder says "The filament lamp must be better because it uses more power." Evaluate this statement. Answers: (a) Lamp X efficiency = (8.5 ÷ 10) × 100 = 85%. Lamp Y efficiency = (5.4 ÷ 60) × 100 = 9%. (b) Lamp X wasted power = 10 − 8.5 = 1.5 W. Lamp Y wasted power = 60 − 5.4 = 54.6 W. (c) The statemen
Stretch & Challenge (Grade 8-9):
- Synoptic links: explain how p2 conservation and efficiency connects to another Combined Science (Trilogy) topic you have studied
- Real-world: research one real-world use or example of p2 conservation and efficiency
- Critical: "What are the limitations of the models used in p2 conservation and efficiency?"
Plenary (5 minutes)
Assessment Criteria
- Got it: Confident explanation + correct worked examples
- Getting there: Main points OK, needs support with detail
- Not yet: Confused on key concepts - re-run Lesson 2
Homework & Consolidation
- Consolidation: Re-answer any Lesson 3 practice questions answered incorrectly (20 mins)
- Retrieval: Write flashcards for the key terms: Conservation of energy, Efficiency, Useful energy (10 mins)
- Exam practice: One past-paper question on p2 conservation and efficiency from the board websites (15 mins)
- Extension: Explain p2 conservation and efficiency to someone else in your own words (10 mins)