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Conservation and dissipation of energy
A clear exam answer should show that Conservation and dissipation of energy sits inside 4.1 Energy and should be revised as a connected set of physical ideas, not as isolated definitions. Context: the topic brings together Energy transfers in a system, Efficiency and asks students to move between description, calculation, graph interpretation and explanation where appropriate. Key Concept: identify the quantity, model or interaction first, then state how evidence or a mathematical relationship supports it. Worked Example: for a typical exam item, name the relevant principle, use the given data or diagram, and finish with a sentence that interprets the physical consequence. Exam Focus: keep command words visible; describe asks for features, explain asks for a causal link, calculate requires equation, substitution and unit, and evaluate needs evidence. Common Mistake: avoid using a nearby concept as a substitute, such as confusing field with force, current with potential difference, speed with acceleration, or red-shift evidence with the Big Bang model itself.
23
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10
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10
Questions
90 min
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Syllabus checklist
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23 objective pages available
Energy transfers in a system13 objectives
- State that energy can be transferred usefully, stored or dissipated but cannot be created or destroyed.
- Describe examples of energy transfers in a closed system with no net change to total energy.
- Explain that energy is often dissipated into less useful stores during system changes.
- Use the term wasted energy to describe energy transferred in less useful ways.
- Explain how lubrication reduces unwanted energy transfers by reducing friction.
- Explain how thermal insulation reduces unwanted energy transfers by heating.
- Describe how thermal conductivity affects the rate of energy transfer by conduction.
- Describe how wall thickness affects the rate of cooling of a building.
- Describe how thermal conductivity of wall materials affects the rate of cooling of a building.
- Required practical activity 2 (physics only): investigate the effectiveness of different materials as thermal insulators.
- Required practical activity 2 (physics only): investigate factors that may affect thermal insulation properties.
- Identify apparatus and techniques used in thermal insulation practical work.
- Interpret data from thermal conductivity or insulation investigations.
Efficiency10 objectives
- Calculate efficiency using useful energy output divided by total energy input.
- Calculate efficiency using useful power output divided by total power input.
- Recall and apply both efficiency equations.
- Express efficiency as a decimal.
- Express efficiency as a percentage.
- Convert efficiency values between decimals and percentages.
- Explain why no real energy transfer is perfectly efficient.
- Identify useful and wasted energy transfers in a device or process.
- (HT only) Describe ways to increase the efficiency of an intended energy transfer.
- (HT only) Explain how reducing unwanted energy transfers improves efficiency.
Key terms
Exam tips
- Understand Energy Conservation: Explain Energy transfers in a system clearly: energy cannot be created or destroyed; it can only be transferred or transformed.
- Understand Closed Systems: Familiarize yourself with examples of energy transfers in closed systems, ensuring you can explain how energy remains constant despite changes.
Common mistakes
- Misunderstanding Energy Conservation: Remember that energy cannot be created or destroyed; it can only be transferred, stored, or dissipated.
- Misunderstanding Closed Systems: Emphasize that in a closed system, energy is conserved and can only be transferred or transformed, not created or destroyed.
Practice preview
- A machine uses 500 J of energy input and produces 400 J of useful energy output. What is the efficiency of the machine?
- If a device has an efficiency of 90% and uses 2000 J of energy, how much useful energy does it produce?
- A light bulb has an efficiency of 75% and consumes 60 W of power. What is the useful power output of the light bulb?
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