Efficiency

    OCR
    GCSE

    Efficiency quantifies the performance of a physical system by comparing useful output energy (or power) to total input energy (or power). Defined by the ratio Useful Output / Total Input, it is a dimensionless quantity expressed as a decimal (0.0–1.0) or percentage (0–100%). Candidates must apply the Principle of Conservation of Energy to explain that efficiency cannot exceed 100% in standard mechanical systems, as energy is inevitably dissipated to the surroundings, typically as thermal energy due to friction or electrical resistance. Mastery requires calculating this ratio, interpreting Sankey diagrams, and proposing specific mechanisms to reduce dissipation, such as lubrication or thermal insulation.

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    Objectives
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    Exam Tips
    4
    Pitfalls
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    Key Terms
    4
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for correct recall and substitution into the equation: Efficiency = Useful energy output / Total energy input
    • Credit responses that explicitly link reduced friction (via lubrication) or reduced thermal dissipation (via insulation) to increased efficiency
    • Award 1 mark for identifying that energy is dissipated to the thermal store of the surroundings, not just 'lost'
    • Reject any calculation resulting in an efficiency greater than 1 or 100%; award 0 marks for the final answer in such cases

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly calculated the value, but check if the question asked for a percentage or a decimal format"
    • "Your explanation of 'lost heat' needs to be more precise—use 'dissipated to the surroundings' to gain the mark"
    • "You identified the useful energy correctly; now explain how lubrication increases the proportion of useful output"
    • "Remember that efficiency cannot exceed 1; check your division steps to ensure Input is the denominator"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for correct recall and substitution into the equation: Efficiency = Useful energy output / Total energy input
    • Credit responses that explicitly link reduced friction (via lubrication) or reduced thermal dissipation (via insulation) to increased efficiency
    • Award 1 mark for identifying that energy is dissipated to the thermal store of the surroundings, not just 'lost'
    • Reject any calculation resulting in an efficiency greater than 1 or 100%; award 0 marks for the final answer in such cases

    Examiner Tips

    Expert advice for maximising your marks

    • 💡Always perform a sanity check: if your result is greater than 1 (or 100%), you have divided the wrong way round
    • 💡When explaining how to improve efficiency, specify the mechanism: 'lubrication reduces friction' or 'insulation reduces thermal transfer'
    • 💡If the question gives power (Watts) instead of energy (Joules), the efficiency formula applies identically: Useful Power Output / Total Power Input

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Inverting the equation by dividing total input energy by useful output energy, resulting in a value > 1
    • Confusing 'wasted energy' with 'useful energy output' when extracting numerical data from the question stem
    • Stating that energy is 'destroyed' or 'disappears' rather than being dissipated or transferred to a less useful store
    • Failing to convert the decimal answer to a percentage when the answer line explicitly requires a % unit

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Conservation of Energy and Dissipation
    Calculation of Efficiency (Energy and Power)
    Sankey Diagrams and Energy Flow
    Methods of Improving Efficiency (Insulation, Lubrication, Streamlining)

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Explain
    Suggest
    State
    Compare

    Practical Links

    Related required practicals

    • •{"code":"PAG P1","title":"Investigation of factors affecting insulation","relevance":"Demonstrates practical methods to reduce thermal dissipation and improve thermal efficiency"}

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