Ohm's Law

    OCR
    GCSE

    Ohm's Law states that the current flowing through an ohmic conductor is directly proportional to the potential difference across it, provided that physical conditions, specifically temperature, remain constant. This linear relationship is mathematically expressed as V = IR, defining resistance as the ratio of potential difference to current. Candidates must distinguish between ohmic conductors, which produce linear I-V graphs passing through the origin, and non-ohmic components like filament lamps and diodes where resistance varies. Mastery requires the ability to analyze circuit data, interpret I-V characteristics, and apply the formula in both series and parallel circuit contexts.

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    Objectives
    3
    Exam Tips
    4
    Pitfalls
    4
    Key Terms
    5
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that current is directly proportional to potential difference
    • Award 1 mark for the essential condition: 'at constant temperature'
    • Credit calculation of resistance using the ratio V/I from specific data points
    • Award 1 mark for identifying a straight line through the origin as evidence of an Ohmic conductor
    • Credit responses that link increased resistance to increased ion-electron collisions in non-Ohmic contexts

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly calculated resistance, but check your significant figures against the data provided"
    • "Your definition is incomplete; you must mention 'constant temperature' to gain the mark"
    • "Good identification of the linear relationship. Now explain what the gradient represents for this specific graph axes configuration"
    • "You have confused the variable resistor's role; remember it is used to vary the current/p.d. across the component being tested"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that current is directly proportional to potential difference
    • Award 1 mark for the essential condition: 'at constant temperature'
    • Credit calculation of resistance using the ratio V/I from specific data points
    • Award 1 mark for identifying a straight line through the origin as evidence of an Ohmic conductor
    • Credit responses that link increased resistance to increased ion-electron collisions in non-Ohmic contexts

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When calculating resistance from a graph, ensure you check which quantity is on the y-axis; for an I-V graph, R = 1/gradient
    • 💡Memorise the condition 'constant temperature' as a non-negotiable part of the definition
    • 💡In 6-mark practical questions, explicitly state how to vary potential difference (e.g., using a variable resistor)

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Omitting the phrase 'at constant temperature' when defining Ohm's Law
    • Confusing the gradient of an I-V graph (1/R) with the gradient of a V-I graph (R)
    • Incorrectly identifying filament lamps or diodes as Ohmic conductors because they conduct electricity
    • Describing the relationship merely as 'positive correlation' instead of 'direct proportionality'

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Proportionality of current and potential difference
    The equation V = IR and resistance calculations
    I-V characteristics (Ohmic vs Non-Ohmic)
    Experimental verification using standard test circuits

    Likely Command Words

    How questions on this topic are typically asked

    State
    Calculate
    Describe
    Explain
    Plot

    Practical Links

    Related required practicals

    • •{"code":"PAG P3","title":"Investigation of resistance","relevance":"Investigating I-V characteristics of circuit elements"}

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