Resistance

    OCR
    GCSE

    Resistance quantifies the opposition to the flow of electric current within a circuit component, defined mathematically as the ratio of potential difference to current ($R=V/I$). At the microscopic level, it results from collisions between delocalized electrons and vibrating lattice ions, transferring kinetic energy to the thermal store of the component. Candidates must analyze how physical dimensions (length, cross-sectional area) and environmental factors (temperature, light intensity) alter resistance in both ohmic and non-ohmic conductors. Mastery of this concept is essential for calculating total resistance in series and parallel configurations and determining power dissipation.

    0
    Objectives
    3
    Exam Tips
    3
    Pitfalls
    5
    Key Terms
    4
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for defining resistance as the ratio of potential difference across a component to the current flowing through it (R = V/I)
    • Credit responses that explain the increase in resistance of a filament lamp due to increased lattice ion vibrations impeding electron flow
    • Award 1 mark for calculating total resistance in a series circuit by summing individual resistances
    • Candidates must identify that adding resistors in parallel decreases total resistance because it provides multiple paths for current flow

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly calculated the resistance, but check your units — did you convert mA to A?"
    • "Good description of the graph shape. To improve, explain *why* the gradient changes in terms of ion vibration and electron collisions"
    • "You identified the correct formula. Now, ensure you rearrange it correctly before substituting the values"
    • "Excellent use of the term 'ohmic conductor'. Remember to specify that temperature must remain constant for Ohm's Law to hold"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for defining resistance as the ratio of potential difference across a component to the current flowing through it (R = V/I)
    • Credit responses that explain the increase in resistance of a filament lamp due to increased lattice ion vibrations impeding electron flow
    • Award 1 mark for calculating total resistance in a series circuit by summing individual resistances
    • Candidates must identify that adding resistors in parallel decreases total resistance because it provides multiple paths for current flow

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When describing the I-V characteristic of a diode, explicitly state that current only flows in the forward bias direction and is negligible in reverse bias
    • 💡For 6-mark Level of Response questions on practical design, list specific control variables (e.g., wire material, temperature) to access Level 3
    • 💡Check the axes of I-V graphs carefully; if Voltage is on the y-axis, the gradient is Resistance; if Current is on the y-axis, the gradient is 1/R

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Stating that resistance decreases in a filament lamp as voltage increases, rather than increasing due to heating effects
    • Failing to convert current from milliamperes (mA) to amperes (A) before substituting into V = I × R
    • Believing that adding a resistor in parallel increases the total resistance of the circuit, confusing it with series rules

    Key Terminology

    Essential terms to know

    Ohm's Law and the V=IR relationship
    Microscopic models of resistance (electron-ion collisions)
    I-V characteristics of components (Resistor, Filament Lamp, Diode)
    Series and Parallel resistance rules
    Factors affecting resistance (Length, Area, Temperature, Material)

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Describe
    Explain
    Suggest
    Plot

    Practical Links

    Related required practicals

    • •{"code":"PAG P3","title":"Investigation of resistance","relevance":"Investigating factors affecting resistance of a wire (length, thickness)"}
    • •{"code":"PAG P4","title":"I-V characteristics","relevance":"Measuring current and voltage for resistor, lamp, and diode"}

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