Current, potential difference and resistance

    AQA
    GCSE

    Electric current is defined as the rate of flow of electrical charge, driven by a potential difference which represents the work done or energy transferred per unit charge. Resistance quantifies the opposition to this current flow, defined by the ratio of potential difference to current, and is intrinsically linked to the physical dimensions and material properties of the conductor. Ohm's Law establishes the linear relationship between current and potential difference in ohmic conductors at constant temperature, serving as the fundamental principle for analyzing charge conservation in series circuits and potential distribution in parallel configurations.

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    Objectives
    3
    Exam Tips
    4
    Pitfalls
    5
    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 the rate of flow of electrical charge ($I = Q/t$)
    • Credit responses that explain resistance in a filament lamp increases because metal ions vibrate more vigorously, impeding electron flow
    • Award 1 mark for correct substitution into $V = IR$, ensuring current is converted from mA to A where necessary
    • For diode I-V graphs, award marks for showing zero current in the reverse bias direction and a sharp rise in current above the threshold voltage
    • Award 1 mark for identifying that an ohmic conductor has a constant resistance provided the temperature remains constant

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that current is the rate of flow of electrical charge ($I = Q/t$)
    • Credit responses that explain resistance in a filament lamp increases because metal ions vibrate more vigorously, impeding electron flow
    • Award 1 mark for correct substitution into $V = IR$, ensuring current is converted from mA to A where necessary
    • For diode I-V graphs, award marks for showing zero current in the reverse bias direction and a sharp rise in current above the threshold voltage
    • Award 1 mark for identifying that an ohmic conductor has a constant resistance provided the temperature remains constant

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When asked to describe a graph, separate the answer into two parts: describe the shape (e.g., 'linear through origin') and then state the physical meaning (e.g., 'constant resistance')
    • 💡In circuit calculations, always write down the equation, then the substitution, then the final answer with units; AQA awards compensatory marks for working even if the final answer is wrong
    • 💡For thermistors and LDRs, remember the acronyms TURD (Temperature Up, Resistance Down) and LURD (Light Up, Resistance Down) to avoid reversing the relationships

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing the placement of ammeters (must be series) and voltmeters (must be parallel) in circuit diagrams
    • Stating that resistance decreases in a filament lamp as potential difference increases; candidates fail to link heating effects to increased resistance
    • Forgetting to convert time into seconds when using $Q = It$, or current into Amperes when using $V = IR$
    • Describing the I-V curve of a diode as 'linear' after the threshold voltage; it is exponential/non-linear

    Key Terminology

    Essential terms to know

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Explain
    Draw
    Describe
    Determine
    Compare

    Practical Links

    Related required practicals

    • •{"code":"Required Practical 3","title":"Resistance","relevance":"Investigation of how resistance of a wire varies with length and combinations of resistors in series/parallel"}
    • •{"code":"Required Practical 4","title":"I-V Characteristics","relevance":"Constructing circuits to measure and plot data for a resistor, filament lamp, and diode"}

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