Potential Difference (Voltage)

    OCR
    GCSE

    Potential difference (p.d.) is defined as the work done or energy transferred per unit charge passing between two points in a circuit, measured in Volts (V) where 1 V = 1 J/C. It represents the electrical pressure that drives current against resistance, governed by the relationship V = IR (Ohm's Law) for ohmic conductors. Candidates must distinguish between potential difference across a component, which transfers energy out of the electrical store, and electromotive force (e.m.f.), which transfers energy into the circuit. Mastery requires the application of Kirchhoff's Voltage Law to analyze voltage distribution in series and parallel circuit configurations.

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
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    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for defining potential difference as work done (or energy transferred) per unit charge.
    • Credit responses that explicitly state or draw the voltmeter connected in parallel across the component.
    • Award 1 mark for correct substitution of values into the equation V = I × R prior to final calculation.
    • In series circuits, credit the explanation that the total potential difference of the supply is shared between components.
    • Award 1 mark for identifying that potential difference is the same across each branch in a parallel circuit.

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You have correctly calculated the value, but you must show the substitution step to guarantee method marks."
    • "Your circuit diagram places the voltmeter in series; remember, it must 'hug' the component in parallel to measure the difference across it."
    • "Avoid saying 'voltage goes'; use precise terminology like 'there is a potential difference across the resistor'."
    • "Excellent use of the equation. To reach the top band, explain *why* the potential difference drops when resistance decreases in this series circuit."

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for defining potential difference as work done (or energy transferred) per unit charge.
    • Credit responses that explicitly state or draw the voltmeter connected in parallel across the component.
    • Award 1 mark for correct substitution of values into the equation V = I × R prior to final calculation.
    • In series circuits, credit the explanation that the total potential difference of the supply is shared between components.
    • Award 1 mark for identifying that potential difference is the same across each branch in a parallel circuit.

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When asked to define potential difference, use the formula E = QV to recall the definition: 'Energy transferred per unit charge'.
    • 💡In calculation questions involving standard form, always convert non-standard units (e.g., 50mV = 0.05V) before substituting into equations.
    • 💡For 6-mark circuit analysis questions, structure your answer by comparing V and I behavior separately: 'In series, I is constant, V splits; in parallel, V is constant, I splits'.

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Drawing the voltmeter in series with the component, treating it like an ammeter.
    • Stating that 'voltage flows' through a circuit; candidates must understand that current flows and potential difference is measured across two points.
    • Failing to convert prefixes such as millivolts (mV) or kilovolts (kV) into Volts (V) before applying formulae.
    • Confusing the potential difference across a component with the e.m.f. of the source in complex circuit explanations.

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Energy transferred per unit charge (V = E/Q)
    Ohm's Law and resistance relationships (V = IR)
    Voltage distribution in series vs parallel circuits
    Measurement using voltmeters in parallel

    Likely Command Words

    How questions on this topic are typically asked

    Define
    Calculate
    Explain
    Draw
    Suggest

    Practical Links

    Related required practicals

    • •{"code":"PAG P3","title":"Investigation of I-V characteristics","relevance":"Measuring potential difference across components to determine resistance"}
    • •{"code":"PAG P4","title":"Investigation of series and parallel circuits","relevance":"Verifying rules for potential difference distribution in different circuit topologies"}

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