OCR · A-Level · Physics

    Course version: H556 · Version from Sept 2015

    (e) analysis of circuits with components, including both series and parallel

    Full specification wording

    Series and parallel circuits: (e) analysis of circuits with components, including both series and parallel

    Analysing circuits with components in both series and parallel requires applying Kirchhoff’s laws and the rules for total resistance.

    Checked against the OCR H556 specification (opens in a new tab) on .

    • Point 5 of 6
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Analysing circuits with components in both series and parallel requires applying Kirchhoff’s laws and the rules for total resistance. For a circuit with a series part and a parallel part, first simplify the parallel section to a single equivalent resistance using 1/R = 1/R₁ + 1/R₂ + ... , then add series resistances. Use Ohm’s law (V = IR) to find currents and voltages.

    For example: A 6.0 V battery connected to a 4.0 Ω resistor in series with a parallel pair of 6.0 Ω and 3.0 Ω resistors: the parallel pair has equivalent resistance 2.0 Ω, total resistance 6.0 Ω, total current 1.0 A, and the p.d. across the parallel pair is 2.0 V. Multiple-choice questions may ask for a current, voltage, or equivalent resistance.

    What earns the marks

    • Identify series and parallel sections in a circuit diagram.
    • Calculate equivalent resistance of parallel resistors using 1/R = 1/R₁ + 1/R₂ + ... .
    • Add series resistances to find total resistance.
    • Apply Ohm’s law (V = IR) to find current or potential difference.
    • Use Kirchhoff’s laws to check current and voltage distributions.

    Common mistakes and how to put them right

    • Misunderstanding: treat the entire circuit as series or parallel without simplifying sections.

      Put it right: Break the circuit into series and parallel parts and simplify step by step.

    • Misunderstanding: apply the series resistance formula to parallel resistors.

      Put it right: Use the reciprocal formula for parallel sections.

    • Misunderstanding: assume current is the same through all components in a mixed circuit.

      Put it right: Current is the same only in series sections; it divides in parallel branches.

    • Misunderstanding: forget to include internal resistance of the battery if given.

      Put it right: Include it as a series resistance if the question provides it.

    Exam tips

    • In multiple-choice questions, redraw the circuit to clearly identify series and parallel sections.
    • Calculate equivalent resistances step by step, and keep track of units.
    • Use Ohm’s law and Kirchhoff’s laws to find unknown values; check that your answers are physically reasonable.

    Try the lesson

    The start of the Series and parallel circuits: (e) analysis of circuits with components, including both series and parallel lesson, made from checked teaching notes for this specification point.

    Listen

    Analysing circuits with components in both series and parallel requires applying Kirchhoff’s laws and the rules for total resistance. For a circuit with a series part and a parallel part, first simplify the parallel section to a single equivalent resistance using 1/R = 1/R₁ + 1/R₂ + ... , then add series resistances. Use Ohm’s law (V = IR) to find currents and voltages. For example, a 6.0 V battery connected to a 4.0 Ω resistor in series with a parallel pair of 6.0 Ω and 3.0 Ω resistors: the parallel pair has equivalent resistance 2.0 Ω, total resistance 6.0 Ω, total current 1.0 A, and the p.d. across the parallel pair is 2.0 V. Multiple-choice questions may ask for a current, voltage, or equivalent resistance. A strong answer earns marks for these points. Identify series and parallel sections in a circuit diagram. Calculate equivalent resistance of parallel resistors using 1/R = 1/R₁ + 1/R₂ + ... Add series resistances to find total resistance. Apply Ohm’s law (V = IR) to find current or potential difference. A common slip: Misunderstanding: treat the entire circuit as series or parallel without simplifying sections. Instead, break the circuit into series and parallel parts and simplify step by step. Exam tip: In multiple-choice questions, redraw the circuit to clearly identify series and parallel sections.

    See it

    A top-down flowchart with six nodes. The first node reads 'Circuit with series and parallel sections' and points down to 'Identify series and parallel sections'. That points to 'Simplify parallel section: 1/R = 1/R₁ + 1/R₂ + ...', which points to 'Add series resistances to find total resistance'. That points to 'Apply Ohm's law (V = IR) to find current or p.d.', which finally points to 'Use Kirchhoff's laws to check current and voltage distributions'.
    The step-by-step method for analysing a mixed series and parallel circuit, from identifying sections to checking with Kirchhoff's laws.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Simplify parallel section: 1/R = 1/R₁ + 1/R₂ + ...
    2. 2. Identify series and parallel sections
    3. 3. Circuit with series and parallel sections
    4. 4. Use Kirchhoff's laws to check current and voltage distributions
    5. 5. Apply Ohm's law (V = IR) to find current or p.d.
    6. 6. Add series resistances to find total resistance

    Key words

    Say what each word means before you turn the card.

    Circuit analysis

    Try answering aloud or in your head before turning the card.

    Series and parallel combinations

    Try answering aloud or in your head before turning the card.

    Ohm’s law

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    Kirchhoff’s laws

    Try answering aloud or in your head before turning the card.

    See every definition
    Circuit analysis
    Analysing circuits with components in both series and parallel requires applying Kirchhoff’s laws and the rules for total resistance.
    Series and parallel combinations
    For a circuit with a series part and a parallel part, first simplify the parallel section to a single equivalent resistance, then add series resistances.
    Ohm’s law
    Use V = IR to find currents and voltages.
    Kirchhoff’s laws
    These are used to check current and voltage distributions.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart with six nodes. The first node reads 'Circuit with series and parallel sections' and points down to 'Identify series and parallel sections'. That points to 'Simplify parallel section: 1/R = 1/R₁ + 1/R₂ + ...', which points to 'Add series resistances to find total resistance'. That points to 'Apply Ohm's law (V = IR) to find current or p.d.', which finally points to 'Use Kirchhoff's laws to check current and voltage distributions'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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