OCR · A-Level · Physics

    Course version: H556 · Version from Sept 2015

    E.m.f. and p.d: (a) potential difference (p.d.); the unit volt

    Potential difference measures the energy transferred from electrical energy to other forms per unit charge as charge passes between two points in a circuit.

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

    • Point 1 of 5
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Potential difference measures the energy transferred from electrical energy to other forms per unit charge as charge passes between two points in a circuit. If a component transfers 12 J of energy when 4 C of charge passes through it, the p.d. across it is 12 J ÷ 4 C = 3 J C⁻¹, which is 3 V. The volt is therefore the joule per coulomb: 1 V = 1 J C⁻¹. In a series circuit the p.d. is shared between components; in a parallel circuit the p.d. across each branch is the same. A voltmeter is connected in parallel across the component being measured, because it must compare the energy at two points.

    What earns the marks

    • Potential difference is the energy transferred per unit charge between two points in a circuit.
    • The unit of p.d. is the volt (V), where 1 V = 1 J C⁻¹.
    • A p.d. of 3 V means 3 J of energy is transferred per coulomb of charge passing between the two points.
    • A voltmeter is connected in parallel with the component so it measures the difference in electrical potential energy per unit charge across that component.
    • In a series circuit the total p.d. is shared between components; in a parallel circuit the p.d. across each branch is equal.

    Common mistakes and how to put them right

    • Confusing p.d. with current.

      Put it right: Current is the rate of flow of charge measured in amperes, whereas p.d. is energy transferred per unit charge measured in volts.

    • Connecting a voltmeter in series.

      Put it right: A voltmeter must be in parallel with the component, because it compares the energy per unit charge at two points.

    • Thinking the volt is a base SI unit.

      Put it right: The volt is a derived unit equal to 1 J C⁻¹, equivalent to 1 kg m² s⁻³ A⁻¹.

    Exam tips

    • Write the unit as V and, when asked to show it is derived, substitute 1 V = 1 J C⁻¹ and expand the joule into base units.
    • Use W = VQ to check numerical answers: rearrange to V = W ÷ Q and confirm the unit is J C⁻¹.
    • Read the circuit diagram carefully to decide whether the p.d. is across a single component or shared between several in series.

    Try the lesson

    The start of the E.m.f. and p.d: (a) potential difference (p.d.); the unit volt lesson, made from checked teaching notes for this specification point.

    Listen

    Potential difference measures the energy transferred from electrical energy to other forms per unit charge as charge passes between two points in a circuit. If a component transfers 12 J of energy when 4 C of charge passes through it, the p.d. across it is 12 J ÷ 4 C = 3 J C⁻¹, which is 3 V. The volt is therefore the joule per coulomb: 1 V = 1 J C⁻¹. In a series circuit the p.d. is shared between components; in a parallel circuit the p.d. across each branch is the same. A voltmeter is connected in parallel across the component being measured, because it must compare the energy at two points. A strong answer earns marks for these points. Potential difference is the energy transferred per unit charge between two points in a circuit. The unit of p.d. is the volt (V), where 1 V = 1 J C⁻¹. A p.d. of 3 V means 3 J of energy is transferred per coulomb of charge passing between the two points. A voltmeter is connected in parallel with the component so it measures the difference in electrical potential energy per unit charge across that component. Exam tip: Write the unit as V and, when asked to show it is derived, substitute 1 V = 1 J C⁻¹ and expand the joule into base units.

    See it

    A top-down flowchart. The first node reads 'Energy transferred W between two points' and points to 'Charge Q passes between the points', which points to 'p.d. V = W ÷ Q'. From this node four branches lead out: one to 'Unit: volt (V)', which continues to '1 V = 1 J C⁻¹'; one to 'Example: 12 J ÷ 4 C = 3 J C⁻¹ = 3 V'; one to 'Voltmeter connected in parallel across the component'; one to 'Series circuit: p.d. shared between components'; and one to 'Parallel circuit: p.d. across each branch is the same'.
    It shows how potential difference is calculated from energy transferred and charge, its unit, and how it behaves in series and parallel circuits and with a voltmeter.

    Key words

    Say what each word means before you turn the card.

    Potential difference

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

    The volt

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    Voltmeter

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    Series and parallel p.d.

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    See every definition
    Potential difference
    The energy transferred per unit charge between two points in a circuit.
    The volt
    The unit of p.d., equal to the joule per coulomb: 1 V = 1 J C⁻¹.
    Voltmeter
    Connected in parallel across the component being measured, because it must compare the energy at two points.
    Series and parallel p.d.
    In a series circuit the p.d. is shared between components; in a parallel circuit the p.d. across each branch is the same.

    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. The first node reads 'Energy transferred W between two points' and points to 'Charge Q passes between the points', which points to 'p.d. V = W ÷ Q'. From this node four branches lead out: one to 'Unit: volt (V)', which continues to '1 V = 1 J C⁻¹'; one to 'Example: 12 J ÷ 4 C = 3 J C⁻¹ = 3 V'; one to 'Voltmeter connected in parallel across the component'; one to 'Series circuit: p.d. shared between components'; and one to 'Parallel circuit: p.d. across each branch is the same'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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