AQA · GCSE · Physics

    Course version: 8463 · Version from Sept 2016

    Students should be able to explain how the power transfer in any circuit…

    Full specification wording

    Power: Students should be able to explain how the power transfer in any circuit device is related to the potential difference across it and the current through it, and to the energy changes over time: power = potential di f f erence × current P = V I power = current × resistance P = I R power, P, in watts, W potential difference, V, in volts, V current, I, in amperes, A (amp is acceptable for ampere) resistance, R, in ohms, Ω

    Power is the energy transferred each second.

    Checked against the AQA 8463 specification (opens in a new tab) on .

    • Point 3 of 3
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Power is the energy transferred each second. Each coulomb passing through a device transfers V joules, and I coulombs pass every second, so the energy per second is P = V I. Substituting V = I R for a resistor gives the second form, P = I² R. Note that the current is squared, so doubling the current quadruples the power dissipated, which is why thick cables are used for large currents. Choose the equation that matches the data: P = V I when you are given potential difference and current, P = I² R when you are given current and resistance. A 230 V kettle drawing 8.7 A transfers 230 × 8.7 = 2001 W, about 2 kW, and a 3 A current in a 5 Ω element gives P = 3² × 5 = 45 W.

    What earns the marks

    • Selecting P = V I or P = I² R to match the quantities given.
    • Correct substitution of the values into the chosen equation.
    • The calculated power with the unit watts, W.
    • A correct rearrangement, such as I = P ÷ V, when the power is the quantity supplied.

    Common mistakes and how to put them right

    • Using P = I R instead of P = I² R because the square is easy to miss in printed text.

    • Forgetting to square the current, so a 3 A current in 5 Ω is given as 15 W.

    • Mixing kilowatts and watts in the same calculation without converting.

    • Quoting the answer in joules because power and energy have been confused.

    Exam tips

    • Write the equation, substitute, then evaluate; the substitution mark is awarded even when the arithmetic slips.
    • Check the equation sheet for the squared term, because the printed specification line is easy to misread.
    • Convert kilowatts to watts before substituting, and state the unit with every answer.

    Try the lesson

    The start of the Power: Students should be able to explain how the power transfer in any circuit device is related to the potential difference across it and the current through it, and to the energy changes over time: power = potential di f f erence × current P = V I power = current × resistance P = I R power, P, in watts, W potential difference, V, in volts, V current, I, in amperes, A (amp is acceptable for ampere) resistance, R, in ohms, Ω lesson, made from checked teaching notes for this specification point.

    Have a go first

    A lamp is connected to a 12 V supply and a current of 2 A flows through it. Using P = V I, what is the power transferred by the lamp?

    Listen

    Power is the energy transferred each second. Each coulomb passing through a device transfers V joules, and I coulombs pass every second, so the energy per second is P = V I. Substituting V = I R for a resistor gives the second form, P = I² R. Note that the current is squared, so doubling the current quadruples the power dissipated, which is why thick cables are used for large currents. Choose the equation that matches the data: P = V I when you are given potential difference and current, P = I² R when you are given current and resistance. A 230 V kettle drawing 8.7 A transfers 230 × 8.7 = 2001 W, about 2 kW, and a 3 A current in a 5 Ω element gives P = 3² × 5 = 45 W. A strong answer earns marks for these points. selecting P = V I or P = I² R to match the quantities given. correct substitution of the values into the chosen equation. the calculated power with the unit watts, W. a correct rearrangement, such as I = P ÷ V, when the power is the quantity supplied. Exam tip: Write the equation, substitute, then evaluate; the substitution mark is awarded even when the arithmetic slips.

    See it

    A top-to-bottom flowchart with nine boxes. The first box states that power is the energy transferred each second. An arrow leads to a box saying each coulomb transfers V joules, then to a box saying I coulombs pass every second, then to a box giving the energy per second as P = V I. An arrow leads to a box saying substitute V = I R for a resistor, then to a box giving the second form P = I² R, then to a box noting the current is squared, then to a box saying doubling the current quadruples the power dissipated, and finally to a box saying why thick cables are used for large currents.
    It shows how the energy transferred each second leads to P = V I, and how substituting V = I R gives P = I² R, so doubling the current quadruples the power dissipated.

    Key words

    Say what each word means before you turn the card.

    power

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

    P = V I

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

    P = I² R

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

    See every definition
    power
    The energy transferred each second, measured in watts, W.
    P = V I
    Power equals potential difference multiplied by current, used when you are given potential difference and current.
    P = I² R
    Power equals current squared multiplied by resistance, used when you are given current and resistance.

    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-to-bottom flowchart with nine boxes. The first box states that power is the energy transferred each second. An arrow leads to a box saying each coulomb transfers V joules, then to a box saying I coulombs pass every second, then to a box giving the energy per second as P = V I. An arrow leads to a box saying substitute V = I R for a resistor, then to a box giving the second form P = I² R, then to a box noting the current is squared, then to a box saying doubling the current quadruples the power dissipated, and finally to a box saying why thick cables are used for large currents.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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    Test your understanding of Power: Students should be able to explain how the power transfer in any circuit device is related to the potential difference across it and the current through it, and to the energy changes over time: power = potential di f f erence × current P = V I power = current × resistance P = I R power, P, in watts, W potential difference, V, in volts, V current, I, in amperes, A (amp is acceptable for ampere) resistance, R, in ohms, Ω

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