AQA · GCSE · Physics

    Course version: 8463 · Version from Sept 2016

    Electrical power is transferred from power stations to consumers using the…

    Full specification wording

    The National Grid: Electrical power is transferred from power stations to consumers using the National Grid.

    The grid's job is to move power, and power is the product of potential difference and current, P = V I. A station generating 500 MW can deliver that power as a huge current at a low potential…

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

    • Point 2 of 4
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    The grid's job is to move power, and power is the product of potential difference and current, P = V I. A station generating 500 MW can deliver that power as a huge current at a low potential difference or as a small current at a very high one. The cables have resistance, and the power wasted heating them is P = I² R, which depends on the square of the current and not on the potential difference. So the grid transmits at 400 kV: raising the potential difference from 25 kV to 400 kV cuts the current by a factor of 16 for the same transmitted power, and cuts the wasted heating by 16², which is 256. That is why long-distance lines always run at high potential difference and must be held out of reach on pylons.

    What earns the marks

    • The transmitted power being the product of potential difference and current, P = V I.
    • A higher transmission potential difference giving a smaller current for the same power.
    • The power wasted in the cables depending on the square of the current, P = I² R.
    • Less energy being dissipated to the surroundings, so the transmission is more efficient.

    Common mistakes and how to put them right

    • Saying a higher potential difference reduces the wasted energy without mentioning the smaller current.

    • Stating that the potential difference decreases at the step-up transformer.

    • Claiming no energy at all is transferred to the surroundings from the cables.

    • Using P = I R rather than P = I² R when arguing about cable losses.

    Exam tips

    • Answer in the order potential difference up, current down, I² R losses down, efficiency up.
    • Never write that the step-up transformer reduces potential difference, because mark schemes refuse the whole answer.
    • Write 'reduces the energy transferred to the surroundings' rather than 'stops energy being lost'.

    Try the lesson

    The start of the The National Grid: Electrical power is transferred from power stations to consumers using the National Grid. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which system is used to transfer electrical power from power stations to consumers?

    Listen

    The grid's job is to move power, and power is the product of potential difference and current, P = V I. A station generating 500 MW can deliver that power as a huge current at a low potential difference or as a small current at a very high one. The cables have resistance, and the power wasted heating them is P = I² R, which depends on the square of the current and not on the potential difference. So the grid transmits at 400 kV: raising the potential difference from 25 kV to 400 kV cuts the current by a factor of 16 for the same transmitted power, and cuts the wasted heating by 16², which is 256. That is why long-distance lines always run at high potential difference and must be held out of reach on pylons. A strong answer earns marks for these points. the transmitted power being the product of potential difference and current, P = V I. a higher transmission potential difference giving a smaller current for the same power. the power wasted in the cables depending on the square of the current, P = I² R. less energy being dissipated to the surroundings, so the transmission is more efficient. Exam tip: Answer in the order potential difference up, current down, I² R losses down, efficiency up.

    See it

    A top-down flowchart. The first box reads 'Power station generates power' and points to 'Transmitted power P = V I'. That points to 'Raise potential difference to 400 kV'. From there one arrow leads to 'Smaller current for the same power', which leads to 'Cable heating P = I² R depends on square of current', then to 'Less energy dissipated to the surroundings', and finally to 'Transmission is more efficient'. A second arrow from 'Raise potential difference to 400 kV' leads to 'High potential difference held out of reach on pylons'.
    It shows how raising the transmission potential difference lowers the current, which cuts the I² R heating losses and makes transmission more efficient.

    Key words

    Say what each word means before you turn the card.

    P = V I

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

    I² R losses

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

    high potential difference

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

    transmission efficiency

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

    See every definition
    P = V I
    The transmitted power is the product of potential difference and current.
    I² R losses
    The power wasted heating the cables depends on the square of the current, P = I² R.
    high potential difference
    Transmitting at a high potential difference gives a smaller current for the same power, so less energy is dissipated to the surroundings.
    transmission efficiency
    Less energy being dissipated to the surroundings means the transmission is more efficient.

    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 box reads 'Power station generates power' and points to 'Transmitted power P = V I'. That points to 'Raise potential difference to 400 kV'. From there one arrow leads to 'Smaller current for the same power', which leads to 'Cable heating P = I² R depends on square of current', then to 'Less energy dissipated to the surroundings', and finally to 'Transmission is more efficient'. A second arrow from 'Raise potential difference to 400 kV' leads to 'High potential difference held out of reach on pylons'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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