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Physics · Electricity

The National Grid

Describe the National Grid and explain why using step-up and step-down transformers makes the transfer of energy efficient.

  • 6 key terms
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Warm-up

Answer each one, then check.

  1. 1

    What does a power station generate?

    Show answerHide answer

    Electricity

  2. 2

    What happens to a wire when a current flows?

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    It heats up

  3. 3

    Write the equation for power using current and resistance.

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    \(P = I^2R\)

  4. 4

    What does step-up mean?

    Show answerHide answer

    Increases the potential difference

  5. 5

    What is a cable?

    Show answerHide answer

    A wire used to carry current

Learning Objectives

  1. 1Describe the National Grid as a system of cables and transformers.
  2. 2State the roles of step-up and step-down transformers.
  3. 3Explain why the National Grid is an efficient way to transfer energy.

THE NATIONAL GRID

Step-up transformers increase the potential difference for transmission, which reduces the current and so the energy wasted as heat in the cables.

The National Grid is a system of cables and transformers linking power stations to consumers.

Why High pd Means Efficient

Follow the logic.

  1. 1 Power = pd × current

    For the same power, a higher pd means a smaller current.
  2. 2 Less current

    A smaller current means less heating in the cables.
  3. 3 Energy wasted

    Power loss in the cable is \(P = I^2R\), so halving the current cuts the loss to a quarter.
  4. 4 Step-up transformers

    Increase the pd before the cables.
  5. 5 Step-down transformers

    Decrease the pd to a safe value for homes (230 V).

Comparing Losses

100 kW is transmitted through cables of total resistance 2.0 Ω. Compare the power lost at 1000 V with the power lost at 10 000 V.

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  1. 1 Current at 1000 V \(I = P \div V = 100\,000 \div 1000 = 100\) A
  2. 2 Power lost \(P = I^2R = 100^2 \times 2.0 = 20\,000\) W
  3. 3 Current at 10 000 V \(I = 100\,000 \div 10\,000 = 10\) A
  4. 4 Power lost \(10^2 \times 2.0 = 200\) W

Answer20 kW lost at 1000 V but only 200 W lost at 10 000 V: high pd wastes far less energy.

Key Points

For an exam answer.

  • Efficient

    High pd, low current means little energy wasted as heat in the cables.

  • Step-up

    Used at the power station end.

  • Step-down

    Used near homes and factories.

  • Safety

    The high pd is reduced to 230 V before homes.

Follow the Grid

Put these in the correct order: homes, step-up transformer, power station, step-down transformer, transmission cables.

1. Start at the power station.

2. End at homes.

A good answer shows: Power station, step-up transformer, transmission cables, step-down transformer, homes.

Can I...?

  1. 1Describe the National Grid.
  2. 2State what a step-up transformer does.
  3. 3State what a step-down transformer does.
  4. 4Explain why high pd is used.
  5. 5Link current to energy wasted.
  6. 6Compare power losses.
  7. 7Draw or complete a grid diagram.
  8. 8Explain why 230 V is needed in homes.

Summary & Exam Focus

  • Step-up transformers increase pd, step-down decrease it.
  • High pd means low current for the same power.
  • Low current means less energy wasted as heat in the cables.
  • The National Grid is efficient.

Exam focus

Explain why the National Grid transmits electricity at a high potential difference. (3 marks) (3 marks)

Say higher pd means lower current, so less heating.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

National Grid
A system of cables and transformers linking power stations to consumers.
Transformer
A device that changes the size of an alternating pd.
Step-up transformer
Increases the pd.
Step-down transformer
Decreases the pd.
Transmission cables
Cables that carry electricity over long distances.
Efficient
Wasting little energy.

Practice questions

Have a go at each one before you open its answer.

  1. Question 1 Complete 2 marks

    Name the two types of transformer used in the National Grid and state what each does.

    Show answerHide answer

    Model answer

    Step-up transformers increase the potential difference; step-down transformers decrease the potential difference.

    Mark scheme

    • Step-up increases pd — 1 mark
    • Step-down decreases pd — 1 mark
  2. Question 2 Use the diagram 3 marks

    The diagram shows part of the National Grid. (a) Name the parts labelled A and B. (b) State why B is needed near homes.

    A power station, transformer A, transmission cables, transformer B and a house.
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    Model answer

    (a) A is a step-up transformer; B is a step-down transformer. (b) To decrease the pd to a safe value for homes (230 V).

    Mark scheme

    • A: step-up transformer — 1 mark
    • B: step-down transformer — 1 mark
    • Reduces the pd to a safe value for homes — 1 mark
  3. Question 3 Explain 3 marks

    Explain why using a high potential difference to transmit electricity is efficient.

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    Model answer

    For the same power, a high potential difference means a smaller current. A smaller current causes less heating in the cables, so less energy is wasted.

    Mark scheme

    • High pd means lower current for the same power — 1 mark
    • Lower current means less heating of the cables — 1 mark
    • So less energy wasted — 1 mark
  4. Question 4 Calculate 4 marks

    A power of 100 kW is transmitted at 10 000 V through cables of total resistance 2.0 Ω. Calculate the power wasted as heat in the cables.

    Show answerHide answer

    Model answer

    \(I = 100\,000 \div 10\,000 = 10\) A; \(P = I^2R = 10^2 \times 2.0 = 200\) W

    Mark scheme

    • Rearranges to I = P ÷ V — 1 mark
    • 10 A — 1 mark
    • Correct substitution into I²R — 1 mark
    • 200 W — 1 mark
  5. Question 5 Explain 2 marks

    The potential difference in the transmission cables is much higher than in homes. Explain why the potential difference in homes is reduced.

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    Model answer

    230 V is safer for use in homes and works with appliances designed for it; a step-down transformer is used to decrease the pd.

    Mark scheme

    • Safe for homes or appliances designed for 230 V — 1 mark
    • Step-down transformer decreases the pd — 1 mark

Quick check

  1. A step-up transformer...

    1. Aincreases the pd
    2. Bdecreases the pd
    3. Cincreases the current
    4. Dstores energy
    Show answerHide answer

    A: increases the pd

    It is used at the start of the transmission cables.

  2. High pd transmission means the current is...

    1. Ahigh
    2. Blow
    3. Czero
    4. Dalternating only
    Show answerHide answer

    B: low

    P = VI: the same power with a higher V means a smaller I.

  3. A step-down transformer is used...

    1. Aat the power station
    2. Binside a battery
    3. Cnear homes
    4. Dbetween cells
    Show answerHide answer

    C: near homes

    It reduces the pd for use in homes.

  4. Which loses less energy as heat in cables?

    1. Ahigh current
    2. Bhigh resistance only
    3. Ca short cable
    4. Dlow current
    Show answerHide answer

    D: low current

    P = I²R.

  5. The National Grid links...

    1. Apower stations to consumers
    2. Bhomes to homes
    3. Cbatteries to lamps
    4. Dcells to resistors
    Show answerHide answer

    A: power stations to consumers

    It is a system of cables and transformers.

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