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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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Teacher resources

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Student handouts

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Warm-up

Answer each one, then check.

  1. 1

    What does a power station generate?

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    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?

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    Increases the potential difference

  5. 5

    What is a cable?

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    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.

Questions and answers

10 questions set on this lesson, with the mark schemes and model answers open.

1. Exam question Complete 2 marks Easier

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

Mark scheme — 2 marks available

  • Step-up increases pd — 1 mark
  • Step-down decreases pd — 1 mark

Model answer

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

2. Exam question Use the diagram 3 marks Easier

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.

Mark scheme — 3 marks available

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

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).

3. Exam question Explain 3 marks Easier

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

Mark scheme — 3 marks available

  • 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

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.

4. Exam question Calculate 4 marks Easier

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.

Mark scheme — 4 marks available

  • Rearranges to I = P ÷ V — 1 mark
  • 10 A — 1 mark
  • Correct substitution into I²R — 1 mark
  • 200 W — 1 mark

Model answer

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

5. Exam question Explain 2 marks Easier

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

Mark scheme — 2 marks available

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

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.

6. Multiple choice 1 mark Easier

A step-up transformer...

  1. A increases the pd Correct
  2. B decreases the pd
  3. C increases the current
  4. D stores energy

Why: It is used at the start of the transmission cables.

7. Multiple choice 1 mark Core

High pd transmission means the current is...

  1. A high
  2. B low Correct
  3. C zero
  4. D alternating only

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

8. Multiple choice 1 mark Core

A step-down transformer is used...

  1. A at the power station
  2. B inside a battery
  3. C near homes Correct
  4. D between cells

Why: It reduces the pd for use in homes.

9. Multiple choice 1 mark Core

Which loses less energy as heat in cables?

  1. A high current
  2. B high resistance only
  3. C a short cable
  4. D low current Correct

Why: P = I²R.

10. Multiple choice 1 mark Stretch

The National Grid links...

  1. A power stations to consumers Correct
  2. B homes to homes
  3. C batteries to lamps
  4. D cells to resistors

Why: It is a system of cables and transformers.