OpenRevise GCSE

Viewing as

Student Teacher

Teacher view: planning notes, the answers to every question, and the teacher copies of the files.

Physics · Electricity

Mains electricity and safety

Explain the difference between direct and alternating potential difference, and describe the three-core cable and the dangers of live wires.

  • 6 key terms
  • All boards
Download the teacher pack · 5 files

Teacher resources

The teacher copies: slides with the questions built in, the answers, and anything else attached to this lesson for whoever is teaching it.

Student handouts

The same files the students see, to print or hand out.

Warm-up

Answer each one, then check.

  1. 1

    What is the unit of frequency?

    Show answerHide answer

    Hertz (Hz)

  2. 2

    What colour is the earth wire?

    Show answerHide answer

    Green and yellow stripes

  3. 3

    What does a fuse do?

    Show answerHide answer

    Melts and breaks the circuit if the current is too large

  4. 4

    What does AC stand for?

    Show answerHide answer

    Alternating current

  5. 5

    What is the potential difference of the mains?

    Show answerHide answer

    About 230 V

Learning Objectives

  1. 1State the frequency and potential difference of UK mains.
  2. 2Explain the difference between dc and ac.
  3. 3Identify the live, neutral and earth wires by colour and function.
  4. 4Explain why a live wire can be dangerous even when a switch is open, and the dangers of connecting live to earth.

MAINS ELECTRICITY

Mains electricity in the UK is an alternating supply of about 230 V with a frequency of 50 Hz.

Direct pd (dc) always pushes charge in the same direction. Alternating pd (ac) repeatedly changes direction.

The Three-Core Cable

Learn the colour, name and job of each wire.

  • Live

    Colour: Brown. Job: Carries the alternating pd from the supply. Potential difference: About 230 V

  • Neutral

    Colour: Blue. Job: Completes the circuit. Potential difference: At or close to 0 V

  • Earth

    Colour: Green and yellow stripes. Job: Safety wire: stops the appliance becoming live. Potential difference: 0 V; only carries a current if there is a fault

Why the Live Wire Is Dangerous

Explain why touching a live wire can be dangerous even when the switch in the appliance is open.

Show the solutionHide the solution
  1. 1 Switch position If the switch is in the neutral wire, the live wire stays connected to the appliance
  2. 2 Potential difference The live wire is at about 230 V compared with earth
  3. 3 Danger A person touching it can provide a path to earth, and a current flows through the person

AnswerThe live wire is at 230 V even when the switch is open (if the switch is in the neutral); touching it can give a dangerous current.

Live and Earth

Explain the dangers of a connection between the live wire and earth.

Show the solutionHide the solution
  1. 1 Low resistance path A very large current can flow to earth
  2. 2 Effects The wires heat up, which can cause a fire; a person may be electrocuted
  3. 3 Earth wire It gives a safe path so a fuse blows or the trip operates

AnswerA connection between live and earth can cause a large current, overheating, fires or electric shock.

Safety Tips

Exam-ready reasons.

  • Fuse and switch

    They go in the live wire so the appliance is disconnected from 230 V.

  • Earth wire

    Connected to the metal case so that if the live wire touches it, a large current flows and the fuse melts.

  • Double-insulated appliances

    Have no earth wire because they have a plastic case.

Plug Detective

A plug's wires are brown, blue and green and yellow. State the name of each wire and its job. Then say why a switch should go in the live wire.

1. Name each wire.

2. Say what it does.

A good answer shows: Brown: live, carries 230 V. Blue: neutral, completes the circuit. Green and yellow: earth, safety wire. A switch in the live wire disconnects the appliance from the 230 V supply.

Can I...?

  1. 1State the frequency and pd of UK mains.
  2. 2Explain dc and ac.
  3. 3Name the three wires and their colours.
  4. 4State the job of each wire.
  5. 5Say the live wire is at 230 V.
  6. 6Explain why a live wire is dangerous with the switch open.
  7. 7Explain the dangers of live-to-earth connections.
  8. 8Explain the purpose of the earth wire.

Summary & Exam Focus

  • UK mains: 230 V, 50 Hz, ac.
  • Live: brown; neutral: blue; earth: green and yellow.
  • Earth wire only carries current if there is a fault.
  • Switches go in the live wire.

Exam focus

Give the colour of the live wire and state its potential difference compared with earth. (2 marks) (2 marks)

Brown, and about 230 V.

Key terms

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

Direct pd (dc)
A pd that always pushes charge in the same direction.
Alternating pd (ac)
A pd that repeatedly changes direction.
Frequency
The number of cycles per second, in hertz.
Live wire
The wire carrying the alternating pd, at about 230 V.
Neutral wire
The wire that completes the circuit, at about 0 V.
Earth wire
The safety wire, at 0 V, connected to the metal case.

Questions and answers

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

1. Exam question State 2 marks Easier

State the frequency and the approximate potential difference of the UK mains supply.

Mark scheme — 2 marks available

  • 50 Hz — 1 mark
  • 230 V — 1 mark

Model answer

50 Hz and about 230 V.

2. Exam question Complete 3 marks Easier

The diagram shows the wires in a three-pin plug labelled A, B and C. Give the name of each wire.

A three-pin plug with the three wires labelled A, B and C.

Mark scheme — 3 marks available

  • Earth — 1 mark
  • Neutral — 1 mark
  • Live — 1 mark

Model answer

A: earth (green and yellow), B: neutral (blue), C: live (brown).

3. Exam question Explain 2 marks Easier

Explain the difference between direct potential difference and alternating potential difference.

Mark scheme — 2 marks available

  • Direct: one direction — 1 mark
  • Alternating: changes direction repeatedly — 1 mark

Model answer

A direct pd always causes charge to flow in the same direction. An alternating pd repeatedly changes direction.

4. Exam question Explain 3 marks Easier

The earth wire in a kettle is connected to its metal case. Explain the purpose of the earth wire.

Mark scheme — 3 marks available

  • Safety wire — 1 mark
  • Stops the case becoming live — 1 mark
  • Only carries current if there is a fault, so the fuse melts — 1 mark

Model answer

It is a safety wire. If the live wire touches the case, a large current flows through the earth wire, which stops the case from becoming live, and the fuse melts.

5. Exam question Explain 4 marks Easier

A person touches the live wire in a broken appliance, but the switch in the appliance was off. Explain why the person could still receive an electric shock, and why connecting the live wire to earth is dangerous.

Mark scheme — 4 marks available

  • Live wire is at about 230 V — 1 mark
  • Switch may be in the neutral wire, so live remains connected — 1 mark
  • Current flows through the person to earth — 1 mark
  • Live to earth: very large current causing heating, fire or shock — 1 mark

Model answer

The live wire is at about 230 V, and if the switch is in the neutral wire the live wire is still connected to the supply, so the person provides a path to earth and a current flows through them. A connection between live and earth gives a very large current which can cause a fire or shock.

6. Multiple choice 1 mark Easier

The frequency of UK mains is...

  1. A 230 Hz
  2. B 50 Hz Correct
  3. C 5 Hz
  4. D 500 Hz

Why: 50 cycles each second.

7. Multiple choice 1 mark Core

The colour of the neutral wire is...

  1. A brown
  2. B green and yellow
  3. C blue Correct
  4. D red

Why: Neutral is blue.

8. Multiple choice 1 mark Core

The earth wire is...

  1. A brown
  2. B blue
  3. C black
  4. D green and yellow Correct

Why: Green and yellow stripes.

9. Multiple choice 1 mark Core

Alternating pd...

  1. A changes direction repeatedly Correct
  2. B is always in one direction
  3. C is 0 V
  4. D has no frequency

Why: That is what alternating means.

10. Multiple choice 1 mark Stretch

The live wire is at about...

  1. A 0 V
  2. B 230 V Correct
  3. C 12 V
  4. D 50 V

Why: It carries the alternating pd from the supply.