Mains electricity and safety - Teacher Slides.pptx

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Mains electricity and safety

Electricity

Lesson 5 of 10

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Warm-up

Electricity

Lesson 5 of 10

Answer each one, then check.

1

What is the unit of frequency?

2

What colour is the earth wire?

3

What does a fuse do?

4

What does AC stand for?

5

What is the potential difference of the mains?

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Warm-up - Answers

Electricity

Lesson 5 of 10

1

What is the unit of frequency?

Hertz (Hz)

2

What colour is the earth wire?

Green and yellow stripes

3

What does a fuse do?

Melts and breaks the circuit if the current is too large

4

What does AC stand for?

Alternating current

5

What is the potential difference of the mains?

About 230 V

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Learning Objectives

Electricity

Lesson 5 of 10

1

State the frequency and potential difference of UK mains.

2

Explain the difference between dc and ac.

3

Identify the live, neutral and earth wires by colour and function.

4

Explain why a live wire can be dangerous even when a switch is open, and the dangers of connecting live to earth.

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Electricity

Lesson 5 of 10

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.

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Direct and Alternating pd

Electricity

Lesson 5 of 10

Alternating pd changes direction 50 times each second.

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A Three-pin Plug

Electricity

Lesson 5 of 10

Colour coding makes the wires easy to identify.

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The Three-Core Cable

Electricity

Lesson 5 of 10

Learn the colour, name and job of each wire.

Wire

Colour

Job

Potential difference

Live

Brown

Carries the alternating pd from the supply

About 230 V

Neutral

Blue

Completes the circuit

At or close to 0 V

Earth

Green and yellow stripes

Safety wire: stops the appliance becoming live

0 V; only carries a current if there is a fault

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Why the Live Wire Is Dangerous

Electricity

Lesson 5 of 10

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

1

Switch position

If the switch is in the neutral wire, the live wire stays connected to the appliance

2

Potential difference

The live wire is at about 230 V compared with earth

3

Danger

A person touching it can provide a path to earth, and a current flows through the person

ANSWER

The 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.

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Live and Earth

Electricity

Lesson 5 of 10

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

1

Low resistance path

A very large current can flow to earth

2

Effects

The wires heat up, which can cause a fire; a person may be electrocuted

3

Earth wire

It gives a safe path so a fuse blows or the trip operates

ANSWER

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

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Safety Tips

Electricity

Lesson 5 of 10

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.

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Key Terms

Electricity

Lesson 5 of 10

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.

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Electricity

Lesson 5 of 10

YOUR TASK

Plug Detective

10 minutes

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.

WHAT 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.

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Can I...?

Electricity

Lesson 5 of 10

State the frequency and pd of UK mains.

Explain dc and ac.

Name the three wires and their colours.

State the job of each wire.

Say the live wire is at 230 V.

Explain why a live wire is dangerous with the switch open.

Explain the dangers of live-to-earth connections.

Explain the purpose of the earth wire.

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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)

Brown, and about 230 V.

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Exam Practice: Mains electricity and safety

Electricity

Lesson 5 of 10

Answer all questions. Use the mark allocation as a guide to how much to write. · 15 minutes

Question 1 · 2 marks · State

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

Question 2 · 3 marks · Complete

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

Question 3 · 2 marks · Explain

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

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Exam Practice: Mains electricity and safety (cont.)

Electricity

Lesson 5 of 10

Question 4 · 3 marks · Explain

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

Question 5 · 4 marks · Explain

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…

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Question 1 · 2 marks · State

Electricity

Lesson 5 of 10

“

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

HOW TO ANSWER IT

Command word: State. Worth 2 marks, so plan before writing.

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Question 1 · mark scheme

Electricity

Lesson 5 of 10

2 marks available. Award a mark for each point made.

50 Hz

1 mark

230 V

1 mark

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Question 2 · 3 marks · Complete

Electricity

Lesson 5 of 10

The diagram shows the wires in a three-pin plug labelled A, B and C. Give the name of each wire. (3 marks)

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Question 2 · mark scheme

Electricity

Lesson 5 of 10

3 marks available. Award a mark for each point made.

Earth

1 mark

Neutral

1 mark

Live

1 mark

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Question 3 · 2 marks · Explain

Electricity

Lesson 5 of 10

“

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

HOW TO ANSWER IT

Command word: Explain. Worth 2 marks, so plan before writing.

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Question 3 · mark scheme

Electricity

Lesson 5 of 10

2 marks available. Award a mark for each point made.

Direct: one direction

1 mark

Alternating: changes direction repeatedly

1 mark

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Question 4 · 3 marks · Explain

Electricity

Lesson 5 of 10

“

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

HOW TO ANSWER IT

Command word: Explain. Worth 3 marks, so plan before writing.

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Question 4 · mark scheme

Electricity

Lesson 5 of 10

3 marks available. Award a mark for each point made.

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

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Question 5 · 4 marks · Explain

Electricity

Lesson 5 of 10

“

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.

HOW TO ANSWER IT

Command word: Explain. Worth 4 marks, so plan before writing.

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Question 5 · mark scheme

Electricity

Lesson 5 of 10

4 marks available. Award a mark for each point made.

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

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