AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER
Mains electricity and safety
Electricity · Lesson 5 of 10
Warm-up
Answer each one, then check.
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
Learning Objectives
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.
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.
Direct and Alternating pd
|
Alternating pd changes direction 50 times each second. |
Graphs of direct pd as a flat line and alternating pd as a wave crossing the axis.
A Three-pin Plug
|
Colour coding makes the wires easy to identify. |
A three-pin plug with brown live, blue neutral and green and yellow earth wires labelled.
The Three-Core Cable
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 |
Why the Live Wire Is Dangerous
|
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.
Live and Earth
|
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.
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.
Key Terms
|
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. |
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. |
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...?
☐ 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.
Summary
✓ 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. |