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AQA GCSE Physics · Topic 4.2 Electricity
Domestic Uses and Safety: a.c., d.c. and the Three-Core Cable
In this lesson you'll learn to: explain the difference between direct and alternating
current, recall that UK mains is a.c. at 230 V and 50 Hz, name the live, neutral
and earth wires with their colours and potentials, and explain why the live wire is dangerous even when
a switch is off.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
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Start here — have a play
Don't read anything yet. On tab 1, watch the charges and the trace for the cell, then flip the cell. Try a.c. through a diode, then plain a.c. Each time, ask: does the current ever change direction? On tab 2, click each wire in the plug to see its trace, then press Test me. On tab 3, watch the cell and mains symbols being drawn, then test yourself on them.
Supply
p.d. across the bulb against time
The rule: it is the current that decides. If the current only ever flows one way, it is direct current (d.c.), even if the p.d. goes up and down. If the current keeps changing direction, it is alternating current (a.c.). Slowed right down here: real mains changes direction 100 times a second.
Potential against time
Click a wire, or
Click a wire inside the plug
Learn these: UK mains is a.c. at 230 V and 50 Hz. Live is brown, neutral is blue, earth is green and yellow.
Symbol
Exam tip: draw symbols with a ruler and keep the gaps. A cell is one long thin line and one short thick line. The a.c. supply is a circle with one wave inside it: the wave is what tells the examiner the current keeps changing direction.
Traces are drawn to scale; the movement of the charges is slowed down so you can see it.
▲ The traces are to scale. The charges are slowed right down: real mains reverses 100 times a second.
Spotted the difference? Now learn the facts.
This topic is mostly recall: two definitions, two numbers, three colours, and one explanation of why the live wire is dangerous. The notes below have every one.
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Revision notes
4.2.3.1 Direct and alternating potential difference
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Direct current (d.c.)
A current that flows in one direction only. It comes from a direct (steady) potential difference, such as a cell or battery.
Trace: a flat, straight line.⇄
Alternating current (a.c.)
A current that repeatedly changes direction. It comes from an alternating potential difference, which keeps changing direction. Mains electricity is a.c.
Trace: a wave above and below zero.Draw the symbols
Questions on a.c. and d.c. often show a circuit, so know the two supplies on sight. Use a ruler and keep the gaps.
Cell (d.c.)
Battery (d.c.)
a.c. supply (mains)
Lamp: don't mix it up
⭐ How to remember them
Cell: one long thin line (the + side) and one short thick line. A battery is two or more cells joined long-short-long-short.
a.c. supply: a circle with one wave inside. The wave is the clue: the p.d. keeps reversing. A circle with a cross is a lamp.
⭐ Learn these two numbers
UK mains electricity is an a.c. supply at a frequency of 50 Hz and about 230 V.
50 Hz means 50 complete cycles every second, so one cycle takes 1 ÷ 50 = 0.02 s (20 ms). The live wire actually peaks at about 325 V; 230 V is its effective value, and 230 V is the number to write.
4.2.3.2 Mains electricity: the three-core cable
Most appliances connect to the mains with a three-core cable. The insulation on each wire is a different colour so you can tell them apart.
| Wire | Colour | Potential | Its job |
|---|---|---|---|
| Live | brown | about 230 V | Carries the alternating p.d. from the supply. Connected to the fuse. |
| Neutral | blue | about 0 V | Completes the circuit. |
| Earth | green and yellow stripes | 0 V | A safety wire that stops the appliance becoming live. It only carries a current if there is a fault. |
⭐ Why the live wire is dangerous
Your body is at 0 V (earth potential). The live wire is at about 230 V. Touching it creates a large potential difference across your body, so a current flows through you to earth. That can give a fatal electric shock.
This is true even when the switch is off: the switch breaks the circuit, but the live wire is still at 230 V, so you can still complete a circuit to earth through your body.
Any connection between the live wire and earth is dangerous: a big current flows, which can cause a fire.
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How to write it in the exam
The 3-mark "why is it dangerous" question always wants the same three links. Write all three, in order.
The three links for "why is the live wire dangerous?"
The live wire is at about 230 V and a person is at 0 V,
so there is a large potential difference between them,
so a current flows through the person to earth.
the two potentialsthe p.d.the current
Exam-style question · a.c. and d.c. · 2 marks
A torch runs from a battery and a kettle runs from the mains. Describe the difference between the current in the torch and the current in the kettle.
- The torch has direct current: it flows in one direction only (1)
- The kettle has alternating current: it repeatedly changes direction (1)
Exam-style question · recall · 2 marks
Give the potential difference and the frequency of the UK mains supply.
- 230 V (1)
- 50 Hz (1)
Where marks go missing: writing 325 V, or 60 Hz (that is the USA).
Exam-style question · safety · 3 marks
A student switches off a lamp at the wall and says the cable is now completely safe to touch where the insulation is damaged. Explain why the student is wrong.
- The live wire is still at about 230 V when the switch is off (1)
- The student is at 0 V, so there would be a large p.d. across their body (1)
- So a current would flow through them to earth, giving an electric shock (1)
Check your answer has all 3 links: the two potentials, the p.d., the current through the person.
📘 Now do it in your workbook
Define a.c. and d.c. and sketch their traces, label the plug and the three-core cable with colours and potentials, then answer the 6-mark hairdryer question.
AQA GCSE Physics Workbook · ELECTRICITY · page 12The full workbook covers the whole of Topic 4.2 Electricity, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
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Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Page 12 of the workbook has the a.c. and d.c. definitions, the plug to label and the 6-mark hairdryer question, with full answers at the back.
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