SPEC 8463 • 4.2.3 • PAPER 1 ⭐
AC, DC and the UK Mains Supply
Two definitions, two numbers, three wires. Easy marks every series — but examiners flag the same recall errors paper after paper.
AC vs DC — The Two Definitions
The spec section is titled "Direct and alternating potential difference" — so define both in terms of pd (and the current follows).
DC
Direct potential difference: a pd with fixed polarity — one terminal stays positive, the other stays negative.
Current flows in one direction only. Produced by cells and batteries.
AC
Alternating potential difference: a pd whose polarity periodically reverses.
Current periodically reverses direction. Produced by the mains supply and alternators.
DC pd stays positive (above zero). AC pd oscillates between positive and negative — the wave crosses the zero line each half-cycle.
Where Each Comes From
TypeSourceTypical values
DCCells and batteries1.5 V – 9 V (small portable supplies)
ACMains supply (and any alternator)230 V, 50 Hz in the UK
Spec point
The spec wants you to explain the difference between direct and alternating potential difference. The minimum credit-worthy answer: "DC flows in one direction; AC periodically reverses direction."
UK Mains: 230 V, 50 Hz
Two recall numbers. Both required. Both come up every series.
230 V
Potential difference (live to earth)
50 Hz
Frequency (cycles per second)
Period from frequency
Time for one full cycle: T = 1 ÷ f = 1 ÷ 50 = 0.02 s = 20 ms. The current direction reverses 50 times per second.
Spec wording
"The domestic electricity supply has a frequency of 50 Hz and is about 230 V." Use "230 V" in answers — the spec word is "about", so 230 is the number you write.
Examiner Alert
Jun 2024 Paper 1H · Q05.1
72% of students scored 2/2; 15% scored 1/2. The two main errors are both flagged by AQA.
"A number of students thought mains p.d. was 240 V. It was officially changed to 230 V in 1987 and this is the value given in the specification. However, it was more common for students to get the p.d. correct and the frequency incorrect." — AQA Examiner Report, Jun 2024 Paper 1H
Two killer errors
"240 V" — outdated value. UK was officially harmonised to 230 V in 1987. Always write 230 V.
Wrong frequency — actually more common than the 240 V error. Students remember the pd and forget 50 Hz. Lock both in together.
Three-Core Cable — Colour Code
Domestic appliances connect to the mains through a three-core cable. Each wire is insulated and colour-coded.
Brown — Live wire. Carries the alternating potential difference from the supply.
Blue — Neutral wire. Completes the circuit. At, or close to, earth potential (0 V).
Green & yellow stripes — Earth wire. Safety wire. At 0 V; carries no current except during a fault.
Memorise verbatim
Marks are awarded for the exact spec wording. "Brown is the live one" can lose marks if you confuse the colours under pressure. The colour-to-function mapping must be perfect.
Real 1-mark question style
Jun 2024 P1H Q10.1 asked: "What colour is the insulation covering the live wire in a three-core cable?" 72% scored. The answer is just "brown" — straight recall, no explanation needed. Don't overthink one-word answers.
Live Wire — Brown
Spec wording
"The live wire carries the alternating potential difference from the supply."
Colour: brown
Carries the AC supply pd (oscillates ~±230 V about earth potential)
Potential difference between live and earth ≈ 230 V
The fuse is fitted in the live wire — so a fault current breaks the live connection
Always dangerous
The live wire is at 230 V relative to earth whenever the supply is connected — even when the appliance is switched off, even when the fuse hasn't blown. Treat any exposed live conductor as live.
Neutral Wire — Blue
Spec wording
"The neutral wire completes the circuit. The neutral wire is at, or close to, earth potential (0 V)."
Colour: blue
Provides the return path for current back to the supply
Held at, or very close to, 0 V (earth potential) by the supplier
Carries current whenever the appliance is on
Common confusion
"Neutral carries no current" is wrong — neutral carries the same current as live when the appliance is running. What's near zero is the potential difference, not the current.
Earth Wire — Green & Yellow Stripes
Spec wording
"The earth wire is a safety wire to stop the appliance becoming live. The earth wire is at 0 V, it only carries a current if there is a fault."
Colour: green and yellow stripes
Connected to the metal case of the appliance
At 0 V under normal conditions
Carries no current unless a fault occurs
Common wrong answer
"The earth wire grounds the current" is too vague to score. The mark-scheme answer is "safety wire to stop the appliance becoming live." Use that phrase.
The UK Three-Pin Plug
Inside the plug (cover removed, wiring view): three pins connected to three coloured wires. The fuse sits in series with the live wire.
View from inside the plug — wires visible. Fuse always in series with the live (brown) wire.
The Fuse — Circuit Symbol
You've seen the fuse in the plug diagram. AQA also expects you to draw its circuit symbol — a 1-mark recall task on real papers.
A rectangle with a horizontal line through it. Wires extend from each end.
Jun 2024 P1H · Q10.2
"Draw the circuit symbol for a fuse." 66% of students scored — meaning 1 in 3 lost the easy mark. AQA's examiner report flagged two specific failure modes.
"Students who made the box for the answer into a fuse symbol did not score the mark, nor did a fuse symbol added to any side of the box." — AQA Examiner Report, Jun 2024
Drawing rules
Draw the symbol INSIDE the answer box, not as the box itself
The rectangle of the fuse symbol must be your own drawing — separate from the printed box outline
Don't attach the symbol to the side of the box
Include the wires extending from each end
Spec Safety Point 1 — Live Is Always Dangerous
Spec-required explanation
"Students should be able to explain that a live wire may be dangerous even when a switch in the mains circuit is open."
Why
The live wire is at 230 V relative to earth as long as the supply is connected — regardless of whether the appliance switch is open or closed. Opening the switch breaks the circuit (so no current flows in normal use), but the live wire is still at 230 V.
If a person touches the live wire while also in contact with earth (e.g. standing on a damp floor, or touching a tap), current flows through them to earth. They get a shock — possibly fatal — even though the appliance is "off".
4-mark version (specimen P1H Q13.1)
The full chain — used when the question is worth 4 marks:
The live wire is at 230 V (relative to earth), even when the switch is open. (1)
The earth (e.g. ground / pipes / damp floor) is at 0 V. (1)
The person is connected to earth — so there's a large potential difference across the person. (1)
This causes a current to flow through the person — an electric shock, possibly fatal. (1)
Spec Safety Point 2 — Earth Wire + Fuse Protection
How the earth wire and fuse work together to protect a user from a metal-cased appliance with a fault.
3-mark PEE answer
If the live wire becomes loose and touches the metal case, the case becomes live (at 230 V).
The earth wire provides a low-resistance path from the case to ground. A large current flows from live → case → earth wire → ground.
This large current melts (blows) the fuse in the live wire, breaking the circuit. The case is no longer live, so the user is protected.
Why both are needed
The earth wire diverts the fault current safely. The fuse then disconnects the live supply. One alone wouldn't protect the user — the earth wire on its own would conduct indefinitely; the fuse alone wouldn't blow if there were no fault path.
Spec Safety Point 3 — Live ↔ Earth Connection
Spec-required explanation
"Students should be able to explain the dangers of providing any connection between the live wire and earth."
What happens
If the live wire is connected to earth (intentionally or by accident — e.g. touching live while standing on the floor), there's a huge pd of 230 V across a very low resistance path.
By I = V ÷ R, this gives a very large current. The consequences:
Electric shock — if the path is through a person, the current can stop the heart
Heating of wires — large current heats the wires; can cause a fire
Damage to the appliance or supply circuitry
Why a fuse helps but isn't a guarantee
The fuse will blow if the current rises high enough. But fuses take time to melt — fast enough to protect wiring from fire, but a person can still receive a serious shock in milliseconds.
Weak vs Strong Answers
The classic colour confusion — and the vague-safety-language trap.
Weak (0/2)
"The brown wire is neutral and the blue wire is the live one."
Swapped colours. Loses both marks instantly.
Strong (2/2)
"The brown wire is live; the blue wire is neutral; the green-and-yellow striped wire is earth."
Weak (0/3)
"The earth wire grounds the electricity so you don't get shocked."
No mechanism. No mention of fuse. Vague.
Strong (3/3)
"If the live wire touches the case, the earth wire carries a large current to ground, which blows the fuse and breaks the circuit."
Mark Scheme Language
The exact phrases AQA mark schemes credit. Memorise these — paraphrasing loses marks.
ConceptMark-scheme phrase
Live wire role"carries the alternating potential difference from the supply"
Neutral wire role"completes the circuit" / "at, or close to, earth potential (0 V)"
Earth wire role"safety wire to stop the appliance becoming live" / "only carries current if there is a fault"
Live still dangerous"at 230 V relative to earth even when the switch is open"
Fuse mechanism"large current melts the fuse / blows the fuse, breaking the circuit"
Practice — Two Quick Recall Hits
Q1. State the values of the potential difference and frequency of the UK domestic mains electricity supply.2 marks
1
pd = 230 V(1 mark)
2
frequency = 50 Hz(1 mark)
!
Both values required. Per Jun 2024 examiner report, frequency-wrong is more common than pd-wrong.
Q2. A washing machine is plugged into the mains and switched off at the appliance switch. Explain why a person could still get an electric shock if they touch the live wire inside.3 marks
1
The live wire is still at 230 V relative to earth, even when the switch is open. (1 mark)
2
The person is connected to earth (e.g. standing on the floor), which is at 0 V. (1 mark)
3
A pd exists across the person, causing a current to flow through them — an electric shock. (1 mark)
Recap
Definitions
DC = one direction; AC = periodically reverses.
UK Mains values
230 V & 50 Hz. Period 20 ms.
Wires
Brown = Live; Blue = Neutral; Green/Yellow = Earth.
Three safety points the spec requires you to explain
1
Live is always dangerous — at 230 V even when the switch is open.
2
Earth + fuse — earth diverts fault current; fuse blows and disconnects supply.
3
Live-to-earth connection — very large current, risk of shock and fire.
Checkpoint
You can now:
Distinguish AC from DC by direction of current flow.
State UK mains as 230 V and 50 Hz — both numbers, not one.
Identify the three wires by colour and state each wire's role using mark-scheme language.
Explain why a live wire is dangerous even when the switch is open .
Explain the earth + fuse protection mechanism in PEE form.
Explain the dangers of any live-earth connection .
Avoid the Jun 2024 traps — " 240 V " and forgetting the frequency.
Next lesson
2.08 — Electrical Power. Two recall equations (P = VI and P = I²R), a decision rule for which to use, and the squared-current trap students consistently miss.
Next: AC vs DC — The Two Definitions