Overview
What you’ll learn
- Distinguish a.c. from d.c. and quote UK mains values: about 230 V at 50 Hz.
- Name the three plug wires, their colours and their jobs, and explain why the live wire is always dangerous.
- Explain how fuses, circuit breakers, earthing and double insulation work together as a safety system.
- Use P = V I, P = I² R and E = P t fluently, in joules and kilowatt-hours, with costs.
- Explain the National Grid's high-voltage, low-current strategy using P = I² R.
- Describe step-up and step-down transformers qualitatively and why transformers need a.c.
Specification checklist (AQA Trilogy 8464 · Edexcel 1SC0 · OCR Gateway J250)
Statements are paraphrased from the shared DfE combined science subject content and grouped by tier. Foundation content is studied by every student; Higher statements are examined on Higher-tier papers only. This chapter covers only what Combined Science (double award) examines — separate-science extras live on the GCSE Biology, Chemistry and Physics hubs. QuizQuip is not affiliated with AQA, Pearson Edexcel or OCR.
Foundation — every student
- 24.1Describe direct potential difference (d.c.) as a p.d. that keeps the same polarity, and alternating potential difference (a.c.) as a p.d. that repeatedly reverses polarity; recall that UK mains is a.c. at about 230 V with a frequency of 50 Hz.
- 24.2Describe the functions of the live, neutral and earth wires, their colour coding (brown, blue, green-and-yellow stripes), and explain why the live wire is dangerous even when a switch in it is open.
- 24.3Explain how fuses, circuit breakers and earthing protect users and cables: a fault current through the earth connection blows the fuse and disconnects the live supply; the switch and fuse are always placed in the live wire.
- 24.4Explain why appliances with plastic (insulating) cases need no earth wire — double insulation — and select an appropriate fuse rating slightly above an appliance's normal operating current.
- 24.5Calculate the power of mains appliances using P = V I, and the energy transferred using E = P t, in both joules and kilowatt-hours, including the cost of domestic electricity from meter readings.
- 24.6Describe the National Grid as the network of cables and transformers that carries electrical power from power stations to consumers, using step-up transformers before transmission and step-down transformers before local supply.
- 24.8Describe how a transformer works only with a.c. — the changing current in the primary coil produces a changing magnetic field that induces a p.d. across the secondary coil — and state that step-up transformers raise voltage (lowering current) while step-down transformers lower it for safe domestic use.
Higher — Higher tier only
- 24.7Explain, using P = I² R, why the Grid transmits power at very high voltage and correspondingly low current: heating losses in the cables scale with the square of the current, so a small current keeps the wasted power small.
Key points
- 1D.c. (cells, batteries) keeps one polarity; a.c. repeatedly reverses — UK mains is a.c. at about 230 V, 50 Hz (50 cycles each second).
- 2Live (brown) carries the alternating supply; neutral (blue) completes the circuit near earth potential; earth (green-and-yellow stripes) is a safety wire bonded to metal cases.
- 3The switch and fuse sit in the live wire so opening them disconnects the appliance from the high-potential supply — the live wire stays dangerous even when the switch is open.
- 4A fuse melts when the current beats its rating and breaks the circuit; a fault to a metal case flows through the earth wire, the big current blows the fuse and the case is made safe.
- 5Plastic-cased appliances are double insulated and need no earth wire; choose a fuse rating just above the normal current (a 3.0 A iron takes a 5 A fuse).
- 6P = V I gives appliance power; P = I² R gives cable heating; E = P t gives energy in joules — and in kilowatt-hours (1 kWh = 3.6 MJ) for bills at ~30 p per kWh.
- 7The National Grid transmits at very high voltage, low current because cable heating is P = I² R — halve the current and you quarter the loss.
- 8Step-up transformers raise the voltage for transmission; step-down transformers return it to about 230 V near homes; transformers only work because a.c. makes a changing magnetic field.
Notes · 1 of 4
A.c., d.c. and the UK mains supply
One polarity or many: reading the two kinds of supply.
Direct potential difference (d.c.) keeps the same polarity: the positive and negative terminals never swap, so the current flows one way round the circuit. Cells and batteries are the everyday examples. On an oscilloscope-style trace, d.c. is a flat horizontal line sitting at its voltage value.
Alternating potential difference (a.c.) repeatedly reverses polarity, surging one way then the other. The UK mains supply is a.c. with a potential difference of about 230 V and a frequency of 50 Hz — the cycle repeats 50 times every second, so the current changes direction 50 times each second. On a trace, a.c. is a wave sweeping positive and negative about zero. Why a.c.? Because transformers step voltage up and down easily, and the Grid depends on that.
| Property | Direct (d.c.) | Alternating (a.c.) |
|---|---|---|
| Polarity | Fixed — terminals never swap | Reverses repeatedly |
| Current direction | One way only | Surges each way in turn |
| Typical source | Cell, battery, USB charger output | UK mains (about 230 V, 50 Hz) |
| Trace shape | Flat horizontal line | Oscillating wave about zero |
Worked example — quoting mains values
Q: State the potential difference and frequency of the UK mains supply, and what the frequency value means. A: About 230 V at a frequency of 50 Hz. Frequency is cycles per second, so the alternating p.d. completes 50 full cycles — and the current reverses direction 50 times — every second. Two quoted values plus the interpretation: three easy marks.
Notes · 2 of 4
Wires, plugs and the safety system
Brown, blue and stripes: three wires, three very different jobs.
A UK plug carries three wires. The live wire (brown) carries the alternating p.d. from the supply — it sits at about 230 V relative to earth and is the dangerous one. The neutral wire (blue) completes the circuit back to the supply, sitting at about 0 V (earth potential) in normal operation. The earth wire (green-and-yellow stripes) is a safety wire with no job in normal use — it is connected to the metal case of an appliance so that a fault current has a low-resistance escape route.
Two safety rules follow. First, the switch and fuse are always placed in the live wire: switching off then truly disconnects the appliance from the high-potential supply. Second, the live wire is dangerous even when the switch is open — the socket side of the switch is still at 230 V, so touching "off" does not mean safe. Fuses and circuit breakers both cut the circuit when the current runs too large: the fuse's thin wire melts once; a circuit breaker trips and can be reset.
If a frayed live wire touches a metal case, the case itself becomes live. The earth wire offers that fault current a low-resistance path straight back to the supply: the current surges far above the fuse rating, the fuse melts, and the dead appliance is disconnected before anyone touches it. Appliances with plastic (insulating) cases cannot go live — they are double insulated, marked with a square-within-a-square symbol, and need no earth wire at all.
Common hazard questions
Never touch any mains wire without switching off at the socket — the neutral sits near 0 V but a miswired plug or a swapped connection can make it live. And a 3 A fuse in an appliance that normally draws 3 A will blow during the startup surge: pick a rating just above normal current, not equal to or below it.
Notes · 3 of 4
Power, energy and the price of electricity
P = VI, P = I²R and E = Pt — plus what a kilowatt-hour really is.
The power of an appliance — energy transferred per second — comes from P = V I. A laptop charger drawing 0.50 A from the 230 V mains transfers P = 230 × 0.50 = 115 W. When you know the current and the cable's resistance instead, the heating in that cable is P = I² R — the equation that will later explain the whole National Grid.
Energy transferred follows E = P t. In joules: a 138 W hair straightener on for 5.0 minutes (300 s) transfers E = 138 × 300 = 41 400 J. Domestic energy is billed in kilowatt-hours: 1 kWh is the energy a 1 kW appliance transfers in 1 hour, and 1 kWh = 1000 W × 3600 s = 3.6 MJ. A 1.2 kW heater left on for 2.5 hours uses 1.2 × 2.5 = 3.0 kWh, which at 30 p per kWh costs 90 p.
| Equation | Gives you | Watch out for |
|---|---|---|
| P = V I | Appliance power from mains values | Current in amps, p.d. in volts |
| P = I² R | Heating in cables and resistors | Current is squared — losses grow fast |
| E = P t (J) | Energy in joules | Time must be in seconds |
| E (kWh) = P (kW) × t (h) | Energy for bills | kW with hours — not W with seconds |
Worked example — fuse rating and energy
Q: A 690 W iron runs on 230 V mains. (a) Find its normal current. (b) Choose a fuse from 3 A, 5 A, 13 A. (c) Find the energy it transfers in 40 minutes. A: (a) I = P ÷ V = 690 ÷ 230 = 3.0 A. (b) The 5 A fuse — the 3 A fuse would blow on the startup surge; 13 A leaves the cable unprotected. (c) E = Pt = 690 × 2400 s = 1 656 000 J ≈ 1.7 MJ (or 0.69 kW × 0.667 h ≈ 0.46 kWh).
Notes · 4 of 4
The National Grid and transformers
Why your electricity crosses the country at 400 000 V.
The National Grid is the network of cables and transformers linking power stations to consumers. Leaving a power station, a step-up transformer raises the voltage to as much as 400 000 V; the same power then travels as a much smaller current. Near homes, a chain of step-down transformers returns the voltage to about 230 V for safe domestic use.
The reason is P = I² R. Cable resistance is fixed by the cable itself, so the heating loss scales with the square of the current. Transmitting 46 kW at 400 000 V means a current of only I = 46 000 ÷ 400 000 ≈ 0.12 A; in a 50 Ω cable that wastes P = 0.12² × 50 ≈ 0.7 W. The same 46 kW at 230 V would need 200 A, wasting 200² × 50 = 2 000 000 W — two megawatts of pure heat. High voltage, low current, tiny losses.
Worked example — the cost of transmitting at low voltage (Higher)
Q: A cable of resistance 15 Ω carries 4.0 A. Find the power wasted, then the new loss if the current doubles. A: P = I²R = 4.0² × 15 = 16 × 15 = 240 W. Doubling the current squares it: 8.0² × 15 = 64 × 15 = 960 W — four times the loss. Always square the factor: half the current → a quarter of the loss.
Transformers only work with a.c.: the alternating current in the primary coil creates a changing magnetic field in the iron core, and that changing field induces an alternating p.d. across the secondary coil. A step-up transformer has more turns on the secondary (voltage up, current down); a step-down transformer has fewer. D.c. in the primary gives a steady field, which induces nothing — which is exactly why the Grid runs on a.c.
Revise
Key terms
The words examiners expect you to use precisely.
- Direct potential difference (d.c.)
- A p.d. that keeps the same polarity, driving current one way — as from a cell or battery.
- Alternating potential difference (a.c.)
- A p.d. that repeatedly reverses polarity; UK mains is a.c. at about 230 V, 50 Hz.
- Hertz (Hz)
- The unit of frequency: one cycle per second. UK mains completes 50 cycles each second.
- Live wire
- The brown wire carrying the alternating supply at about 230 V relative to earth; the dangerous one.
- Neutral wire
- The blue wire that completes the circuit, sitting near earth potential (about 0 V) in normal operation.
- Earth wire
- The green-and-yellow safety wire bonded to metal cases, giving fault current a low-resistance path that blows the fuse.
Show 4 more key termsShow fewer key terms
- Double insulation
- Insulating-plastic casing (square-within-a-square symbol) that removes the need for an earth wire.
- Kilowatt-hour (kWh)
- The energy a 1 kW appliance transfers in 1 hour; 1 kWh = 3.6 MJ, the billing unit on domestic meters.
- National Grid
- The network of cables and transformers carrying electrical power from power stations to consumers at very high voltage.
- Transformer
- Two coils on an iron core; a changing primary current induces an alternating p.d. across the secondary. Step-up raises voltage, step-down lowers it.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Calling UK mains "direct current at 230 V"
Mains p.d. reverses polarity 50 times a second — that is the definition of a.c.
Instead: UK mains is alternating, about 230 V at 50 Hz; cells and batteries are the d.c. sources.
Mistake: Mixing up the brown and blue wires
Swapped live and neutral leave the appliance "off" but still at 230 V inside.
Instead: Brown = live (the dangerous supply wire); blue = neutral (near 0 V); striped = earth.
Show 10 more mistakesShow fewer mistakes
Mistake: Thinking the neutral wire is safe to touch
Near 0 V only in a correctly wired circuit; a fault or swapped wiring can make it live.
Instead: Treat every mains conductor as dangerous; switch off at the socket before touching anything.
Mistake: Putting the switch or fuse in the neutral wire
Opening the neutral leaves the whole appliance connected to the live supply at 230 V.
Instead: Switch and fuse always break the live wire, disconnecting the high potential.
Mistake: Choosing a fuse equal to the normal current
Startup surges push the current briefly above its running value, so a 3 A fuse blows in a 3.0 A appliance.
Instead: Pick the next rating above normal current: a 3.0 A iron takes a 5 A fuse.
Mistake: Confusing kW and kWh
A kilowatt is a power (rate); a kilowatt-hour is an energy (amount).
Instead: kWh = kW × hours. A 2 kW heater run for 3 h uses 6 kWh of energy.
Mistake: Using watts with hours in E = Pt
Mixing units gives answers out by factors of thousands.
Instead: Joules need watts with seconds; kilowatt-hours need kilowatts with hours.
Mistake: Saying doubling the current doubles cable heating
Heating is P = I²R — the current is squared.
Instead: Double the current → four times the loss; halve it → a quarter.
Mistake: Saying the Grid uses high voltage "because electricity travels faster"
Current travels at the same drift speed everywhere; the Grid cares about wasted power, not speed.
Instead: High voltage → low current for the same power → P = I²R losses shrink.
Mistake: Saying transformers create energy
A step-up transformer raises voltage but lowers current — power in ≈ power out.
Instead: Transformers transfer power, trading voltage for current; none is created.
Mistake: Expecting the earth wire to carry current in normal use
In a healthy appliance the earth wire carries nothing.
Instead: The earth wire is a standby path: only a fault sends current down it, and that surge blows the fuse.
Mistake: Reading a.c. and d.c. traces with the axes swapped
The vertical axis shows p.d.; the horizontal axis shows time — mixing them swaps amplitude for frequency.
Instead: Height of the wave = peak p.d.; number of waves per second on the time base = frequency.
Check yourself
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Explain a.c., d.c. and the UK mains supply
UK mains supplies electricity as
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Practise
Topical questions
12 exam-style questions with model answers and mark schemes.
A set of structured and calculation questions matched to this chapter, in three tiers, each with a model answer, mark scheme and examiner tip. Higher-tier questions are tagged.
Practise topical questionsExam practice
Practice paper
A 30-mark paper in the style of a Combined Science paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 24 Practice Paper
30 marks · 40 min · 5 pages
6 multiple-choice questions and structured questions with answer lines. A.c. and d.c. supplies, UK mains values, live/neutral/earth wiring, fuses, earthing and double insulation, appliance power with P = V I and P = I² R, energy in joules and kilowatt-hours with costs, and the National Grid with step-up and step-down transformers.
Mark scheme
PROChapter 24 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
How does this chapter fit the real exams?
Physics Paper 1
1 h 15 · 70 marks (Trilogy)
Energy, electricity, particle model, atomic structure (chapters 21–26)
Physics Paper 2
1 h 15 · 70 marks (Trilogy)
Forces, waves, magnetism and electromagnetism (chapters 27–30)
Tiers
Foundation (grades 1-1 to 5-5) · Higher (4-3 to 9-9)
Two grades are awarded together; the 21 required practicals are examined on the written papers
Want a full paper? Try the Biology Paper 1-style (Foundation), Biology Paper 1-style (Higher), Biology Paper 2-style (Foundation), Biology Paper 2-style (Higher), Chemistry Paper 1-style (Foundation), Chemistry Paper 1-style (Higher), Chemistry Paper 2-style (Foundation), Chemistry Paper 2-style (Higher), Physics Paper 1-style (Foundation), Physics Paper 1-style (Higher), Physics Paper 2-style (Foundation), Physics Paper 2-style (Higher), Required practicals practice sets.
Questions
Frequently asked questions
What are the UK mains values I must quote?
About 230 V at 50 Hz. The frequency is the examiner's favourite follow-up: 50 hertz means 50 full cycles per second, so the current reverses direction 50 times each second. Cells and batteries, by contrast, are d.c. — one polarity, one direction.
Why is the live wire dangerous even when the appliance is switched off?
Because the switch breaks the live wire on the appliance side only. The socket side — and everything up to that break — is still connected to the 230 V supply. That is exactly why switches and fuses are always placed in the live wire: opening it truly disconnects the high potential.
How do the earth wire and fuse work as a team?
If a live wire touches a metal case, the earth wire gives the fault current a low-resistance path back to the supply. The current surges far above the fuse rating, the fuse wire melts, and the circuit breaks — disconnecting the live supply before the case can harm anyone.
What is a kilowatt-hour, really?
It is an amount of energy, not a power: the energy a 1 kW appliance transfers in one hour. 1 kWh = 1000 W × 3600 s = 3.6 MJ. Bills multiply kilowatts by hours for kWh, then by the price per kWh — a 2 kW heater for 3 h costs 6 × 30 p = £1.80 at 30 p per kWh.
Why does the Grid transmit at 400 000 V instead of 230 V?
Cable heating is P = I² R. For a fixed power, raising the voltage cuts the current (P = V I), and because the current is squared in the loss formula, the waste collapses — transmitting 46 kW at 400 000 V needs only about 0.12 A, while the same power at 230 V would need 200 A and waste megawatts. Step-up transformers raise the voltage; step-down transformers bring it back before your home.
Up next
Chapter 25: Particle model and density