Electricity and magnetism · GCSE Physics
Mains electricity
Teacher-written GCSE Physics revision on mains electricity: 230 V a.c. at 50 Hz, live, neutral and earth, fuses and circuit breakers, the national grid, and why transformers need alternating current.
UK mains is about 230 V a.c. at 50 Hz. Live is the dangerous wire. The fuse sits in the live. Earth and plastic cases protect the user if a fault makes the case live.
The important bits
What you need to know
- 1
UK mains is alternating current (a.c.) at about 230 V, frequency 50 Hz — the potential difference reverses 50 times per second. Cells and batteries supply direct current (d.c.).
- 2
A three-core cable has live (brown), neutral (blue) and earth (green/yellow). Live carries the alternating potential; neutral completes the circuit near 0 V; earth is a safety wire.
- 3
The fuse or circuit breaker sits in the live wire and melts or trips if current is too high, disconnecting the live supply before the cable overheats.
- 4
The earth wire connects a metal case to earth. If a live fault touches the case, a large current flows to earth, the fuse blows, and the case cannot stay live.
- 5
Double-insulated appliances have a plastic case and no earth wire; the symbol is a square inside a square. They still need a fuse in the live.
- 6
The national grid uses step-up transformers to increase potential difference and decrease current for transmission, reducing I²R heating in the cables, then step-down transformers for 230 V at homes.
- 7
Transformers only work with a.c., because a changing current is needed to produce a changing magnetic field in the core. That is a reason the mains is a.c.
- 8
Circuit breakers can be reset and act faster than fuses. Residual-current devices (RCDs) trip if live and neutral currents differ, which can mean a leak through a person.
Quotations worth analysing
Short evidence. Real method.
“The fuse is connected in the live wire.”
A fuse in the neutral could blow and leave the appliance still connected to live. Always live-side protection.
“A step-up transformer increases potential difference and decreases current.”
Power is roughly conserved, so if V rises, I falls. Lower current means less heating in the transmission cables.
“Mains electricity is a.c.; a battery supplies d.c.”
Oscilloscope traces: a.c. crosses the axis; d.c. is a steady (or pulsed) one-way potential difference.
Go deeper
Live, fuse, earth is one safety story
The live wire is at 230 V a.c. relative to earth. Touching it can pass a current through you to earth. The fuse is a weak link in that live conductor: too much current, it melts, the live is isolated. If a toaster’s live wire comes loose and touches the metal case, and the case is earthed, a large current flows in the earth wire, the fuse blows, and you do not hold a live case. If there is no earth and the case is metal, the case can sit at 230 V waiting for a hand. Plastic cases (double insulation) remove that path. Draw the three wires into a plug: brown to live (right pin looking at the back? — learn the plug diagram your board uses), fuse in the live, earth to the top pin. Wrong colour is a standard labelling question.
Go deeper
The grid is high voltage so that current can be low
Power stations produce electrical power P = VI. For a given P, a larger V means a smaller I. Heating in kilometres of cable is I²R, so a smaller I slashes the wasted thermal store in the wires. Step-up transformers near the station raise V to hundreds of kilovolts. Step-down transformers in substations bring it to 230 V, which is still dangerous but usable. Transformers need a.c.; a steady d.c. current would not change the magnetic field in the core, so there would be no induced potential difference in the secondary. In six-mark answers, write: step up, lower current, less I²R loss, then step down for safety and appliances. “High voltage travels faster” is not physics.
See the idea in action
A 2.2 kW kettle on 230 V a.c. draws I = P/V = 2200 / 230 = 9.6 A. A 13 A fuse is correct; a 3 A fuse would melt in normal use. The live wire is brown and carries the fuse. If the kettle is double-insulated, there is no earth wire. Separately, a power station delivers 100 MW. At 400 kV, I = P/V = 1.00 × 10⁸ / 4.00 × 10⁵ = 250 A. At 25 kV the current would be 4000 A. Heating in a 2.0 Ω cable would be I²R = 250² × 2.0 = 125 kW at high voltage, versus 32 MW at 25 kV — that is why the grid steps up.
Exam technique
Turn knowledge into marks
For mains safety, name live, fuse, earth and the fault they protect against. State 230 V, 50 Hz, a.c. For the grid, write higher V → lower I → less I²R heating, and say transformers need a.c. Choose fuses from I = P/V.
Common mistakes
Do not give these marks away
- 01
Putting the fuse in the neutral, or saying the earth wire carries the current in normal use.
- 02
Saying the mains is d.c., or that transformers work from a battery without extra electronics.
- 03
Explaining the national grid without lower current and less heating, or choosing a 13 A fuse for a 2 A appliance.
Why does the national grid transmit electrical power at high potential difference?
AHigh voltage makes the current larger so energy arrives faster
BFor a given power, higher V means lower I, which reduces I²R heating in the cables
CTransformers only work at 230 V
DA.c. cannot be used at low voltage
Show the answer
For a given power, higher V means lower I, which reduces I²R heating in the cables. P = VI, so raising V lowers I for the same power. Cable heating is I²R, so lower current means less energy wasted as thermal store in the wires.
Quick questions
If this is the bit you searched
What do live, neutral and earth do?
Live supplies the alternating potential difference. Neutral completes the circuit. Earth is a safety connection from a metal case to the ground so a live fault blows the fuse.
Why do transformers need a.c.?
A changing current in the primary produces a changing magnetic field in the core, which induces a potential difference in the secondary. Direct current would not keep changing.
What is the difference between a fuse and an earth wire?
The fuse melts and breaks the live if current is too high. The earth wire provides a low-resistance path from a metal case so that a fault current is large enough to blow that fuse.
Is UK mains 230 V d.c.?
No. It is about 230 V a.c. at 50 Hz. Batteries are d.c. Oscilloscope traces of mains cross the time axis.