Mains ElectricityEdexcel GCSE Physics: Revision notes
Section 1
Direct current vs alternating current
Electrical energy is transferred from batteries and the a.c. mains to the energy stores of motors and heating devices in domestic appliances.
- Direct current (d.c.): charge moves in one direction only. Cells and batteries supply direct current.
- Alternating current (a.c.): the movement of charge changes direction repeatedly (reverses back and forth).
- In the UK, the domestic mains supply is a.c., with a frequency of 50 Hz and a voltage of about 230 V.
Learn the two key UK mains numbers precisely: 50 Hz frequency, about 230 V — these are common recall marks.
Section 2
What are the three wires in a domestic mains cable?
A standard domestic mains cable contains three wires, each with a specific colour and function:
| Wire | Colour | Function | Typical potential difference |
|---|---|---|---|
| Live | Brown | Carries the alternating potential difference from the supply | Alternates, around 230 V relative to earth |
| Neutral | Blue | Completes the circuit, carrying current back to the supply | Approximately 0 V |
| Earth | Green and yellow | Safety wire — normally carries no current | 0 V |
The live wire alternates between positive and negative potential relative to earth, while the neutral wire stays at (or very close to) 0 V — this is why a person is at risk of a shock if they touch the live wire, since a large potential difference exists between it and the earthed ground/their body.
Section 3
How does the earth wire keep people safe?
- The earth wire is a safety wire connected to the metal casing of an appliance and to the ground. It normally carries no current.
- If a fault causes the live wire to touch the metal casing of an appliance, the earth wire provides a low-resistance path for current to flow safely to earth, rather than through a person who touches the casing.
- This surge of current is large enough to blow a fuse, or trip a circuit breaker, which cuts off the current supply and disconnects the appliance, preventing further danger.
- Switches and fuses should always be connected in the live wire, not the neutral wire — this ensures that when a switch is off or a fuse blows, the live (dangerous) side of the appliance is disconnected, making it safe to touch even though the neutral wire might otherwise still be connected to the supply.
- There is a serious danger in providing any connection between the live wire and earth (for example, through faulty wiring or a person touching a live component), because a large potential difference (about 230 V) drives a current through the connection, which could pass through a person's body and cause a fatal electric shock.
A common error is putting the fuse/switch in the neutral wire — always state it must be in the LIVE wire, otherwise the appliance stays live even when 'off'.
Must Know
- Direct current (d.c.): charge flows in one direction only (cells/batteries). Alternating current (a.c.): direction of charge flow repeatedly reverses.
- UK mains supply: a.c., 50 Hz, about 230 V.
- Three mains wires: live (brown, carries alternating p.d.), neutral (blue, ~0 V, completes circuit), earth (green/yellow, safety, normally no current).
- Switches and fuses must be connected in the live wire.
- The earth wire and fuse/circuit breaker work together to cut off current and prevent electric shock if a fault occurs.
- Any connection between live and earth is dangerous because the large potential difference can drive a fatal current through a person.
That's the notes covered.
Carry on to the next subtopic.