Revision notes · Electricity
Domestic uses and safety
Direct and alternating potential difference4.2.3.1
Definition: Direct current (dc) flows in one direction only — cells and batteries supply dc. Alternating current (ac) continuously changes direction, reversing back and forth — mains electricity is ac.
In the UK, mains electricity is an ac supply with a frequency of 50 Hz and is about 230 V.
Mains electricity4.2.3.2
Most electrical appliances are connected to the mains using a three-core cable, containing a live wire, a neutral wire and an earth wire, distinguished by the colour of their insulation.
| Wire | Colour | Voltage | Role |
|---|---|---|---|
| Live | Brown | ≈230 V (alternating) | Carries the alternating potential difference from the supply — can be dangerous even with the appliance switched off, since current may still be able to flow through it |
| Neutral | Blue | ≈0 V | Completes the circuit, carrying current back to the supply |
| Earth | Green and yellow stripes | 0 V | A safety wire, connected to the case of the appliance and to the ground; only carries a current if there is a fault |
If a fault causes the live wire to touch the metal casing of an appliance, the earth wire gives the current a low-resistance path to earth, causing a large current to flow that blows the fuse (or trips a circuit breaker) and disconnects the live wire — preventing the casing from staying live and causing an electric shock to anyone who touches it.
⚠️ Common mistake: Forgetting that the live wire can still be dangerous even when a device is switched off, if the switch is in the neutral wire rather than the live wire, or if there is a fault — this is why appliances are switched off using the live wire, and why the earth wire and fuse are needed as extra protection.
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