Mains electricity in the UK is an alternating supply of about 230 V at 50 Hz. The potential difference between the live wire and the earth keeps changing direction, so the live wire is a source of current all the time.
This is important for safety: the live wire is always at a potentially lethal voltage, even when an appliance is switched off, because it's still connected to the supply at the socket.
Quick Check: What colour is the live wire in a UK plug, and what is its function?
The live wire is brown. It carries current to the appliance at an alternating potential difference of about 230 V. It is always dangerous, even when the appliance is switched off but still plugged in.
This how it works covers How It Works: Why the Live Wire Is Always Dangerous within Mains Electricity & Safety for GCSE Physics. Revise Mains Electricity & Safety in Electricity for GCSE Physics with 17 exam-style questions and 30 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 7 of 16 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Mains Electricity & Safety
What does AC stand for, and how does it differ from DC?
Explain how a fuse protects an electrical circuit from damage.