How is mains electricity supplied to homes, and how is it wired safely?
Domestic electricity and mains: direct and alternating current, the UK mains supply, the three-core cable and the role of the live, neutral and earth wires.
A focused answer to AQA GCSE Physics 4.2.4, covering the difference between direct and alternating current, the UK mains supply values, and the live, neutral and earth wires in a three-core cable and how they keep appliances safe.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
AQA wants you to distinguish direct from alternating current, state the UK mains supply values, and describe the live, neutral and earth wires in a three-core cable and the safety role of each. This is topic 4.2.4 of the AQA GCSE Physics (8463) specification.
Direct and alternating current
A cell or battery has a fixed positive and negative terminal, so charge always flows the same way: that is direct current. The mains generator instead produces a potential difference that swings from positive to negative and back again times every second, so the current keeps reversing. A useful way to tell them apart is on an oscilloscope: a d.c. supply gives a flat horizontal line, while an a.c. supply gives a wave that oscillates above and below zero. The frequency is the number of complete cycles per second, which for the UK mains is the figure.
The three-core cable
Most appliances connect to the mains through a three-core cable with colour-coded wires. The colours and the insulation around each wire are standardised so that anyone wiring a plug can identify the wires safely:
- Live wire (brown): carries the alternating potential difference from the supply, at about . It is the dangerous wire.
- Neutral wire (blue): completes the circuit, staying at or near . The live and neutral together form the working circuit through the appliance.
- Earth wire (green and yellow): a safety wire at that carries current only if there is a fault, stopping the metal case of an appliance becoming live.
Appliances with a plastic outer case (such as a hairdryer) are often double insulated and do not need an earth wire, because there is no exposed metal that could become live.
Why the live wire is dangerous
Try this
Q1. State the potential difference and frequency of the UK mains supply. [2 marks]
- Cue. About and .
Q2. Explain why the live wire is dangerous to touch. [2 marks]
- Cue. It is at about relative to earth (), so a current would flow through your body to earth.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20184 marksA three-core cable connects a metal-cased toaster to the mains. Describe the function of the earth wire and explain how, together with the fuse, it protects the user if the live wire comes loose and touches the metal case.Show worked answer →
The earth wire is a safety wire connected to the metal case; it is at and normally carries no current (1 mark). If the live wire touches the metal case, the case would become live and dangerous, but the earth wire provides a low-resistance path from the case to earth (1 mark). A very large current then flows from the live wire through the case and earth wire to earth (1 mark). This large current melts the fuse (or trips the circuit breaker), which breaks the live wire and disconnects the appliance, so the case can no longer give the user an electric shock (1 mark). Markers reward the low-resistance path, the large fault current, and the fuse melting to isolate the supply.
AQA 20203 marksCompare direct current and alternating current, and state the potential difference and frequency of the UK domestic mains supply.Show worked answer →
Direct current (d.c.) flows in one direction only and is produced by cells and batteries (1 mark). Alternating current (a.c.) repeatedly reverses direction, so the potential difference alternates between positive and negative values (1 mark). The UK domestic mains supply is alternating current with a potential difference of about and a frequency of , meaning it changes direction times each second (1 mark). Markers reward the directional contrast and both correct mains values. A frequent error is to label the mains as d.c. or to quote or .
Related dot points
- Current, potential difference and resistance: the meaning of each quantity, the charge equation, Ohm's law and the I-V characteristics of resistors, lamps and diodes.
A focused answer to AQA GCSE Physics 4.2.1, covering current as the rate of flow of charge, potential difference, resistance, Ohm's law and the current-voltage characteristics of ohmic resistors, filament lamps and diodes.
- Series and parallel circuits: the rules for current, potential difference and resistance in each arrangement, and how to combine resistors.
A focused answer to AQA GCSE Physics 4.2.2, covering the rules for current, potential difference and resistance in series and parallel circuits and how total resistance changes when components are combined.
- Electrical power and the national grid: the power and energy equations, charge and energy transfer, and why step-up and step-down transformers make transmission efficient.
A focused answer to AQA GCSE Physics 4.2.3 and 4.2.5, covering the electrical power equations, the energy transferred by an appliance, and why the national grid uses step-up and step-down transformers to transmit power efficiently.
- Static electricity: charging by friction, the forces between charges, sparking, and the idea of an electric field around a charged object (separate physics only).
A focused answer to AQA GCSE Physics 4.2.6, covering how insulators are charged by friction through electron transfer, attraction and repulsion between charges, how sparks form, and the concept of an electric field around a charged object.
Sources & how we know this
- AQA GCSE Physics (8463) specification — AQA (2016)