What you'll learn
The electricity in our homes is supplied through the mains, and using it safely depends on understanding the wiring and the safety features built into it. For AQA GCSE Physics you need to understand the difference between direct and alternating current, the UK mains supply, the three wires in a cable and their roles, and the safety features such as earthing and fuses. This guide covers direct and alternating current, the mains supply, the three wires, and electrical safety. By the end you should be able to describe the domestic electricity supply and explain how its safety features protect us.
Key terms and definitions
Direct current (dc) — Current that flows in one direction only, for example from a battery.
Alternating current (ac) — Current that repeatedly changes direction, as in the mains supply.
Mains electricity — The electrical supply to homes, in the UK about 230 V ac at 50 Hz.
Live wire — The wire that carries the alternating potential from the supply.
Neutral wire — The wire that completes the circuit and is near earth potential.
Earth wire — A safety wire that carries current away safely if a fault occurs.
Fuse — A safety device that melts and breaks the circuit if the current is too high.
Earthing — Connecting the metal case of an appliance to the earth wire for safety.
Core concepts
Direct and alternating current
There are two types of current:
- Direct current (dc) flows in one direction only. It is supplied by cells and batteries.
- Alternating current (ac) repeatedly changes direction. It is produced by generators and is the type supplied to our homes.
The UK mains supply is an alternating current with a potential difference of about 230 V and a frequency of 50 Hz (meaning it changes direction 50 times per second).
The three wires in a mains cable
Most mains cables and plugs contain three wires, each with a standard colour and job:
- Live wire (brown) — carries the alternating potential difference from the supply (at about 230 V). This is the dangerous wire.
- Neutral wire (blue) — completes the circuit; it is at or near 0 V (earth potential).
- Earth wire (green and yellow) — a safety wire. It does not normally carry current, but connects the metal case of an appliance to earth so that current can flow safely away if there is a fault.
The potential difference between the live and neutral wires is about 230 V, and it is the live wire that makes mains electricity dangerous.
Why the live wire is dangerous
The live wire is dangerous because it is at a high potential (about 230 V) relative to earth. Your body is at earth potential, so if you touch the live wire, a large potential difference exists between you and earth, and a current flows through you to earth — giving an electric shock. This can happen even if the appliance is switched off, because the live wire may still be connected to the supply. This is why you must never touch the live wire.
Earthing and the metal case
Many appliances have a metal case, which could become dangerous if a fault caused the live wire to touch it — the case would become live, and anyone touching it would get a shock. Earthing prevents this:
- The metal case is connected to the earth wire.
- If the live wire touches the case, a large current flows from the live wire, through the case, to earth via the earth wire.
- This large current melts the fuse (or trips the circuit breaker), disconnecting the appliance and making it safe.
So earthing and the fuse work together to protect the user.
Fuses and circuit breakers
A fuse is a safety device containing a thin wire that melts if the current becomes too high, breaking the circuit and stopping the current. The fuse must be rated slightly higher than the normal operating current, so it does not melt in normal use but melts if a dangerous fault increases the current. A circuit breaker does the same job but works by switching off automatically and can be reset, rather than melting. Both protect against currents that are too high, which could cause overheating and fires.
Double insulation
Some appliances have a plastic (non-conducting) case and are double insulated. These do not need an earth wire, because the case cannot become live — the plastic does not conduct, so even if a wire came loose the user is protected. Double-insulated appliances are marked with a special symbol and have only a live and neutral wire.
Power, energy and choosing a fuse
The power of an appliance is linked to the current it draws, which matters when choosing a fuse. Power is given by power = potential difference × current (P = V × I), so for the mains at 230 V, an appliance's current can be worked out from its power rating: current = power ÷ voltage. For example, a 2300 W heater draws 2300 ÷ 230 = 10 A. The fuse must be rated just above the normal operating current, so it does not blow in normal use but melts if a fault increases the current. A 10 A appliance would typically use a 13 A fuse. Being able to calculate the current from the power, and choose a suitable fuse, is a common exam question that connects electrical safety to the power equation.
Why appliances transfer energy
Domestic appliances work by transferring energy from the mains supply to useful forms. When charge flows through an appliance, work is done and energy is transferred — for example, to heat (in a kettle or heater), to kinetic energy (in a motor), or to light. The amount of energy transferred depends on the power of the appliance and how long it is used: energy = power × time. This is why a high-power appliance used for a long time transfers a lot of energy and costs more to run. Understanding that the mains supplies energy which appliances transfer to useful forms links domestic electricity to the wider ideas of energy transfer and cost, which exams often connect.
Worked examples
Example 1: Comparing dc and ac
State the difference between direct and alternating current, and which is supplied by the mains. Direct current flows in one direction only (for example, from a battery). Alternating current repeatedly changes direction. The mains supply is alternating current, about 230 V at 50 Hz.
Example 2: Identifying a wire's role
What is the job of the earth wire, and does it normally carry current? The earth wire is a safety wire connecting the metal case of an appliance to earth. It does not normally carry current, but if a fault makes the case live, it carries the current safely to earth, melting the fuse and making the appliance safe.
Example 3: How earthing and a fuse work together
Explain how the earth wire and fuse protect a user if the live wire touches a metal case. The case is connected to the earth wire, so if the live wire touches it, a large current flows from live through the case to earth. This large current melts the fuse, which breaks the circuit and disconnects the appliance, so the case is no longer live and the user is protected.
Example 4: Double insulation
Why does a double-insulated appliance not need an earth wire? A double-insulated appliance has a plastic case that does not conduct electricity, so the case cannot become live even if a wire comes loose. Because there is no risk of the case becoming live, an earth wire is not needed.
Common mistakes and how to avoid them
A common error is muddling the wire colours. Learn them: live = brown, neutral = blue, earth = green and yellow. These are frequently tested.
Students often say the earth wire normally carries current. It does not carry current in normal use — it only carries current in a fault, to protect the user. The neutral wire completes the normal circuit.
Another mistake is thinking a switched-off appliance is always safe to touch inside. If it is still plugged in, the live wire may still be at 230 V, so it remains dangerous.
When explaining earthing, remember it works with the fuse: the fault current flows to earth and melts the fuse, disconnecting the appliance. Leaving out the fuse gives an incomplete answer.
Finally, do not confuse a fuse and a circuit breaker with the earth wire. The fuse/circuit breaker breaks the circuit if the current is too high; the earth wire provides the safe path for the fault current.
Exam technique for "Domestic uses of electricity and electrical safety"
Be ready to state the difference between dc and ac and the UK mains values (about 230 V, 50 Hz), and to name the three wires with their colours and roles.
For safety questions, explain why the live wire is dangerous (high potential relative to earth), and explain how earthing and the fuse work together to protect the user when the live wire touches a metal case.
Know what double insulation is and why such appliances need no earth wire. Use precise terms — live, neutral, earth, fuse, earthing — throughout, and give clear cause-and-effect explanations of how the safety features work.
Quick revision summary
- Direct current (dc) flows one way (batteries); alternating current (ac) changes direction — the UK mains is ac, about 230 V, 50 Hz.
- Three wires: live (brown) carries ~230 V, neutral (blue) completes the circuit (~0 V), earth (green/yellow) is a safety wire.
- The live wire is dangerous because it is at a high potential relative to earth, so touching it lets current flow through you to earth.
- Earthing connects a metal case to earth; if the live wire touches the case, a large current flows to earth and melts the fuse, disconnecting the appliance.
- A fuse (or resettable circuit breaker) melts/switches off if the current is too high, preventing overheating and fires.
- Double-insulated appliances have a plastic case and need no earth wire.