Domestic Uses of Electricity

This lesson covers the domestic uses of electricity, including alternating and direct current, the structure of a three-pin plug, the function of the earth wire and fuses, and the role of circuit breakers. This is part of the AQA GCSE Physics specification (4.2.2).

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Alternating Current (a.c.) and Direct Current (d.c.)

There are two types of electric current:

Direct Current (d.c.)

Direct current always flows in the same direction. The current has a constant value (for a steady d.c. supply).

Sources of d.c.:

• Batteries and cells
• Solar cells
• Phone chargers (which convert mains a.c. to d.c.)

Alternating Current (a.c.)

Alternating current continuously changes direction. The current repeatedly reverses — it flows one way, then the other way, in a regular cycle.

Sources of a.c.:

• Mains electricity from the National Grid
• Generators at power stations

UK Mains Supply

The UK mains electricity supply is:

• Alternating current (a.c.)
• 230 V (potential difference)
• 50 Hz (frequency — the current changes direction 50 times per second, giving 50 complete cycles per second)

Property	UK Mains
Type	a.c.
Voltage	230 V
Frequency	50 Hz

Exam Tip: You must remember the three key values for the UK mains supply: a.c., 230 V, 50 Hz. These are frequently tested in multiple-choice questions. Do not confuse 230 V with 240 V — the UK officially uses 230 V (though actual measurements may vary slightly).

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Visualising a.c. and d.c. on an Oscilloscope

An oscilloscope (or CRO — cathode ray oscilloscope) displays voltage against time.

• d.c. appears as a horizontal straight line (constant voltage, no variation with time).
• a.c. appears as a wave (usually a sine wave), showing the voltage continuously rising and falling.

From an a.c. oscilloscope trace, you can determine:

• The peak voltage — the maximum height of the wave above or below the centre line.
• The frequency — by measuring the time period (T) of one complete cycle, then using f = 1/T.

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The Three-Pin Plug

All mains appliances in the UK use a three-pin plug to connect to the mains supply. Understanding the structure of a plug is essential for the exam.

Structure of a Three-Pin Plug

Wire	Colour	Pin Position	Function
Live	Brown	Bottom-right	Carries the alternating current to the appliance (alternates between +230 V and -230 V)
Neutral	Blue	Bottom-left	Completes the circuit; at approximately 0 V
Earth	Green and yellow stripes	Top-centre	Safety wire; carries current to ground if there is a fault

Key Features of a Plug

Feature	Material	Why
Casing	Plastic or rubber	Electrical insulator — prevents electric shock
Pins	Brass	Good conductor of electricity; does not rust; hard and springy for a good connection
Cable grip	Plastic or rubber	Holds the cable firmly so that the wires are not pulled out of their terminals
Fuse	Glass tube with thin wire	Melts and breaks the circuit if the current is too high

Wiring a Plug

• The brown (live) wire connects to the fuse and then to the bottom-right pin.
• The blue (neutral) wire connects to the bottom-left pin.
• The green/yellow (earth) wire connects to the top-centre pin.
• Each wire must be firmly secured in its correct terminal — no bare wire should be visible.
• The outer insulation of the cable should be held securely by the cable grip.

Exam Tip: A popular exam question shows a plug diagram and asks you to identify faults. Common faults include: wires connected to wrong terminals, no earth wire, no cable grip, bare wires visible, wrong fuse rating. Make sure you know the correct wire colours and positions.

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The Live Wire

The live wire is the most dangerous wire in a mains circuit.

• It alternates between approximately +230 V and -230 V.
• Even when an appliance is switched off, the live wire is still connected to the 230 V supply — it is always dangerous.
• Touching a live wire can cause a lethal electric shock because a large potential difference exists between the live wire (230 V) and the earth (0 V), causing a large current to flow through your body.

The neutral wire is at approximately 0 V (it is connected to earth at the power station).

Exam Tip: A common question asks "Why is the live wire dangerous even when the switch is off?" The answer is that the live wire is always at 230 V, so if you touch it, current will flow through your body to earth because of the large p.d. between the live wire and earth (0 V).

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The Earth Wire and Fuses

The Earth Wire

The earth wire is a safety feature. It is connected to the metal casing of an appliance (if it has one) and to the ground via the earth pin of the plug.

How it works (in the event of a fault):

1. If a fault occurs and the live wire touches the metal casing of an appliance, the casing becomes live (at 230 V).
2. Without an earth wire, anyone touching the casing would receive a lethal electric shock.
3. With the earth wire, the current flows through the earth wire to the ground instead of through the person.
4. This large current through the earth wire causes the fuse to melt (blow), breaking the circuit and making the appliance safe.

Fuses

A fuse is a thin wire inside a glass tube or ceramic holder. It is connected in series with the live wire.

How a fuse works:

1. The fuse is rated at a specific current value (e.g., 3 A, 5 A, or 13 A).
2. If the current in the circuit exceeds the fuse rating, the thin wire in the fuse heats up and melts.
3. This breaks the circuit in the live wire, stopping the flow of current and preventing damage or fire.

Choosing the Correct Fuse