Mains Electricity — AC and DC

This lesson covers alternating current (AC), direct current (DC) and the UK mains electricity supply, including the three-pin plug and electrical safety, as required by AQA GCSE Combined Science Trilogy (8464, section 6.2.2).

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AC vs DC

Property	Direct Current (DC)	Alternating Current (AC)
Direction of flow	Always in one direction	Repeatedly reverses direction
Source	Cells, batteries, DC power supplies	Mains supply, generators
Oscilloscope trace	Flat horizontal line	Sine wave
UK mains?	No	Yes

Why Does the UK Use AC?

The main reason the UK mains supply uses AC is that AC can be easily transformed (stepped up or down) to different voltages using a transformer. This is essential for the efficient transmission of electricity through the National Grid (covered in a later lesson).

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The UK Mains Supply

Property	Value
Voltage	~230 V
Frequency	50 Hz

The voltage of 230 V is the root mean square (rms) voltage. The actual peak voltage is about 325 V. For GCSE, you only need to know the 230 V and 50 Hz values.

A frequency of 50 Hz means the current changes direction 50 times per second (technically, it completes 50 full cycles per second).

$f = \frac{1}{T}$f=T1​

Where $f$f is frequency in hertz (Hz) and $T$T is the time period in seconds (s).

For 50 Hz: $T = \frac{1}{50} = 0.02 \text{ s}$T=501​=0.02 s

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The Three-Pin Plug

A UK three-pin plug contains three wires:

Wire	Colour	Function	Voltage
Live	Brown	Carries the alternating current to the appliance; alternates between +325 V and −325 V	~230 V (rms)
Neutral	Blue	Completes the circuit; carries current away from the appliance	~0 V
Earth	Green and yellow stripes	Safety wire — provides a low-resistance path to ground if a fault occurs	0 V

Live Wire — Why Is It Dangerous?

The live wire is always dangerous, even when the appliance is switched off, because:

• It may still be at 230 V relative to earth.
• Touching it completes a circuit through your body to the ground, allowing current to flow through you.
• This can cause electric shock, which can be fatal.

Exam Tip (AQA 8464): A very common exam question is: "Why is the live wire dangerous even when the switch is off?" The answer: the live wire is still at 230 V — if you touch it, a current can flow through your body to earth.

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Electrical Safety Features

Fuse

• A thin wire inside a ceramic holder, connected in the live wire.
• If the current exceeds the fuse rating, the wire melts and breaks the circuit.
• This disconnects the live wire, making the appliance safe.
• Fuses come in standard ratings: 3 A, 5 A and 13 A.
• Choose a fuse with a rating just above the normal operating current.

Earth Wire

• Connected to the metal casing of an appliance.
• If a fault causes the live wire to touch the metal case, a large current flows through the earth wire to the ground.
• This large current blows the fuse, disconnecting the live wire and preventing electric shock.

How the Earth Wire and Fuse Work Together

flowchart TD
    A["Fault: live wire touches metal case"] --> B["Large current flows through earth wire to ground"]
    B --> C["Current exceeds fuse rating"]
    C --> D["Fuse melts / blows"]
    D --> E["Live wire is disconnected — appliance is safe"]

Circuit Breaker

Feature	Detail
Function	Trips (switches off) when current is too high
Advantage over fuse	Faster response and can be reset — does not need replacing
Connected in	Live wire

Residual Current Device (RCD)

Feature	Detail
Function	Detects a difference between the current in the live and neutral wires
How it works	If current is "leaking" (e.g. through a person), the live and neutral currents are unequal, and the RCD trips very quickly
Speed	Much faster than a fuse — typically trips in ~30 ms

Double Insulation

• An appliance with no exposed metal parts.
• Has two layers of insulation — the electrical insulation around the wires and the plastic casing.
• Does not need an earth wire (uses a two-core cable: live and neutral only).
• Marked with the double-insulation symbol (a square inside a square).

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Choosing a Fuse

To select the correct fuse rating:

1. Calculate the normal operating current: $I = \frac{P}{V}$I=VP​
2. Choose the fuse rating that is just above this current.

Worked Example

A 920 W microwave operates on a 230 V supply. Which fuse should be used: 3 A, 5 A or 13 A?

$I = \frac{P}{V} = \frac{920}{230} = 4.0 \text{ A}$I=VP​=230920​=4.0 A

A 3 A fuse would blow during normal use. A 5 A fuse is the correct choice (just above 4 A).

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Oscilloscope Traces

An oscilloscope displays voltage on the y-axis and time on the x-axis.

Supply Type	Oscilloscope Trace
DC	A flat horizontal line (constant voltage)
AC	A sine wave (voltage alternates between positive and negative)

From an AC oscilloscope trace you can determine:

• Peak voltage = maximum height from the centre line × volts/division setting.
• Time period (T) = the time for one complete cycle × time/division setting.
• Frequency = $f = 1/T$f=1/T.

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Common Mistakes

Mistake	Correction
Saying the earth wire "carries the current normally"	The earth wire only carries current during a fault
Putting the fuse in the neutral wire	The fuse must always be in the live wire
Saying double-insulated appliances have an earth wire	Double-insulated appliances use a two-core cable — no earth wire
Confusing AC frequency with voltage	Frequency is how often the current changes direction; voltage is the energy per unit charge

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Summary