Mains Electricity

Domestic mains electricity in the UK is 230 V AC at 50 Hz. A current this high through a person can stop the heart, so appliances use FOUR overlapping safety measures.

1. Insulation. Wires inside an appliance are wrapped in plastic insulation (PVC). This stops the live and neutral wires touching each other or the metal casing, and stops a user touching the live wire by accident.

2. Earthing. Appliances with METAL CASINGS have a third wire — the EARTH wire (green/yellow stripes). The earth wire connects the casing directly to the ground via a low-resistance path. If a fault makes the live wire touch the casing:

• A large current flows from live → casing → earth wire → ground.
• The current is well above the appliance's normal operating current.
• This LARGE current blows the FUSE.
• The fuse breaks the circuit → the appliance is disconnected → the casing is safe.

The user is protected because (i) the large current goes to earth via the wire (much lower resistance than through them), and (ii) the fuse cuts the circuit within a fraction of a second.

3. Double insulation. Appliances with PLASTIC casings (kettles, hairdryers, electric toothbrushes) don't need an earth wire. The plastic casing cannot conduct, so even if a live wire works loose inside, the user touching the casing won't receive a shock. These are marked with a 'double square' symbol.

4. Fuses and circuit breakers. These break the circuit if the current rises above a safe value.

• Fuse = thin wire (in the LIVE wire of the plug) that MELTS if the current exceeds the rated value. One-time use; must be replaced.
• Circuit breaker = an electromagnetic switch that OPENS automatically. Faster than a fuse and resettable.

Picking the right fuse rating. Common ratings are 3 A, 5 A and 13 A. Choose the SMALLEST rating that is ABOVE the appliance's normal operating current ($I = P/V$). E.g. a 2.3 kW kettle on 230 V draws 10 A → fit a 13 A fuse.

When a current flows through a resistor, electrons collide with the LATTICE IONS of the conductor. Each collision transfers kinetic energy from the electron to the ion, increasing the lattice's vibrations — i.e. the resistor's temperature rises.

This is the basis of many domestic appliances:

Appliance	What's heated	Outcome
Kettle	water	boils water for tea/coffee
Toaster	nichrome wire	bread crust browns
Electric oven	heating element	cooks food
Hair dryer	nichrome wire + fan	dries hair
Filament lamp	tungsten filament	glows white-hot

In each case the resistive element is chosen to have:

• High resistance so $P = I^2 R$ is large (lots of heat),
• A high melting point so it survives operating temperature,
• Resistance that doesn't change too much with temperature.

For appliances where heating is NOT wanted (motors, transformers, computer chips), low-resistance components are used and cooling fans / heat sinks remove the heat that does build up.

Power dissipated by an electrical component (spec 2.4):

$P = I \times V$

• $P$ in watts (W)
• $I$ in amperes (A)
• $V$ in volts (V)

Energy transferred by a current (spec 2.5):

$E = I \times V \times t = P \times t$

• $E$ in joules (J)
• $t$ in seconds (s) — convert minutes / hours first.

Domestic energy in kWh. Electricity bills use the kilowatt-hour (kWh), where 1 kWh = 3.6 × 10⁶ J. Calculate: energy (in kWh) = power (in kW) × time (in hours). This isn't on the spec but is useful context.

Choosing a fuse (spec 2.4 application). Given an appliance's power and supply voltage, find the operating current:

$I = \dfrac{P}{V}$

Then pick the SMALLEST standard fuse rating (3 A, 5 A or 13 A) that lies ABOVE the operating current. A 100 W lamp on 230 V draws ~0.43 A → 3 A fuse. A 2 kW heater on 230 V draws ~8.7 A → 13 A fuse.

Edexcel pitfall. Picking a fuse EQUAL to the operating current is wrong — the fuse must allow normal operation without blowing.

Alternating current (AC). The direction of current reverses periodically. Mains supplies are AC.

• UK mains: 230 V (rms), 50 Hz (50 cycles per second).
• USA mains: 110 V (rms), 60 Hz.

AC is used because it can be efficiently STEPPED UP/DOWN by transformers (covered in Topic 6 Magnetism). This minimises transmission losses over long distances.

Direct current (DC). Current flows in ONE direction only. Cells, batteries, USB chargers and most electronic devices use DC.

Recognising on an oscilloscope.

• DC: horizontal straight line (constant voltage with time).
• AC: sine wave oscillating above and below zero.

A typical Edexcel question gives an oscilloscope trace and asks 'AC or DC?' — quote the visual evidence ('the trace oscillates above and below zero' = AC; 'the trace is a horizontal line' = DC).