Domestic Electricity
UK Mains Electricity
UK mains electricity is alternating current AC at 230 V and 50 Hz. AC periodically reverses direction, unlike DC (from batteries) which flows in one direction only. The 50 Hz means the current completes 50 full cycles per second.
Electrical Power and Energy
Power: P = IV = I²R = V²/R (where P is in watts, I in amps, V in volts)
Energy transferred: E = P × t (joules = watts × seconds) or E = VIt
Cost of electricity: energy is sold in kilowatt-hours (kWh). 1 kWh = 1 kW used for 1 hour = 3 600 000 J. Cost = power (kW) × time (hours) × price per kWh.
Efficiency
Efficiency = useful energy output ÷ total energy input (× 100 for %) Or: Efficiency = useful power output ÷ total power input
A 100 W light bulb that emits 10 W of light and 90 W of heat has an efficiency of 10%.
The Three-Pin Plug (WJEC Required Knowledge)
UK three-pin plugs have three connections:
- Live (brown): carries the alternating high voltage (~230 V). Connected to the fuse.
- Neutral (blue): return path, at ~0 V.
- Earth (green/yellow stripe): safety wire connecting the metal casing of an appliance to earth (0 V). If a fault causes the casing to go live, current flows safely to earth, blowing the fuse.
The wires are held in with cable grips. The outer insulation must not be stripped too far. The fuse is always wired into the live wire.
Fuses and Circuit Breakers
A fuse contains a thin wire that melts if current exceeds its rated value, breaking the circuit. Fuses protect cables from overheating, not people directly.
Choosing the correct fuse: select the lowest rated fuse above the normal operating current of the appliance. E.g. if an appliance uses 4.5 A normally, use a 5 A fuse (not a 3 A which would blow immediately, nor a 13 A which gives no protection).
Circuit breakers (MCBs) use an electromagnet or bimetallic strip to disconnect the circuit; they can be reset. RCDs (residual current devices) detect any difference between live and neutral currents (indicating leakage through a person) and disconnect in milliseconds — much faster than a fuse.
Earthing and Double Insulation
Earthing: appliances with metal casings have the casing connected to the earth wire. If a live wire touches the casing, a large current flows to earth, blowing the fuse and cutting power.
Double insulation (marked with a square-within-a-square symbol): the casing is plastic (non-conducting) so there is no risk of the casing going live; no earth wire needed.
⚠Common mistakes
- Live and neutral confused: live is brown and at high voltage; neutral is blue at ~0V. The fuse goes in the live wire — if it were in neutral, the appliance would remain at high voltage even with the fuse blown.
- Efficiency > 1: efficiency can never exceed 100% because you cannot get more useful energy out than you put in (conservation of energy).
- kWh vs joules: exam questions may ask for energy in kWh for cost calculations; convert carefully (1 kWh = 3.6 × 10⁶ J).
- Fuse rating confusion: fuse must blow if current is too high, so choose the lowest rating above normal current.
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