GCSE Physics Mains & grid revision

Direct current (d.c.) flows in one direction only — cells and batteries supply it, and a p.d.–time graph is a flat line. Alternating current (a.c.) reverses direction repeatedly: the p.d. rises, falls, reverses and rises again, tracing a wave. UK mains is a.c. with a p.d. of about 230 V and a frequency of 50 Hz — it completes 50 full cycles every second, swapping direction twice per cycle. The 230 V quoted is an effective value that delivers the same power as a 230 V d.c. supply would; the actual peak of the wave is higher (about 325 V). On an oscilloscope-style trace, d.c. is a flat line and a.c. is a regular wave — the time for one complete wave is one period, and frequency = 1 ÷ period.

The switch and fuse always sit in the live wire. That placement matters: with the switch open, the appliance is disconnected from the dangerous 230 V — if the switch were in the neutral, the appliance would still be live at full mains p.d. even though it is off. A fuse is a thin wire that melts when the current exceeds its rating, cutting the live supply. Modern consumer units use circuit breakers, which trip mechanically and can simply be reset. Earthing completes the protection: if a fault makes the live wire touch a metal case, the case would sit at 230 V and shock anyone who touched it — but the earth wire gives the current a low-resistance escape route, the surge blows the fuse, and the dead appliance announces its fault safely.

• Double-insulated appliances (marked with a square within a square) have plastic cases that cannot become live, so they need no earth wire — just live and neutral.
• Choosing a fuse: work out the appliance’s current with I = P ÷ V, then pick the next fuse rating above it (common ratings: 3 A, 5 A, 13 A).
• Risks of handling mains: touching the live wire (especially with damp skin, which lowers resistance) lets current flow through your body to earth — potentially fatal.

Every appliance transfers energy from the electrical store of the grid into what it does — heat, motion, light, sound. Power is the rate: P = V I (from the mains values) or P = I² R (from the component’s current and resistance). Energy is power × time: E = P t. For domestic quantities the kilowatt-hour is the practical unit — 1 kWh = 3.6 MJ — and bills simply count kWh × price per kWh. A 1.8 kW dishwasher running 4 hours uses 1.8 × 4 = 7.2 kWh, costing 7.2 × 28 p ≈ £2.02 at 28 p/kWh.

The National Grid links every power station to every consumer through a web of cables and transformers. The physics problem: cables waste power as heat at P = I² R — and because the current is squared, a current ten times smaller loses a hundred times less power. So the grid steps the voltage up (to 400 000 V on the biggest lines) using step-up transformers, which lets the same power flow with a much smaller current. Near homes, step-down transformers and local substations bring it back to 230 V for safety. Direct transmission at 230 V across the country would need enormous currents and melt the cables; the high-voltage trick is what makes a national grid affordable.

Transformer calculations — using the ratio of turns on the primary and secondary coils — are Higher tier and are treated fully in chapter 17 (magnetism and electromagnetism); here you need the roles: step-up at the power station end, step-down at the consumer end.

Rubbing an insulator — a balloon on hair, a duster on a polythene rod — transfers electrons between the surfaces. The material that gains electrons becomes negative; the one that loses them becomes positive. Both are left charged because insulators do not let the charge flow away. The rules of force: like charges repel, unlike charges attract. Every charged object is wrapped in an electric field: the region where another charge feels a force. Field lines point away from positive charge and towards negative charge, and the field — and the force — weakens with distance. When the field near a charged object gets strong enough, it tears electrons off air molecules and a spark carries the charge to earth — that is what lightning is, at scale.

The exam favourite is why earthing prevents danger: an earthed object cannot build up charge, because any charge placed on it immediately flows to earth — so no field grows strong enough to spark.