Sec 4 Combined Science (Physics) Practical Electricity notes

Push a current through a high-resistance wire and it warms up — energy transferred from the electrical supply to thermal stores. Put that element inside a metal kettle, an oven cavity, a toaster slot or a convector panel and you have the four workhorses of the kitchen: kettles, ovens, toasters, heaters, plus irons, hair dryers and electric blankets. The same effect in a filament lamp makes it glow — and explains why it wastes most of its energy as heat.

The heating effect doubles as the main hazard of electricity: wherever current flows uninvited — through a damaged cable, a damp surface, an overloaded extension — the same law warms things you never wanted warm.

Power is the rate of energy transfer: P = VI — supply p.d. times current. A 230 V appliance drawing 5 A transfers 1150 W. Energy is power × time: E = VIt = Pt, in joules with time in seconds. Since joules are tiny on mains scales, bills use the kilowatt-hour (kW h) — the energy a 1 kW appliance transfers in 1 hour: 1 kW h = 3.6 MJ, one "unit" on the meter. Cost = kW h × price per unit.

Damaged insulation: the plastic sheath is all that separates a live conductor from your hand; frayed or cracked insulation exposes live metal — touching it drives current through the body. Replace damaged cables, never tape them and forget. Overheating cables: push too much current through a cable (overloaded extension sockets are the classic) and the wiring warms; melted insulation then invites short circuits and fire — match fuses and extensions to the load. Damp conditions: pure water hardly conducts, but the water around your home is full of ions — wet skin and wet floors conduct readily, which is why damp bathrooms and live appliances are a lethal mix; dry hands, dry floors, no mains appliances near baths.

A fuse is a deliberate weak link: a thin wire that melts and breaks the circuit once the current exceeds its rating. Ratings are chosen just above the appliance’s normal current — a 10 A appliance takes a 13 A fuse, not a 30 A one — so it survives normal surges but blows promptly on a fault. A circuit breaker does the same job electromagnetically or thermally: it trips fast and can be reset with a switch, which is why modern consumer units use breakers rather than rewireable fuses.

Earthing protects the user of a metal-cased appliance: the case is connected by the earth wire to the ground. If a live wire breaks loose and touches the case, the earth path lets a huge current flow — instantly blowing the fuse and disconnecting the live supply. Without the earth, the case would sit at mains potential, waiting for a hand. Double insulation removes the need for an earth: appliances with plastic casings (hair dryers, drills marked ⧈) cannot put live parts where the user can touch them, even inside — the plastic is an insulator twice over.

Three wires run to every mains appliance. The live wire (brown) carries the high p.d. from the supply — the dangerous one. The neutral (blue) completes the loop back to the supply, sitting near earth potential. The earth (green and yellow stripes) connects the metal case to the ground and carries no current in normal use. In the three-pin plug: the fuse lives in the plug, in the live pin; the wires are secured by their colour; and no bare copper should peek from under the cord grip.

Why switches and fuses on the live wire? Because opening the live removes the high p.d. from the entire appliance — "off" really means off. Break a circuit in the neutral instead and, though the appliance stops working, the live wire and heating elements remain at mains potential: one slipped finger across a "dead" toaster that is still live ends the lesson.