Mains electricity and power
Physics · GCSE · The School
The three wires
LIVE carries the alternating potential from the supply and is the dangerous one, at about 230 V in the UK. NEUTRAL completes the circuit and sits near zero volts. EARTH carries no current in normal use; it is a safety path so that if the live wire touches a metal case, a large current flows to earth and blows the fuse before anyone touches it. Earth does nothing until something has already gone wrong, which is the point of it.
Power and energy
Power is energy transferred per second, measured in watts: P = IV, and with V = IR you also get P = I²R. Energy is power multiplied by time: E = Pt in joules when time is in seconds. The commonest lost mark is leaving time in minutes — 3 minutes is 180 seconds, and writing that conversion on its own line costs nothing and saves the answer.
Choosing a fuse
A fuse is a deliberately weak link: too much current and its wire melts, breaking the circuit. Work out the normal current with I = P ÷ V, then choose the next fuse ABOVE it from those available — usually 3 A, 5 A or 13 A. Choosing one below means it blows in normal use; choosing far above means it will not blow when it should, which is the dangerous error.
A kettle is rated 2,300 W on a 230 V mains supply. CURRENT: I = P ÷ V = 2,300 ÷ 230 = 10 A. FUSE: 10 A is the normal current, so a 3 A or 5 A fuse would blow every time it boiled — choose 13 A, the next one above. ENERGY for a three-minute boil: convert first, 3 minutes = 180 s, then E = Pt = 2,300 × 180 = 414,000 J, which is 414 kJ. Notice the pattern in all three: write the equation, substitute with units matched, then evaluate. And notice the fuse answer needed the current answer first, which is why exam questions on this topic run in parts — each part hands you the number for the next.
Why some plugs have no earth pin
A DOUBLE-INSULATED appliance — hairdryer, phone charger, most power tools — has a plastic casing that can never become live, so there is no metal case for a fault to reach and no earth wire is fitted. It is marked with a small square inside a square. Wiring an earth into one anyway is not extra safety, it is a design that has removed the hazard the earth wire exists to catch.
Why the National Grid steps voltage up
Power lost in a cable is current squared times resistance, so halving the current quarters the loss. Transformers raise voltage and drop current for transmission, then reverse it for use. The reasoning is the answer — not the transformer, which is only the mechanism.