Sec 4 Physics Practical Electricity notes

Drifting electrons collide constantly with the vibrating ions of a conductor, handing over energy: the wire warms up. That is the heating effect of current, and its size is fixed by two equations: P = VI (volts × amps = watts) and E = VIt (energy = volts × amps × seconds — equivalently E = Pt). A 230 V, 9.0 A kettle converts P = 230 × 9.0 ≈ 2070 W — a two-kilowatt water boiler, on purpose.

Appliances exploit the effect deliberately: kettles, irons, toasters, rice cookers and hair dryers all drive current through high-resistance elements to make heat. Elsewhere the same effect is a pure loss: transmission cables, motor windings and transformers warm up for nothing, which is why thick low-resistance copper and cooling fins exist. Design rule: want heat? Choose a high-resistance element sized for the current; want no heat? Keep resistance low.

Joules are awkward for household quantities, so electricity is billed in kilowatt-hours: the energy of 1 kW running for 1 hour. In joules, 1 kW h = 1000 W × 3600 s = 3.6 × 10⁶ J. Running cost = energy in kW h × tariff per kW h. A 2.0 kW air-con for 3 hours uses 6.0 kW h; at 30 cents per kW h that is $1.80.

The method: convert the appliance power to kilowatts (2000 W → 2.0 kW) and the time to hours (90 min → 1.5 h), multiply for kW h, multiply by the tariff, done. The home meter counts exactly this — every appliance you switch on is a small physics problem with a price tag.

Singapore mains runs three wires. The live wire (brown insulation) carries the supply at about 230 V relative to ground — touch it and you complete a circuit through your body. The neutral wire (blue) completes the circuit back to the supply and sits near 0 V. The earth wire (green-and-yellow stripes) connects metal casings to the ground — it normally carries no current at all; it is a safety escape route, not part of the working circuit.

In a mains plug: live → the fuse terminal, neutral → its own terminal, earth → the top terminal, with the earth wire made slightly longer so that if the cable is yanked, the earth breaks last. The fuse sits in the live wire — and so does the switch — because a broken live line de-energises the appliance: the plug side of the switch is dead at 0 V. A switch on the neutral side would leave the whole lamp live and waiting to shock whoever changes a bulb.

A fuse is a thin wire that melts when the current exceeds its rating, opening the circuit. The rating is chosen just above the appliance’s normal current: a 2300 W kettle on 230 V draws 10 A, so a 13 A fuse is right — a 5 A fuse would blow every morning, a 30 A fuse would never bother to protect anything. A circuit breaker does the same job electromagnetically or thermally: it trips faster and resets with a flick, no spares needed.

Earthing protects against live-to-case faults: the earth wire gives the fault current a low-resistance path, so a huge current surges through earth (not through the person touching the kettle) and blows the fuse in milliseconds. No earth, no escape route: the casing sits at 230 V waiting for a hand. Appliances with plastic (insulating) cases skip the earth wire via double insulation — two independent layers of insulation and no exposed metal — marked by the double-square symbol on the rating plate.

Three classic hazards. Damaged insulation: a frayed flex lets live metal touch anything nearby — shocks and fires come from cables run under carpets or chewed by pets; replace, never tape-and-forget. Overheating cables: too many high-power appliances on one extension lead force more current than the lead’s rating; the cable warms by the heating effect until insulation melts — the fire failure mode, and the reason high-power appliances get wall sockets, not multi-way adaptors. Damp conditions: water lowers resistance and bridges contacts; a hair dryer with wet hands or over a full basin turns the body into the return path.

Each hazard maps to a rule you already know: damaged insulation → the live wire escapes its cage (fuse cannot see a human-scale current, so prevention beats protection); overloaded cables → heating effect scaled up (P = VI burns); damp → your body joins the circuit at low resistance (I = V ÷ R with R collapsed). Explain the hazard by name and the physics earns the marks.