P3.4 Home electricity · KS3 Science

Mains electricity and plugs

Mains electricity in the UK is alternating current (a.c.): the current keeps changing direction. It is 230 V at 50 Hz. Batteries give direct current (d.c.), which flows in one direction.
A three-pin plug has a live wire (brown), which carries the 230 V; a neutral wire (blue), which completes the circuit; and an earth wire (green and yellow stripes), a safety wire that stops the case becoming live if there is a fault.
The fuse is in the live wire. If the current gets too big, the fuse melts and breaks the circuit. Appliances with plastic cases are double insulated and don't need an earth wire.

Power is the rate of energy transfer, measured in watts (W). 1 kW = 1000 W.
Power = energy ÷ time (P = E ÷ t), and power = potential difference × current (P = V × I). A kettle that transfers 180 000 J in 60 s has a power of 3000 W.
Electricity companies charge for energy in kilowatt-hours: energy (kWh) = power (kW) × time (hours).
A 2 kW heater used for 3 hours uses 6 kWh. At 30p per kWh, that costs 6 × 30 = 180p (£1.80).

Non-renewable resources will run out: coal, oil, natural gas and nuclear fuel. Burning fossil fuels releases carbon dioxide, which causes global warming.
Renewable resources won't run out: solar, wind, hydroelectric, tidal, wave, geothermal and biofuels. Some depend on the weather, so they aren't always available.
The National Grid connects power stations to homes. Step-up transformers raise the voltage (to hundreds of thousands of volts), which lowers the current, so less energy is wasted as heat in the cables. Step-down transformers lower it to 230 V for homes.

Rubbing two insulators together transfers electrons from one to the other. The one that gains electrons becomes negatively charged; the one that loses them becomes positively charged.
Like charges repel; unlike charges attract.
Static can cause sparks, which is dangerous when refuelling aircraft, so they are earthed. It is useful in photocopiers and paint sprayers.

Alternating current: Current that keeps changing direction (a.c.).
Direct current: Current that flows in one direction only (d.c.).
Live wire: The brown wire, which carries the alternating potential difference.
Neutral wire: The blue wire, which completes the circuit.
Earth wire: The green and yellow safety wire.
Fuse: A thin wire that melts and breaks the circuit if the current is too high.
Power: The rate of energy transfer, measured in watts (W).
Kilowatt-hour: The energy used by a 1 kW appliance in one hour.
Renewable resource: An energy resource that won't run out, such as wind.
Non-renewable resource: An energy resource that will run out, such as coal.
National Grid: The network of cables and transformers that carries electricity across the country.
Transformer: A device that changes the voltage of an alternating current.
Static electricity: A build-up of electric charge on an insulator.