Practical electricity
Energy, power, electricity costs, safety devices and home wiring for G3 Pure and O-Level Physics.
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Before you begin
This topic is about the electricity you use every day: how appliances turn it into heat, how to work out what they cost to run, and how the wiring in a home keeps you safe from shocks and fires.
Be comfortable with: current, p.d. and resistance from current electricity, and series and parallel circuits from D.C. circuits.
Learning goals
- Explain electrical heating in common appliances
- Apply P = VI
- Apply E = VIt
- Calculate electrical energy and cost in kWh
- Identify the hazard from damaged insulation
- Identify the hazard from overheating cables
- Identify the hazard from damp conditions
- Explain how fuses and circuit breakers protect circuits
- Choose and justify an appropriate fuse rating
- Explain why metal casings are earthed
- Explain why double-insulated appliances do not need an earth wire
- State the meanings and roles of live, neutral and earth
- Describe the wiring of a mains plug
- Explain live-wire placement of switches, fuses and circuit breakers
Syllabus statements covered
- describe the use of the heating effect of electricity in appliances such as electric kettles, ovens and heaters
- recall and apply the relationships P = V I and E = V I t to new situations or to solve related problems
- calculate the cost of using electrical appliances where the energy unit is the kW h
- state the hazards of using electricity in the following situations: (i) damaged insulation (ii) overheating of cables (iii) damp conditions
- explain the use of fuses and circuit breakers in electrical circuits and of fuse ratings
- explain the need for earthing metal casings and for double insulation
- state the meaning of the terms live, neutral and earth
- describe the wiring in a mains plug
- explain why switches, fuses, and circuit breakers are fitted to the live wire.
Lessons
Work through them in order.
Energy & cost
Safety devices & hazards
- Electrical hazardsExplain the shock and fire risks from damaged insulation, overheated cables and damp conditions.
- Fuses and circuit breakersExplain how fuses and circuit breakers cut off excessive current, and choose a suitable fuse rating.
- Earthing and double insulationExplain how earthing a metal casing and double insulation protect users from electric shock.
Wiring details
- Live, neutral and earth in the homeWhat the live, neutral and earth wires do, their colours, and how the supply reaches each circuit in a home.
- Switches and live-wire isolationWhy a light switch, like a fuse or circuit breaker, must be in the live wire to make the appliance safe when it is off.
- Understanding a three-pin mains plugWhere each wire goes in a three-pin plug, why the fuse is on the live wire, and what the cable grip does.
Practise and check
Or choose
Topic reference
Quick reference
| Idea | What to remember |
|---|---|
| Heating effect | current in a heating element raises its internal energy; it heats the water, food or room |
| Power | P = VI, in watts |
| Energy | E = VIt, in joules, with t in seconds |
| Kilowatt-hour | energy (kWh) = power (kW) × time (h); 1 kWh = 3.6 × 10⁶ J |
| Cost | energy in kWh × price of one kWh |
| Live, neutral, earth | brown, at 230 V; blue, about 0 V, completes the circuit; green and yellow, safety |
| Hazards | damaged insulation, overheating cables, damp conditions |
| Fuse rating | the smallest rating above the normal current, I = P/V |
| In the live wire | switches, fuses and circuit breakers |
| Earthing | fault → large current in the earth wire → fuse melts |
| Double insulation | no part you can touch can become live; no earth wire needed |
Core knowledge to remember
- Kettles, ovens and heaters use the heating effect of a current in a resistance wire.
- P = VI and E = VIt; the cost of electricity is found from the energy in kWh.
- Damaged insulation, overheating of cables and damp conditions can each cause an electric shock or a fire.
- A fuse or circuit breaker cuts off a current that is too large; a fuse’s rating should be just above the normal current.
- Metal casings are earthed so that a fault blows the fuse; double-insulated appliances need no earth wire.
- Live is at a high voltage, neutral completes the circuit at about 0 V, and earth is a safety wire.
- In a plug, brown goes to L through the fuse, blue to N, green and yellow to E, and the cable grip holds the outer insulation.
- Switches, fuses and circuit breakers are fitted in the live wire, so that opening them cuts the appliance off from the high voltage.
Common mistakes
- Leaving time in hours in E = VIt, or calling the kWh a unit of power.
- Saying cables overheat because the voltage is too high. The current is too large.
- Choosing a fuse rating far above the normal current “so it won’t blow”.
- Saying the earth wire carries the current in normal use. The neutral wire does.
- Putting a switch or fuse in the neutral wire, which leaves the appliance live when it is off.