Practical electricity
Study G2 Science appliance ratings, power, energy costs and household protection through lessons on electrical safety and mains wiring.
Offline pages
Saved on this device only. Connect at least once every 30 days to confirm Premium. Sign-out or a change of account removes saved copies.
Before you begin
Electricity at home heats water, cooks food and runs every appliance, but it costs money and a fault can kill. The lessons start with the heating effect, electrical power and energy, and the cost of running appliances. They then follow the live, neutral and earth wires into a mains plug and explain how fuses, circuit breakers, earthing and double insulation protect you.
Be comfortable with: power as the rate of energy transfer, and D.C. circuits, because P = VI uses the current through and p.d. across the same appliance.
Learning goals
- Explain electrical heating in common appliances
- Apply P = VI
- Apply E = VIt
- Calculate electrical energy and cost in kWh
- State the meanings and roles of live, neutral and earth
- Describe the wiring of a mains plug
- 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 live-wire placement of switches, fuses and circuit breakers
- Explain why metal casings are earthed
- Explain why double-insulated appliances do not need an earth wire
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.
- Electrical power, energy and costExplain electrical heating, use P = VI and E = VIt, and calculate energy in kWh and the cost of running appliances.
- Live, neutral and mains plugsIdentify the mains conductors and explain the fuse, terminals and cable grip in a plug diagram.
- Electrical hazards and circuit protectionExplain household electrical hazards, choose from stated fuse ratings and show why switches and protection belong in live.
- Earthing and double insulationTrace an earth-fault circuit and compare earthing with double insulation.
Practise and check
Or choose
Topic reference
Quick reference
| Quantity | Relationship | Unit |
|---|---|---|
| electrical power | P = VI | watt, W |
| electrical energy | E = VIt | joule, J |
| energy for billing | energy in kW h = power in kW × time in h | kilowatt-hour, kW h |
| cost | cost = energy in kW h × price per kW h |
| Wire | Colour | Job |
|---|---|---|
| live | brown | carries the current to the appliance at a high voltage |
| neutral | blue | completes the circuit back to the supply |
| earth | green and yellow | a safety wire that carries current only when there is a fault |
Core knowledge to remember
- The heating effect of a current is used in electric kettles, ovens and heaters.
- Hazards of electricity include damaged insulation, overheating of cables and damp conditions.
- A fuse or circuit breaker cuts off the current if it becomes too large. The fuse rating should be just above the normal working current.
- Earthing the metal casing of an appliance lets a fault current flow safely to earth and blow the fuse. A double-insulated appliance has no exposed metal that could become live.
- Switches, fuses and circuit breakers are fitted in the live wire, so that the appliance is disconnected from the high voltage when they open.
Common mistakes
- Forgetting to change watts to kilowatts or minutes to hours when calculating energy in kW h.
- Choosing a fuse rating far above the normal current, or below it.
- Saying the earth wire carries current all the time. It carries current only when there is a fault.
- Fitting a switch or fuse in the neutral wire, which leaves the appliance connected to the live supply when it is off.