SEC G3 Combined Science Physics component K326/K327
CP14: Practical Electricity
Explain mains wiring, safety devices, power ratings, and energy use.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Practical electricity joins useful heating and energy calculations with the wiring decisions that prevent electric shock, cable overheating, and fire.
Heating, power, energy, and cost
Current through a resistive element transfers electrical energy into the element's internal store, producing heating in kettles, ovens, and heaters.
Power equals potential difference times current, and energy equals power times time. Electricity bills commonly use kilowatt-hours, so convert watts to kilowatts and time to hours before multiplying by the tariff.
Hazards and protective devices
Damaged insulation can expose a live conductor, excessive current can overheat a cable, and damp conditions can create an unsafe conducting path through a person.
A fuse melts and a circuit breaker opens when current becomes unsafe. Choose a fuse rating just above the appliance's normal operating current but below a current that could overheat its cable. Earthing gives fault current a low-resistance route from a metal case. Double insulation provides two independent insulating barriers between live parts and an accessible non-conducting case, so one insulation failure does not expose the user and an earth wire is not required.
Mains conductors and plug wiring
The live conductor alternates in potential relative to neutral, the neutral conductor completes the normal circuit near zero potential, and the earth conductor is a safety connection for exposed metal. In a standard plug viewed from the wiring side with the cable entering below, earth connects to the top terminal, neutral to the left, and live through the fuse on the right.
Fit the switch, fuse, and circuit breaker in the live conductor so opening or melting them disconnects the appliance from the live supply. Cutting only the neutral conductor can leave internal parts live.
Formulae and relationships
| Relationship | Use |
|---|---|
| Find electrical power. | |
| Find electrical energy. | |
| Find appliance running cost. |
Worked examples
Example 1: A heater runs for at per kilowatt-hour. Find the cost.
- Energy used .
- Multiply by the tariff.
- Cost .
Answer: .
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Relate appliance power, operating time, electrical energy, and cost using consistent units.
- Explain live, neutral, and earth wiring by function rather than colour alone.
- Match fuses, circuit breakers, earthing, and double insulation to the fault they address.
Official outcome coverage
K326 CP14: 9 mapped outcomes, references CP14(a), CP14(b), CP14(c), CP14(d), CP14(e), CP14(f), CP14(g), CP14(h), CP14(i). Check the official K326 syllabus.
K327 CP14: 9 mapped outcomes, references CP14(a), CP14(b), CP14(c), CP14(d), CP14(e), CP14(f), CP14(g), CP14(h), CP14(i). Check the official K327 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Use appliance ratings and plug diagrams to make a safe energy-use decision without handling live mains equipment. Identify live, neutral and earth functions, place the fuse and switch in the live wire, and justify fuse rating, earthing or double insulation from the fault each controls.
Exam traps and retrieval check
Avoid these traps
- Using seconds with a kilowatt-hour tariff.
- Putting the fuse in the neutral conductor.
- Treating earth as a normal current-carrying return path.
Check from memory
Where does the fuse connect in a plug?
In the live conductor.
Why earth a metal case?
To provide a low-resistance fault path that operates protection.
What energy unit appears on bills?
The kilowatt-hour.
Official Combined Science scope
This shared Combined Physics owner serves both K326 and K327. Electrical heating, power, energy and cost, hazards, protection, earthing, insulation, and mains-plug wiring.