Practical Electricity is the final topic in the electricity block of the O-Level Science (Physics) syllabus. It takes current, potential difference and resistance from Topic 13 and applies them at home: the power an appliance draws, the energy it converts, the cost on the electricity bill, and the safety features built into every plug, kettle and distribution board.
The topic appears every year, usually as one or two Paper 1 multiple-choice questions and a structured Paper 2 question worth around 5 to 8 marks. The questions are predictable and the mark scheme rewards precise wording. If you can calculate power, energy and cost, choose a sensible fuse rating and explain each safety feature in exam-standard sentences, you have banked essentially all the marks this topic offers.
What these notes cover
- •The heating effect of electricity
- •Heating elements and nichrome
- •Electrical power and energy
- •Reading a rating plate
- •The kilowatt-hour and the cost of electricity
- •Electrical hazards: damaged insulation, overheating, damp conditions
- •Fuses and choosing a fuse rating
- •Circuit breakers
- •Earthing and double insulation
- •The three-pin plug: live, neutral and earth
- •Common mistakes and exam technique
Key definitions
- Electrical power
- The rate at which electrical energy is converted to other forms of energy (thermal energy, light, kinetic energy) by a component. It is measured in watts, where 1 W = 1 J/s.
- Kilowatt-hour (kW h)
- The energy used by a 1 kW appliance running for 1 hour. 1 kW h = 1000 W × 3600 s = 3.6 × 10⁶ J = 3.6 MJ. It is a unit of energy, not power.
- Fuse
- A deliberately thin wire sealed in a cartridge, connected in series with the appliance and always in the live wire. If the current rises above its rated value, the wire melts and breaks the circuit.
- Circuit breaker
- An electromagnetic switch that automatically trips when the current through it exceeds a set value, breaking the circuit. Unlike a fuse, it can be reset by flipping its switch.
- Double insulation
- Two separate layers of insulation between the user and any current-carrying part, typically insulated wires inside an insulating casing such as plastic. There is no exposed metal that could become live, so no earth wire is needed.
Key formulas
Heating power in a resistance
P = I²R
P is the thermal power in W, I is the current in A and R is the resistance in Ω. Doubling the current through the same resistance quadruples the heating.
Power and energy
P = E ÷ t, so E = Pt
E is the electrical energy converted in J, t is the time in s and P is the power in W.
Electrical power
P = VI
V is the potential difference in V and I is the current in A. This gives P in W.
Electrical energy
E = VIt
With V in volts, I in amperes and t in seconds, E comes out in joules.
Operating current (for fuse selection)
I = P ÷ V
Use this to find the current from a rating plate such as 230 V, 2200 W. The fuse rating is the next standard rating above this value.
Cost of electricity
Cost = power in kW × time in h × tariff per kW h
Convert first: divide watts by 1000 to get kW and divide minutes by 60 to get hours. Keep the tariff consistent in cents or dollars.
Worked example
An air-conditioner rated at 2.8 kW runs for 7.0 hours every night for a month of 30 nights. The electricity tariff is 29.5 cents per kW h. Calculate the cost for the month, in dollars to 2 decimal places.
- 1Check the units: the power is already in kW (2.8 kW), so no conversion is needed, and the total time is t = 7.0 h/night × 30 nights = 210 h.
- 2Find the energy in kilowatt-hours: E = 2.8 kW × 210 h = 588 kW h.
- 3Convert the tariff to dollars per kW h to avoid a cents/dollars slip: 29.5 cents = $0.295.
- 4Find the cost: Cost = 588 × 0.295 = $173.46, so the month of air-conditioning costs $173.46.
Watch out: Do not mix the two unit systems, e.g. by multiplying watts by hours. This makes the answer wrong by a factor of 1000 or 3600. If a monthly bill comes out as $200 000, a unit was left unconverted.
Common mistakes
Treating the kilowatt-hour as a unit of power
Wrong: The kilowatt-hour measures power, and it can be written as kW/h.
Right: The kilowatt-hour is a unit of energy: the energy converted when a 1 kW appliance runs for 1 hour, equal to 3.6 × 10⁶ J. It is kilowatt times hour, not kilowatts per hour.
Picking any fuse rating bigger than the current
Wrong: Choosing a fuse rated below the operating current, or one far above it to be safe.
Right: Calculate I = P ÷ V, then choose the next standard rating just above it. For example, a kettle drawing 9.6 A takes a 13 A fuse, not a 5 A or a 30 A one.
Giving only half of the earthing explanation
Wrong: The current flows to earth, or the earth wire absorbs the electricity.
Right: The earth wire provides a low-resistance path, so a large current flows from the live casing to the ground. This large current blows the fuse, which disconnects the appliance from the live supply.
Questions students ask
Why must the fuse and switch be in the live wire?
When the switch, fuse or circuit breaker is in the live wire, opening it disconnects the appliance from the 230 V supply. If it were in the neutral wire, the appliance would stay connected to the live wire at 230 V even when switched off. A person touching a faulty appliance could then still receive a fatal electric shock.
How do you choose the correct fuse rating for an appliance?
Work out the normal operating current with I = P ÷ V, using V = 230 V, then choose the next standard rating just above it (3 A, 5 A or 13 A). For example, a 2.4 kW kettle draws 2400 ÷ 230 = 10.4 A, so it takes a 13 A fuse. A rating below the operating current blows in normal use, and one far above it gives no protection.
How does the earth wire protect you from electric shock?
The green-and-yellow earth wire connects the metal casing to the ground and normally carries no current. If the live wire touches the casing, the earth wire provides a low-resistance path, so a large current flows to earth instead of through a person. This large current blows the fuse or trips the breaker, which disconnects the appliance so the casing returns to 0 V.
Why is nichrome used for heating elements?
Nichrome has a high resistivity, so even a short wire has a large resistance and heats up readily. It also has a high melting point, so it can run at very high temperatures without melting. It does not oxidise easily when red-hot, so the element does not burn away or become brittle.
In the full PDF
- •Labelled cable-route diagram with the worked example on heat dissipated along a long, thin supply cable
- •Further worked examples: kettle rating plate, overloaded extension socket, wet-hands shock current, and fuse choice for a kettle and a blender
- •Full cause → consequence chains for damaged insulation, overheating of cables and damp conditions
- •Wire-colour table, step-by-step three-pin plug wiring and model answers to 3-mark exam questions
- •The remaining misconceptions and the complete exam-technique checklist
- •End-of-topic summary of everything you need to know without notes