S3 Physics | Household Electricity, Power and Bills: A Tutor Prep Guide (Tripping, Wiring Safety, kWh Cost)

2026-11-06

Tutor lesson-prep guides · Secondary Physics

Students can recite P equals V times I, but ask why a heater and a microwave trip the breaker together and they answer too much electricity. They have never added the powers and divided by 220.

Objectives — Students can: (1) use P equals V times I to find an appliance current and decide whether a 13 A socket circuit will trip; (2) convert kW to kWh with hours and multiply by a tariff to get a cost; (3) explain the job of each of the three wires and why the fuse and the switch sit on the live wire.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Electrical power	How are P, V and I related	P equals V times I
Units	How many watts in one kilowatt	One thousand
Parallel circuits	How sockets are connected to each other	In parallel, each gets 220 V

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	A tripping incident	Describe a heater and microwave tripping the breaker at home	Guess three possible causes
8-20 min	Three wires	Label live, neutral and earth on a plug diagram	Mark colour and job on the diagram
20-32 min	Add the powers	2000 W plus 1000 W gives 3000 W, over 220 V gives 13.6 A	Repeat it and state why it trips
32-44 min	Trip or not	1500 W plus 1200 W gives 12.3 A, so no trip	Decide three combinations
44-56 min	kWh and cost	2000 W for 3 hours is 6 kWh, times the tariff gives 7.2	Cost two appliances and compare
56-60 min	Exit check	Collect three questions and mark at once	Work independently

03 Teaching script (read it out)

Scene 1: Three wires, three jobs

The three wires in a plug each do one thing: live (brown) carries the voltage in, neutral (blue) carries the current back, earth (green and yellow) only carries current to the ground during a fault, so in normal use it carries none.
That is why the switch and the fuse go on the live wire: wired into the neutral instead, the appliance stays live inside even when switched off, and changing a bulb can still shock you.
What students must answer is why not the neutral, not the colours.

Ask students to repeat: the switch sits on the live wire so the inside is dead when it is off.

Scene 2: Add the powers, then divide

Deciding whether a circuit trips takes three steps: add the powers of the appliances running on the same circuit, divide by 220 V to get the total current, compare with the fuse rating.
A 2000 W heater plus a 1000 W microwave is 3000 W, which over 220 V is 13.6 A, above 13 A, so the fuse blows.
Stress that powers are added, not currents guessed: at the same voltage the two are equivalent, but adding powers first keeps the written steps defensible.

Ask students to do: find the total current for 1500 W plus 1200 W and decide the outcome.

Scene 3: kWh is energy, not power

A bill charges for energy: kWh equals kW times hours. 2000 W is 2 kW, so three hours is 6 kWh, and at an assumed 1.2 per unit the cost is 7.2.
The usual mistake is treating kW and kWh as one thing, then multiplying 2000 straight into the tariff. Have students write power in kW and energy in kWh beside every number.
Add the consequence: a longer use raises kWh but leaves the power unchanged.

Ask students to explain: why a brighter bulb is not always the more expensive one.

04 Question types and worked steps

Type	Example	Steps	Answer
Current from power	A 2000 W appliance at 220 V	Divide power by voltage, giving 9.09 A	State the unit and keep two decimals
Trip decision	Two appliances running together	Add powers, divide by 220 V, compare with 13 A	13.6 A is above 13 A, so it trips
Cost calculation	2000 W for 3 hours	Convert to kW, multiply hours, multiply tariff	6 kWh, cost 7.2
Wiring safety	Which wire takes the fuse	Reason from being dead when switched off	Live, the same wire as the switch

Sanity check (teach this): Sensible-answer check: a current above the fuse rating trips it, so an answer of 0.9 A or 90 A means the units slipped; kWh must exceed kW unless the time is under an hour; a cost of tens of units for a 2000 W appliance over three hours means a decimal error.

05 Independent practice (3 questions)

Question	Answer	What to watch
One appliance	An 800 W kettle at 220 V	Expect 3.64 A; check the division by voltage
Two together	1500 W plus 1200 W, does it trip	12.3 A is below 13 A, so no trip
Cost comparison	1500 W for 2.5 hours	3.75 kWh, then multiply the tariff

06 Common mistakes and what to say

Mistake	Student writes	Say this
Mixing kW and kWh	2000 W for 3 hours is 6000 kWh	2000 W is 2 kW, and you multiply by hours, not by 2000. The answer is 6 kWh.
Fuse on the neutral	It cuts the circuit either way	Switch off and the live is cut, but the appliance still sits on the neutral. Why is that dangerous? The fuse belongs with the switch on the live wire.
Adding currents blindly	9 A plus 4 A is 13 A, just fits	Adding is only valid at the same 220 V; write the power sum first so the steps hold.
Earth carries current	The earth wire carries current all the time	In normal use it carries none; it only takes current away during a fault, which is what separates it from the neutral.

07 Homework (paper, section and question numbers)

Secondary Physics chapter index — Every chapter in the subject
DSE Physics paper — Electricity questions with answers
Redo this chapter — Work out three appliance combinations

08 Exit check (last 5 minutes)

Question	Answer	Decision
Three wires	Live in, neutral back, earth only on a fault	Reversed means replay part two
Trip decision	Add powers, divide by 220 V, compare with 13 A	Comparing powers alone means replay part three
kWh	kW times hours, an energy	Answering in kW means replay part three

09 Tutor FAQ

Is this enough before full DSE circuits?

Yes: this chapter needs only P equals V times I and parallel ideas, the rest comes in S4.

Should real tariffs be used?

Use an assumed value; the marks are for steps and units, and real tariffs change yearly.

Do we cover first aid for shocks?

One line only: switch off the mains first, then call for help; detailed safety belongs to the school lab.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.