S3 Science — Circuits and Resistance: Ohm Law, Series and Parallel Lesson Prep Guide

2026-11-15

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Students can recite voltage equals current times resistance, then add resistances in parallel and treat kWh as kW. This lesson uses voltage sharing and branch currents to keep every answer checkable.

Objectives — Students can: (1) use Ohm law to find current, voltage or resistance with correct units; (2) tell series from parallel and find the equivalent resistance by adding or by reciprocal sum, then check with branch currents; (3) compute power as voltage times current and convert it with time into kWh and cost.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Current and voltage	What are the units of current and of voltage?	Ampere for current, volt for voltage
A simple circuit	One cell and one bulb: how many paths does the current take?	One, so it is a series circuit
Decimals and units	How many watt-hours is 0.18 kWh?	180 Wh, multiply by 1,000

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Two bulbs compared	Show one series and one parallel circuit; ask what happens if one bulb is removed	Say series goes dark, parallel keeps the other lit
5-16 min	Ohm law	Show 12 V and 4 ohm giving 3 A, then ask what resistance gives 2 A	Name the three rearrangements
16-29 min	Equivalent resistance	Work 4 ohm with 6 ohm in series and 4 ohm with 12 ohm in parallel side by side	State that parallel is smaller than the smallest resistor
29-37 min	Power and cost	Show 12 V times 3 A is 36 W, then 5 hours gives 180 Wh and 0.18 kWh	Compute the cost themselves
37-42 min	Independent practice	Three questions on resistance, branch currents and cost; tour asking for the equation	Write the equation before substituting
42-45 min	Exit check	Collect answers and ask whether parallel raises total resistance	Answer no, it always falls

03 Teaching script (read it out)

Scene 1: Ohm law: one equation, three questions

Ohm law is voltage equals current times resistance. For current divide voltage by resistance, for voltage multiply, for resistance divide voltage by current. 12 V across 4 ohm gives 3 A. To get only 2 A you need 12 divided by 2, which is 6 ohm. Stress that resistance belongs to the component: raising the voltage does not change it, only the current changes. That is the sentence students get wrong most often.

Students name the operation for each of the three questions

Scene 2: Series and parallel: voltage shares, current branches

Series has one path, so the current is the same everywhere and the voltage divides in proportion to resistance. 4 ohm and 6 ohm in series make 10 ohm; at 12 V the current is 1.2 A, and the voltages are 4.8 V and 7.2 V, adding to 12 V, which is the check students must do themselves. Parallel has several paths, so the voltage is the same and the current splits. 4 ohm and 12 ohm in parallel give 3 ohm by reciprocal sum, total current 4 A, branches 3 A and 1 A. Parallel equivalent resistance is always smaller than the smallest resistor, which is an instant reasonableness check.

Students explain why parallel resistance is below the smallest resistor

Scene 3: Power and cost: from watts to kilowatt-hours

Power is voltage times current: 12 V times 3 A is 36 W, meaning 36 joules every second. Energy used is power times time, so 36 W for 5 hours is 180 Wh, which is 0.18 kWh after dividing by 1,000. The bill is kWh times the tariff, so 0.18 times 1.2 is 0.216. Students mix up W and kWh; ask which one contains time, because only the energy has time inside it.

Students explain the difference between W and kWh

04 Question types and worked steps

Type	Example	Steps	Answer
Find the current	A 12 V battery across a 4 ohm resistor. Current?	1) Write current equals voltage over resistance. 2) 12 divided by 4	3 A
Voltage in series	4 ohm and 6 ohm in series across 12 V. Voltage on each?	1) Equivalent resistance 10 ohm. 2) Current 1.2 A. 3) Multiply by 4 and by 6	4.8 V and 7.2 V, summing to 12 V
Parallel resistance	4 ohm and 12 ohm in parallel. Equivalent resistance?	1) Add the reciprocals: one quarter plus one twelfth. 2) Invert	3 ohm, smaller than 4 ohm
Electricity cost	36 W for 5 hours at 1.2 per kWh. Cost?	1) 36 times 5 is 180 Wh. 2) Divide by 1,000 for 0.18 kWh. 3) Multiply by 1.2	About 0.216

Sanity check (teach this): Three checks. Direction of resistance: series must exceed the largest value and parallel must fall below the smallest, so a reversed answer means recompute. Conservation: series voltages add to the supply voltage and parallel branch currents add to the total. Units: power in W, energy in kWh, time in hours, because putting minutes where hours belong inflates energy sixty times. Numerically 1 kWh equals 3.6 MJ, which cross-checks any joule to kilowatt-hour conversion.

05 Independent practice (3 questions)

Question	Answer	What to watch
What resistance gives 2 A from a 12 V supply?	6 ohm	Whether the equation was rearranged before substituting
4 ohm and 12 ohm in parallel across 12 V. Total current?	4 A	Whether 3 ohm was used instead of adding the two resistors
A 36 W lamp runs 5 hours a day for 30 days. Energy in kWh?	5.4 kWh	Whether the daily and monthly steps were separated

06 Common mistakes and what to say

Mistake	Student writes	Say this
Adding resistances in parallel	4 ohm and 12 ohm in parallel make 16 ohm	Parallel gives more paths, so the total must be below 4 ohm
Resistance changed by voltage	A bigger voltage means a bigger resistance	Resistance belongs to the component, the current is what changes
W treated as kWh	36 W for 5 hours is 180 kWh	Divide by 1,000 for kilowatt-hours, look at the unit
Branch currents not checked	Total 4 A with branches of 3 A and 4 A	The branches must add to the total, otherwise something is wrong

07 Homework (paper, section and question numbers)

Solubility and crystallisation, question 2 — Both want the relationship written before numbers
Reflection and refraction, question 3 — Both read values off a diagram before computing
Pressure, upthrust and floating, question 3 — Both have a conservation step that checks the answer

08 Exit check (last 5 minutes)

Question	Answer	Decision
Is parallel equivalent resistance larger or smaller than the smallest resistor?	Always smaller	Answering larger means parallel is still being treated as series
12 V across 4 ohm. Current?	3 A	Answering 48 A means multiplication was used
36 W for 5 hours in kWh?	0.18 kWh	Answering 180 kWh means the division by 1,000 was missed

09 Tutor FAQ

Should students measure resistance with a meter?

That happens in the practical. This lesson first makes them compute resistance from voltage and current, then check the meter reading against it.

Do S3 students need internal resistance or emf?

No. S3 treats the supply as ideal. Internal resistance and emf belong to S4 Physics and would only confuse the class here.

Should mixed circuits be included?

Only a two-step version with parallel inside series. Three or more steps is S4 work; S3 needs two clean stages combined once.

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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.