S3 Science | Circuits and Resistance: a Tutor Prep Guide | Ohm law, series and parallel rules, power and fuse choice

2026-11-09

Tutor lesson-prep guides · Secondary Science

Students can recite voltage equals current times resistance, then fall apart on networks: they treat current as water that runs out and voltage as the same everywhere. Two lines separate them: series shares one current, parallel shares one voltage.

Objectives — By the end the student can: (1) compute 12 volts across 24 ohms as 0.5 amperes and write all three rearrangements of Ohm law; (2) combine 6 and 12 ohms in series to 18 ohms, giving 0.667 amperes and voltage shares of 4 and 8 volts; (3) combine the same pair in parallel to 4 ohms, giving 3 amperes total with branches of 2 and 1; (4) compute power as voltage times current, 12 volts and 0.5 amperes giving 6 watts and 0.018 kilowatt hours over 3 hours, and explain why a fuse is rated slightly above the working current.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Fractions and reciprocals	What is the sum of the reciprocals of 6 and 12	One quarter, so the parallel value is 4 ohms
Unit prefixes	How many milliamperes is 0.5 amperes	500 milliamperes
Power	12 volts driving 0.5 amperes: power	6 watts, voltage times current

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Hook	Ask why one dead bulb kills a whole string of fairy lights; collect instincts	Write two instincts to compare later
8-20 min	Ohm law	Use a variable resistor and record three voltage and current pairs	Get 0.5 amperes and write three rearrangements
20-34 min	Series	Connect 6 and 12 ohms, measure each voltage	Get 18 ohms, 0.667 amperes, 4 and 8 volts
34-46 min	Parallel	Rewire the same pair in parallel and measure currents	Get 4 ohms, 3 amperes, branches 2 and 1
46-54 min	Power and fuses	A 12 volt 24 ohm lamp: power and energy over 3 hours	Get 6 watts and 0.018 kilowatt hours
54-60 min	Exit check	Mark three questions at once, name the act each error came from	Work independently and self-assess

03 Teaching script (read it out)

Scene 1: Ohm law: three quantities, one relation

Ohm law fixes the ratio of voltage to current in one conductor. Twelve volts across 24 ohms gives 0.5 amperes.
Method: have the student write voltage equals current times resistance, then derive the other two forms on paper, because questions ask for each in turn.
Stress that resistance belongs to the conductor, set by length, thickness, material and temperature, never by the current flowing.

Student should repeat: in one conductor, voltage divided by current is resistance.

Scene 2: Series: one current, voltages add

Six and twelve ohms in series make 18 ohms. Twelve volts divided by 18 ohms is 0.667 amperes, and both resistors carry that same current.
Voltage divides in proportion to resistance: 4 volts across 6 ohms and 8 volts across 12 ohms, adding to 12.
That is exactly why one dead bulb kills the string: series offers a single path, so one break stops all current.

Student should repeat: series shares one current and splits voltage by resistance.

Scene 3: Parallel: one voltage, currents add, then power

The same pair in parallel has 12 volts across both, an equivalent 4 ohms, 3 amperes total, and branches of 2 and 1 amperes.
The equivalent is always smaller than any branch; make the student check that 4 is below both 6 and 12.
Power is voltage times current: 12 volts and 0.5 amperes is 6 watts, so 3 hours is 0.018 kilowatt hours. A fuse must sit slightly above the working current: too high protects nothing, too low keeps blowing.

Student should repeat: parallel shares one voltage and sums the branch currents.

04 Question types and worked steps

Type	Example	Steps	Answer
Ohm law for current	12 volts across 24 ohms: current	Voltage divided by resistance, in amperes	0.5 amperes
Series resistance and voltage	6 and 12 ohms in series across 12 volts	Add resistances, find current, multiply back	18 ohms, 0.667 amperes, 4 and 8 volts
Parallel resistance and current	6 and 12 ohms in parallel across 12 volts	Reciprocals summed and inverted, then branches	4 ohms, 3 amperes, 2 and 1 amperes
Power and energy	A 12 volt 24 ohm lamp running 3 hours	Find current, multiply by voltage, then by hours	6 watts, 0.018 kilowatt hours

Sanity check (teach this): Sanity check: a series total always exceeds the largest resistor, a parallel total is always below the smallest; series has exactly one current, so two different branch currents mean an error; voltage shares must add to the supply; parallel branch currents must add to the total; power from voltage times current must match current squared times resistance.

05 Independent practice (3 questions)

Question	Answer	What to watch
9 and 18 ohms in series across 27 volts: total, current, each voltage	27 ohms, 1 ampere, 9 and 18 volts	Check resistances are added and voltage divides in proportion
Same pair in parallel across 27 volts: equivalent and total current	6 ohms, 4.5 amperes	The result must be below 9 ohms and currents must sum
A 12 volt 3 watt lamp: working current, and a sensible fuse rating	0.25 amperes, use 0.5 amperes	Check watts are divided by volts and the fuse sits above the current

06 Common mistakes and what to say

Mistake	Student writes	Say this
Current gets used up	Current passes the first resistor and half is left for the second	Series current is identical everywhere; voltage is what changes. Measure at three points to prove it.
Adding parallel resistors	6 and 12 ohms in parallel make 18 ohms	Use reciprocals: the answer is 4 ohms, below the smallest branch.
Resistance set by current	A bigger current means a bigger resistance	Resistance comes from length, thickness, material and temperature; current rises because voltage rises or resistance falls.
Fuse rated exactly at working current	A 3 ampere circuit takes a 3 ampere fuse	Leave headroom, usually one step up, or inrush current will blow it and protection is lost.

07 Homework (paper, section and question numbers)

Science chapter index — All chapters in the subject
Redo this chapter — Recompute networks with questions 1, 3 and 5
Exam-style long questions — See how a circuit diagram is paired with a data table

08 Exit check (last 5 minutes)

Question	Answer	Decision
12 volts across 24 ohms: current	0.5 amperes	Answering 288 means voltage was multiplied by resistance; reteach act one
6 and 12 ohms in parallel: equivalent resistance	4 ohms	Answering 18 means it was treated as series; reteach act three
A 12 volt 0.5 ampere lamp: power	6 watts	Answering 24 means the current was squared; reteach the power definition

09 Tutor FAQ

When does Ohm law fail?

A filament changes resistance a lot as it heats, so its voltage and current are not strictly proportional. School questions still use Ohm law, but students should know it is an approximation.

Why does parallel resistance fall?

Each extra branch gives current another path, so total current rises at the same voltage, which means the equivalent resistance falls.

Which wire should carry the fuse?

The live wire. On the neutral side a blown fuse still leaves the appliance live, which is just as dangerous.

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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.