DSE Physics | Series and Parallel Circuits: Tutor Lesson Plan

2026-11-03

Tutor lesson-prep guides · Secondary Physics

Students see two resistors and add them, even in parallel. This lesson holds three rules: series shares one current, parallel shares one voltage, and the equivalent resistance comes from the formula rather than a feeling. Power and the electricity bill close the loop.

Objectives — Students can: (1) state that series current is the same everywhere and parallel branches share the supply voltage; (2) find 10 ohm, 1.2 A and the 4.8 V / 7.2 V split for 4 ohm and 6 ohm in series; (3) find 2.4 ohm, 5.0 A total and 3.0 A / 2.0 A branches in parallel; (4) use P = VI for 14.4 W and turn 1.2 kW for 3 hours into 3.6 kWh costing 4.32 dollars.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Ohm law	What current flows through 4 ohm at 12 V?	3.0 A
Series current	Does the current change before and after a resistor in series?	No, it is identical
Parallel voltage	Do parallel branches share the same voltage?	Yes, the supply voltage
Power	What is the power for 12 V at 1.5 A?	18 W

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Opening question	Ask what happens to other lamps when one switch opens	Distinguish parallel from series
8-18 min	Series	Draw 4 ohm and 6 ohm on one 12 V cell	Mark current direction and size
18-28 min	Parallel	Rework the same pair in parallel	Calculate the equivalent and justify it
28-40 min	Four question types	Work one of each and compare formulas	Name the formula used each time
40-50 min	Power and cost	A 1.2 kW kettle for 3 hours	Convert to kWh then multiply the tariff
50-60 min	Independent practice	Three questions in eight minutes	Check and label each error

03 Teaching script (read it out)

Scene 1: Add in series, but test parallel against the smallest resistor

4 ohm and 6 ohm in series give 10 ohm, larger than either, because the current passes two obstacles in turn. In parallel they give 2.4 ohm, smaller than 4 ohm, because there is a second path. After every parallel calculation, make the student say whether the answer is smaller than the smallest resistor; if not, it is wrong. This check blocks the add-them-up error better than repeating the formula.

Ask the student to say whether two 4 ohm resistors in parallel give more or less than 4 ohm.

Scene 2: Series shares voltage in ratio, parallel shares current in inverse ratio

The 12 V splits across 4 ohm and 6 ohm in the ratio 4:6, giving 4.8 V and 7.2 V, which add to 12 V. In parallel both resistors sit at 12 V and the current splits in inverse ratio: 3.0 A through 4 ohm and 2.0 A through 6 ohm, 5.0 A in total. Build the habit of adding the voltage drops and the branch currents back to the supply as a free check.

Have the student state that voltage drops add to the supply and branch currents add to the total.

Scene 3: Power and the bill are two ends of the same line

P = VI gives 12 V times 1.2 A, or 14.4 W, which also equals I squared R. At home, 1.2 kW for 3 hours is 3.6 kWh, and at 1.2 dollars per unit the bill is 4.32 dollars. Units are where most marks are lost: kW times hours gives kWh, so watts must be divided by a thousand. Deliberately write 1200 W times 3 hours as 3600 kWh and let the student catch it.

Ask the student to correct 1200 W for 3 hours and name the unit that was wrong.

04 Question types and worked steps

Type	Example	Steps	Answer
Series equivalent and current	4 ohm and 6 ohm in series on 12 V	Add resistances, divide voltage, mark current as common	10 ohm, 1.2 A
Parallel equivalent and splitting	The same pair in parallel on 12 V	Use the product over sum formula, then V over R per branch	2.4 ohm, 5.0 A
Voltage divider	Voltage across 6 ohm in series	Find current first, then V = IR, then check the sum	7.2 V
Power and cost	1.2 kW for 3 hours at 1.2 per unit	Use kW directly, kWh = kW times h, then multiply	3.6 kWh, 4.32

Sanity check (teach this): Order check: a series equivalent of 10 ohm must exceed 6 ohm and a parallel equivalent of 2.4 ohm must fall below 4 ohm, so the two cannot both be wrong in the same direction. Series power 14.4 W equals 12 V times 1.2 A; parallel power 60 W equals 12 V times 5.0 A. A bill near 4.32 is right for a kettle; 432 means a missing factor of a hundred.

05 Independent practice (3 questions)

Question	Answer	What to watch
Current for 4 ohm and 12 ohm in series on 8 V	0.50 A	Do they add first
Equivalent of 4 ohm and 12 ohm in parallel	3.0 ohm	Is it below 4 ohm
Power in 12 ohm at 6 V	3.0 W	Do they use V squared over R

06 Common mistakes and what to say

Mistake	Student writes	Say this
Adding parallel resistances	“4 ohm and 6 ohm in parallel make 10 ohm”	“A second path makes current flow easier, so resistance must fall; 10 ohm is bigger than 4 ohm, which says the extra path made it harder.”
Mixing up voltage and current sharing	“Series current splits in inverse ratio”	“Series is one path with one current; the voltage is what splits, and it splits in direct ratio.”
Confusing W and kW	“1200 W for 3 hours uses 3600 units”	“Divide watts by a thousand first; 3600 units would run a home for months, does that look right?”
One formula, no check	“P = VI gives 0.5 W”	“12 V times 1.2 A cannot be 0.5 W; check whether milliamps were entered as amps.”

07 Homework (paper, section and question numbers)

Physics chapter index — Revise forces and motion first, then questions 1 and 2
Work, energy and power — Question 3 needs P = VI with kW first
Lenses and optical instruments — Multiple choice only, to drill unit conversion

08 Exit check (last 5 minutes)

Question	Answer	Decision
Equivalent of 4 ohm and 6 ohm in series	10 ohm	Error: reciprocal used for series
Equivalent of the same pair in parallel	2.4 ohm	Error: added instead
Voltage across 6 ohm in series	7.2 V	Error: inverse ratio used
Cost of 1.2 kW for 3 hours at 1.2 per unit	4.32	Error: watts not converted

09 Tutor FAQ

Why does parallel reduce the total resistance?

A second branch is a second channel, so the total current rises at the same voltage; more current at the same voltage means a smaller equivalent resistance.

Should internal resistance be included?

Treat cells as ideal at S4 and mention that real cells have internal resistance, which is why lamps dim as a battery ages.

When do we use P = VI and when P = I squared R?

Use VI when voltage and current are known, and I squared R when current and resistance are known. All three forms agree.

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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.