DSE Physics | Resistance and Circuits Tutor Prep Guide

DSE Physics | Resistance and Circuits Tutor Prep Guide | Ohm Law and Networks

2026-11-01

Tutor lesson-prep guides · Secondary Physics

Students multiply every resistor they see, and think two parallel paths must mean a larger resistance, so their answers are always too big. What they lack is not a formula but the two pictures: voltage divides along a series chain, current divides between parallel branches. This lesson builds both pictures on one 12 V supply before any formula appears.

Objectives — Students can: (1) use V = IR in all three rearrangements; (2) tell series from parallel and find the equivalent resistance; (3) use voltage and current division to get branch values; (4) use P = VI and W = Pt and check the units.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Current and voltage	same current in series, same voltage in parallel	use this to locate parts
Fractions	reciprocals	1/R = 1/R1 + 1/R2
Units	W, kW, J, kWh	1 kWh = 3.6 MJ

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Mark the circuit	colour nodes and current	parallel shares two nodes
8-20 min	Ohm law	three rearrangements	12 V / 10 ohm = 1.2 A
20-34 min	Equivalent resistance	add in series, reciprocals in parallel	parallel = 4 ohm
34-46 min	Division	voltage and current	0.8 A and 0.4 A
46-54 min	Power and energy	P = VI, W = Pt	14.4 W total
54-60 min	Exit check	collect and mark	independent work

03 Teaching script (read it out)

Scene 1: First move: split the circuit into a series block and a parallel block

Ask students to colour every piece of wire that has no component. Two ends with the same colour are at the same potential. The 6 ohm and 12 ohm resistors share both colours, so they are in parallel; that pair then sits in series with the 6 ohm resistor on the main path.
Parallel part: 1/R = 1/6 + 1/12 = 0.1667 + 0.0833, so R = 4 ohm. Series addition: total = 4 + 6 = 10 ohm. Ask: why must a parallel equivalent be smaller than the smallest branch? (More paths means more total current at the same voltage.)

Task: circle the parallel block on your own diagram and write its equivalent value.

Scene 2: Division: voltage goes to the series part, current to the parallel part

Total current I = V / R = 12 / 10 = 1.2 A, which is the current through the main path and through the 6 ohm resistor. The parallel block takes 1.2 x 4 = 4.8 V, so the 6 ohm branch carries 4.8 / 6 = 0.8 A and the 12 ohm branch carries 4.8 / 12 = 0.4 A, which adds to 1.2 A.
Check from the voltage side: 4.8 + (1.2 x 6 = 7.2) = 12.0 V, the supply voltage. Remind: the smaller branch resistance takes the larger share of current, the opposite of the gut feeling. Ask: if the 6 ohm branch breaks, does the total current rise or fall? (Fall, one path is gone.)

Task: recompute the total current with that branch removed.

Scene 3: Power and energy: name the component before using a formula

Whole circuit power P = VI = 12 x 1.2 = 14.4 W; the 6 ohm resistor alone dissipates I squared R = 1.2 squared x 6 = 8.64 W. Running for 5 minutes (300 s) uses W = Pt = 14.4 x 300 = 4320 J.
For a single component always use that component own V and I; substituting the main-path values is the most common arithmetic error in this topic.

Task: change 5 minutes to 2 hours and give the energy in kWh. Ask: why does adding a lamp in parallel raise the total power but not the brightness of the others? (The voltage across each stays the same.)

04 Question types and worked steps

Type	Example	Steps	Answer
Equivalent resistance	given a mixed network	parallel first, then series	10 ohm
Branch current	given the supply voltage	total then divide	0.8 A, 0.4 A
Voltage division	two resistors in series	V proportional to R	split by ratio
Power and energy	given a time	P = VI, W = Pt	4320 J

Sanity check (teach this): A parallel equivalent must be smaller than the smallest branch (4 smaller than 6); a larger value means the series rule was used by mistake. Branch currents must add to the total current, so a gap above 0.1 A means a rounding or reading error. Voltages around a loop must add to the supply voltage, otherwise a section was skipped. With W in joules the time must be in seconds; with kWh the power must be in kW. Check the order of magnitude too: household appliances draw roughly 0.1 to 10 A, so 120 A signals a unit slip.

05 Independent practice (3 questions)

Question	Answer	What to watch
Equivalent resistance of 6 ohm parallel 12 ohm	4 ohm	reciprocals then invert
Total current once the 6 ohm resistor is added	1.2 A	12 / 10
Energy used by the whole circuit in 300 s	4320 J	W = Pt

06 Common mistakes and what to say

Mistake	Student writes	Say this
Adding parallel resistors	answers 18 ohm	Parallel opens another path, so the total is smaller than the smallest branch.
Using the supply voltage on a branch	answers 2.0 A	That branch only sees 4.8 V, not 12 V.
Using the main current for one component power	answers 17.3 W	Each component needs its own current and voltage.
Leaving the time in minutes	answers 72 J	In W = Pt the time must be in seconds.

07 Homework (paper, section and question numbers)

Secondary Physics chapter index — questions 4 and 7 are the same type
Prep guide contents — other chapters in this subject
DSE Physics practice papers — electricity set, questions 1 to 6

08 Exit check (last 5 minutes)

Question	Answer	Decision
Equivalent resistance of 6 ohm parallel 12 ohm	4 ohm	all three right means move on to power
Current rule for series and parallel	same in series, splits in parallel	one wrong means reteach next lesson
Energy in 300 s	4320 J	two wrong means return to act one

09 Tutor FAQ

Why is a parallel equivalent smaller than the smallest branch?

A parallel branch gives charge an extra path, so the total current rises at the same voltage and the equivalent resistance falls.

Should the internal resistance of the battery be counted?

Not when the question says ideal battery; if r is given, use E = I(R + r).

What decides how bright a lamp is?

Its actual power; in parallel each lamp keeps the same voltage, so adding lamps does not dim the others.

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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.