DSE Physics | Circuits and Internal Resistance Lesson-Prep Guide (Tutor Edition)

DSE Physics | Circuits and Internal Resistance Lesson-Prep Guide (Tutor Edition) | Ohm Law, Series, Parallel and Power

2026-11-01

Tutor lesson-prep guides · Secondary Physics

Students know V = IR but forget the battery has resistance the moment a question mentions it. This lesson makes non-ideal batteries the first line of every calculation.

Objectives — Students can (1) use E = I(R + r) to find current and terminal voltage; (2) tell series from parallel equivalent resistance and find branch currents; (3) use P = I squared R for power and energy and explain why transmission uses high voltage.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Fractions	What is 1/6 + 1/3?	1/2
Units	How many amperes is 1 mA?	0.001 A
Power	How are P, V and I related?	P = VI

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Ohm law	V = IR for metallic conductors	Write the three forms
10-24 min	Internal resistance	E = I(R + r), V = E – Ir	I = 1.2 A
24-38 min	Series	Resistances add, current is shared	9.5 ohm
38-50 min	Parallel	Reciprocals add, voltage is shared	2.0 ohm
50-56 min	Power and energy	P = I squared R, E = Pt	820.8 J
56-60 min	Exit check	Collect and mark three questions	Work independently

03 Teaching script (read it out)

Scene 1: A battery is not ideal: write r first

EMF E = 12 V and internal resistance r = 0.5 ohm. With an external 9.5 ohm the total is 10.0 ohm, so I = E / (R + r) = 12 / 10.0 = 1.2 A. The terminal voltage is V = E – Ir = 12 – 1.2 x 0.5 = 11.4 V, an efficiency of 95%.

Ask them to compute: with r raised to 1.5 ohm, what is the current? (12 / 11.0, about 1.09 A.)

Scene 2: Series and parallel: two sentences

In series the current is the same everywhere and resistances add: 4 + 5.5 = 9.5 ohm, with voltage shared in proportion. In parallel the voltage is the same and reciprocals add: 1/6 + 1/3 gives 2.0 ohm. Adding r gives 2.5 ohm, so I = 4.8 A and the terminal voltage is 9.6 V; the branches take 1.6 A and 3.2 A, summing to 4.8 A.

Ask them to explain: why a parallel combination is always smaller than its smallest branch (more paths).

Scene 3: Power: one answer, three routes

P = VI = I squared R = V squared / R must give the same number on the same resistor. For 9.5 ohm: P = 1.2 squared x 9.5 = 13.68 W. In 60 seconds that is 13.68 x 60 = 820.8 J. A 1000 W appliance running 5 hours uses 5 kWh, the unit on an electricity bill.

Remind them: decide which resistor the question means before computing power; the battery burns I squared r too.

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Current with internal resistance	E = I(R + r)	1.2 A
Calculation	Terminal voltage	V = E – Ir	11.4 V
Calculation	Parallel equivalent resistance	1/R = 1/R1 + 1/R2	2.0 ohm
Calculation	Power and energy	P = I squared R, E = Pt	13.68 W, 820.8 J

Sanity check (teach this): Terminal voltage is always below the EMF unless r is zero; the total current always exceeds any single branch current; a parallel equivalent is always smaller than its smallest branch (2.0 is below 3.0). If branch currents do not add to the total, the calculation used the EMF instead of the terminal voltage.

05 Independent practice (3 questions)

Question	Answer	What to watch
Current with E=12 V, r=0.5 ohm, R=9.5 ohm	1.2 A	add r to the denominator
Terminal voltage of the same circuit	11.4 V	E – Ir
Equivalent resistance of 6 and 3 ohm in parallel	2.0 ohm	add reciprocals

06 Common mistakes and what to say

Mistake	Student writes	Say this
Ignoring internal resistance	Answering I = 1.26 A	The battery uses some of the EMF, so add r.
Adding parallel resistors	Answering 9 ohm	Parallel gives more paths, so the total falls.
Using the EMF on a branch	Answering 2.0 A	A branch sees the terminal voltage, not E.
Power on the wrong resistor	Reporting total power as external power	Ask which component, then use its own R.

07 Homework (paper, section and question numbers)

DSE Physics practice paper (electricity) — Section A circuit calculations
Secondary Physics chapter index — All chapters

08 Exit check (last 5 minutes)

Question	Answer	Decision
Terminal voltage with E=12 V, r=0.5, R=9.5 ohm	11.4 V	Three correct means move on to electromagnetism
Equivalent resistance of 6 and 3 ohm in parallel	2.0 ohm	One wrong means re-teach parallel
Power in 9.5 ohm at 1.2 A	13.68 W	Two wrong means restart scene one

09 Tutor FAQ

When can internal resistance be ignored?

Only when the question says ideal cell or r is negligible; in DSE write r first anyway.

Why does parallel resistance drop?

More paths carry more current at the same voltage, so V/I falls.

When is P = VI unsafe?

On non-ohmic loads such as motors, where input power is not all dissipated as heat.

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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.