DSE Physics | Electromagnetic induction lesson plan (for tutors)

DSE Physics | Electromagnetic induction lesson plan (for tutors) | Where 8 V comes from

2026-11-03

Tutor lesson-prep guides · Secondary Physics

Students see a coil turn and write “there is a current”, then stall when asked how many volts. This lesson does one job: turn “how fast the flux changes” into a formula they can substitute into, then handle direction.

Objectives — Students can: (1) compute flux with BA and EMF with N x flux change over time (200 turns, 0.020 m2, field falling from 0.40 T to 0.10 T in 0.15 s gives 8.0 V); (2) use Lenz law to state the current direction; (3) explain that 240 V mains is rms with a peak near 339 V at 50 Hz, a period of 0.02 s.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Field lines	Do closer field lines mean a stronger field?	Yes
Area units	How many m2 is 20 cm2?	0.0020 m2
Ohm law	What current flows when 8 V drives 4 ohm?	2 A

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Demonstration	Push a magnet into a coil; the meter kicks, then returns to zero	Say “change is required”
10-24 min	Flux	Flux is BA; compute the change of 0.0060 Wb	They get 0.0060 Wb
24-38 min	Faraday law	EMF = 200 x 0.0060 / 0.15 = 8.0 V	Get 8.0 V with units
38-48 min	Lenz law	Oppose the change, then apply the right-hand rule	State opposite directions for in and out
48-55 min	Generator	Peak 339 V, rms 240 V, period 0.02 s	Explain why rms is used
55-60 min	Exit check	Mark on the spot	Three correct

03 Teaching script (read it out)

Scene 1: No change, no EMF

Flux is BA. A 0.020 m2 coil in 0.40 T goes from 0.0080 Wb to 0.0020 Wb, a change of 0.0060 Wb. Then EMF = N x change / time = 200 x 0.0060 / 0.15 = 8.0 V. The sentence to repeat: a stationary magnet gives zero change and therefore zero EMF, no matter how strong the field is.

Ask them to: halve the time and give the new EMF (16 V).

Scene 2: Lenz law: direction from opposition

Push the N pole in: the near end of the coil becomes N and repels it. Pull it out: the near end becomes S and holds on. Answer in three steps: which way was the flux, is it rising or falling, what current produces the opposing flux. Only “it opposes the change” usually earns half marks.

Ask them: which pole faces the magnet as it is pulled out?

Scene 3: Generator and rms values

A rotating coil has flux varying as cosine, so the EMF is sinusoidal. Mains 240 V is rms, so the peak is 240 x root two, about 339 V, at 50 Hz with a period of 0.02 s. rms is used because it equals the direct current that gives the same heating, so power is V squared over R. Double the speed and both frequency and peak double.

Ask them to compute: 8 V across 4 ohm gives 2 A and 16 W.

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	EMF from N x flux change / time	Find flux change, then divide	8.0 V
Decision	Direction of induced current	Three Lenz steps	Oppose the change
Graph	Meaning of the slope of a flux-time graph	Slope equals EMF over N	Zero slope, zero EMF
Application	Peak and rms of a generator	Peak = root two times rms	339 V

Sanity check (teach this): 8.0 V is an average; instantaneous values need the slope. Flux in webers, time in seconds, and never drop the 200 turns. For direction, always ask first whether the flux is rising or falling.

05 Independent practice (3 questions)

Question	Answer	What to watch
EMF for 200 turns, flux change 0.0060 Wb in 0.15 s	8.0 V	Use N x change over time
Current into a 4.0 ohm load	2.0 A	Divide EMF by resistance
Peak of a 240 V rms supply	339 V	Multiply by root two

06 Common mistakes and what to say

Mistake	Student writes	Say this
Dropping the turns	Reporting 0.04 V	“Each of the 200 turns is in series, so multiply.”
Using milliseconds as seconds	Writing time as 15	“Convert to 0.15 s.”
Calling 240 V the peak	Peak written as 240 V	“240 is rms; the peak is root two times that.”
Only writing “opposes”	No pole named	“Name the pole at the near end to finish the answer.”

07 Homework (paper, section and question numbers)

DSE physics practice paper (electromagnetism) — Section B induction and generator questions
Secondary physics chapter index — All chapters

08 Exit check (last 5 minutes)

Question	Answer	Decision
EMF for 200 turns with flux change 0.0060 Wb in 0.15 s	8.0 V	All three right: move to transformers
Peak of a 240 V rms supply	339 V	One wrong: reteach rms
Current with a stationary magnet	Zero	Two wrong: restart act one

09 Tutor FAQ

A strong field but no motion: why no current?

Induction depends on the rate of change of flux, not on field strength.

Why do mains use rms?

It matches direct current in heating, so power formulas apply directly.

Is Lenz law about energy?

Yes: the induced current does exactly the work you put in pushing the magnet.

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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.