DSE Physics | Electromagnetic Induction Lesson Prep (for tutors)

DSE Physics | Electromagnetic Induction Lesson Prep (for tutors) | Flux, Faraday and Lenz

2026-11-03

Tutor lesson-prep guides · Secondary Physics

Students can recite that induced emf is proportional to the rate of change of flux, then answer “induced current” for a coil moving uniformly in a uniform field. This lesson narrows every case with one question: has the flux changed at this instant?

Objectives — Students can: (1) compute the flux through a coil using BA and say when it is zero; (2) compute induced emf with N times change in flux over change in time; (3) use Lenz law to give the direction of the induced current with a stated reason.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Flux	When is BA zero	When the coil plane is parallel to the field
Units	One weber in tesla square metres	1
Right hand	What the three fingers of the right-hand rule give	Field, current, force

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Diagnose	Three diagrams: changed or not	Give the reason
10-24 min	Flux	BA and the angle	Flux = 0.01 Wb
24-36 min	Faraday	Emf = N x change in flux over time	Emf = 10 V
36-48 min	Lenz	Direction and energy	Four-step direction
48-56 min	Mixed	Graphs and prose	One long question
56-60 min	Exit check	Three questions collected	Independent

03 Teaching script (read it out)

Scene 1: Flux: ask how much field passes through

Flux = BA cos(theta), where theta is the angle between the field and the normal to the coil. When the plane is parallel to the field, theta is 90 degrees and flux is zero; perpendicular gives the maximum.
Example: B = 0.40 T, A = 0.025 m squared, perpendicular, so flux = 0.40 x 0.025 = 0.01 Wb.
Make them answer: rotate the coil by 30 degrees, does the flux rise or fall (it falls, since cos 30 is less than 1).

Tutor move: use a hand as the coil and turn it slowly, asking for more or fewer field lines.

Scene 2: Faraday: a rate, not an amount

Emf = N x change in flux divided by time taken. A coil of 200 turns loses 0.01 Wb in 0.20 s, so emf = 200 x 0.01 / 0.20 = 10 V.
Same change in less time gives more emf; no change gives zero emf however large the flux.
Trap: reading the time as “seconds later” instead of “seconds taken”.

Tutor move: write “faster means bigger” under the time symbol and demo it twice.

Scene 3: Lenz: four steps, no memorising

Step one, the original field direction. Step two, is the flux rising or falling. Step three, the induced field opposes the change (opposite if rising, same if falling). Step four, use the right-hand grip to get the current.
Energy view: the induced current always opposes the motion that caused it, which is why generating electricity takes effort.
Warning: direction questions need the reason; a bare “clockwise” usually earns half marks.

Tutor move: one four-step sheet, no skipping, every single time.

04 Question types and worked steps

Type	Example	Steps	Answer
Flux	Find or compare flux	Identify B, A and the angle	Units Wb, state the angle
Emf	Find emf or average current	Emf = N x flux change over time, then divide by R	Time is the duration of change
Direction	Lenz law direction	Four steps	Reason required
Graph	Emf from a flux-time graph	The slope is the rate	Zero slope means zero emf

Sanity check (teach this): Ask two things of every answer: did the flux change, and how long did it take. No change means zero emf, whatever the field strength or turn count. A direction that does not oppose the change is wrong. Conventional current is opposite to electron flow, so say which you are giving.

05 Independent practice (3 questions)

Question	Answer	What to watch
B = 0.40 T, A = 0.025 m squared, perpendicular, flux	0.01 Wb	BA
200 turns, flux falls to zero in 0.20 s, emf	10 V	N x change over time
A coil leaving a field, what does the current do	Opposes the motion (attracts)	Opposes relative motion

06 Common mistakes and what to say

Mistake	Student writes	Say this
Big flux means big emf	Strong static field answered as induced current	“No change, no emf. Size is irrelevant.”
Wrong time	Using the time after rather than the duration	“That is a clock reading; I need how long the change took.”
Direction without reason	Just “clockwise”	“Half the marks are in the reason.”
Ignoring turns	Single-turn formula on 200 turns	“Every turn contributes, so multiply by N.”

07 Homework (paper, section and question numbers)

Induction calculations (in-house) — Questions 1-4: flux and emf
Direction practice (in-house) — Questions 5-7: four steps with reasons

08 Exit check (last 5 minutes)

Question	Answer	Decision
Coil moving uniformly inside a uniform field, emf	0	Wrong: reteach “has it changed”
200 turns, 0.01 Wb lost in 0.20 s	10 V	Wrong: reteach the time
Lenz law in one sentence	The induced current opposes the change in flux	Wrong: reteach the energy view

09 Tutor FAQ

Why does a generator need force

The induced current opposes the motion, so mechanical energy becomes electrical.

Does direct or alternating current matter

Faraday law is the same; the direction reverses as the flux rises and falls.

Why an iron core in a transformer

It guides the flux so nearly all of it links the secondary coil.

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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.