DSE Physics: Electromagnetic Induction and Transformers Prep Guide (for tutors)

DSE Physics: Electromagnetic Induction and Transformers Prep Guide (for tutors) | Flux, induced EMF and turns ratio

2026-11-02

Tutor lesson-prep guides · Secondary Physics

Students can recite that a changing magnetic field makes electricity, then freeze when a coil rotates or a transformer appears. This lesson uses three formulas only: flux equals BA cos theta, EMF equals N times change in flux over time, and Vp over Vs equals Np over Ns.

Objectives — Students can: (1) compute magnetic flux from field, area and angle, then induced EMF from the rate of change and the number of turns; (2) use Lenz law to state the direction of the induced current and link it to energy conservation; (3) compute secondary voltage and current from the turns ratio and explain why high-voltage transmission cuts power loss.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Magnetic field	What field-line spacing shows	Field strength
Percentages	A fall from 40 percent to zero	Forty percent
Power	How to use P equals VI	A 100 W lamp at 220 V draws 0.45 A

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Magnetic flux	Field, area and angle into one formula	Get 0.0050 Wb
10-22 min	Induced EMF	Rate of change times turns	Get 20 V
22-34 min	Lenz law	Oppose the change, then find the current	Top of coil becomes a north pole
34-44 min	AC generator	Flux and its rate of change are out of phase	Say when EMF peaks
44-56 min	Transformers and transmission	Turns ratio, constant power, I squared R	Get 11 V and 5,000 W
56-60 min	Exit check	Collect three questions and mark live	All three right

03 Teaching script (read it out)

Scene 1: Flux measures what passes through, not what is nearby

Flux equals B times A times cos theta, where theta is the angle between the field and the normal to the coil.
With B equal to 0.5 T, A equal to 0.02 square metres and theta equal to 60 degrees, cos theta is 0.5, so flux is 0.0050 Wb.
When the coil lies parallel to the field, theta is 90 degrees and flux is zero; flux is largest when the coil is perpendicular to the field.
Induced EMF depends only on the rate of change: EMF equals N times change in flux divided by time. With 200 turns, area 0.05 square metres and B falling from 0.4 T to zero in 0.2 s, the change in flux is 0.020 Wb and the EMF is 20 V.
A steady field gives zero EMF no matter how strong it is.

Ask the student: what is the flux when the coil is parallel to the field (zero).

Scene 2: Lenz law is energy conservation in disguise

The induced current always creates a field that opposes the change in flux: pushing a north pole in makes the top of the coil a north pole; pulling it out makes the top a south pole.
So the current reverses between pushing in and pulling out, and work is needed both ways, turning mechanical energy into electrical energy.
In an AC generator, flux is zero when the coil is parallel to the field but the rate of change is greatest, so the EMF peaks there; when the coil is perpendicular, flux is greatest and the EMF is zero.
Peak flux and peak EMF never happen together, and that phase difference is the most examined trap.

Ask the student: is the current the same direction when the magnet goes in and comes out (no, it reverses).

Scene 3: A transformer changes voltage and current, never power

For an ideal transformer, Vp over Vs equals Np over Ns, which also equals Is over Ip, and Vp times Ip equals Vs times Is.
With 1000 turns against 50, the ratio is 20 to 1, so 220 V in gives 11 V out.
On a 100 W load the secondary current is 100 divided by 11, that is 9.1 A, while the primary current is 100 divided by 220, that is 0.45 A.
For transmission, sending 11,000 W at 220 V needs 50 A, and with 2.0 ohm cables the loss is 5,000 W; at 11 kV the current is 1 A and the loss only 2 W.
Step-up transmission exists to shrink the current, not to serve the customer.

Ask the student: does stepping up make the current larger or smaller (smaller).

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Magnetic flux	Flux equals B A cos theta, angle measured from the normal	0.0050 Wb
Calculation	Induced EMF	EMF equals N times change in flux over time	20 V
Explanation	Lenz law direction	Oppose the change first, then use the right-hand grip rule	Opposite for insertion and removal
Calculation	Transformer voltage and current	Vp over Vs equals Np over Ns and power is constant	11 V and 9.1 A

Sanity check (teach this): Flux is measured in webers, that is tesla times square metre; EMF depends on the rate of change, so a steady field gives zero.
Theta is measured from the normal, and cos 90 degrees is zero, so never use the angle between the coil plane and the field.
An ideal transformer keeps power constant: a 20-fold voltage rise means a 20-fold current fall.
Real transformers lose energy to copper heating and eddy currents, so only a question that says ideal allows the clean formula.

05 Independent practice (3 questions)

Question	Answer	What to watch
Flux when B is 0.5 T, A is 0.02 square metres and theta is 60 degrees	0.0050 Wb	BA cos theta with cos 60 equal to 0.5
EMF for 200 turns with flux change 0.020 Wb in 0.2 s	20 V	N times change in flux over time
Secondary voltage for 1000 to 50 turns at 220 V	11 V	220 times 50 over 1000

06 Common mistakes and what to say

Mistake	Student writes	Say this
Assuming a field alone makes EMF	A coil in a magnetic field has electricity	If flux does not change, the rate is zero and so is the EMF.
Using the wrong angle	The coil is parallel to the field so flux is largest	Theta is from the normal; parallel means 90 degrees and zero flux.
Expecting a transformer to boost power	Stepping up raises the power too	Ideal transformers keep power constant, so voltage up means current down.
Guessing the current direction	The current is clockwise, with no reason	State what Lenz law opposes, then apply the grip rule.

07 Homework (paper, section and question numbers)

Practice: flux and induced EMF calculations — Write flux first, then its rate of change
Practice: transformers and transmission loss — Hold on to constant power
All lesson-prep guides — Compare the ten-section structure

08 Exit check (last 5 minutes)

Question	Answer	Decision
Flux when B is 0.5 T, A is 0.02 and theta is 60 degrees	0.0050 Wb	All three right: move to AC circuits
EMF for 200 turns and flux change 0.020 Wb in 0.2 s	20 V	One wrong: recompute the rate
Secondary voltage for 1000 to 50 at 220 V	11 V	Two wrong: back to act three

09 Tutor FAQ

Do students need to memorise the formulas?

Three matter: flux equals BA cos theta, EMF equals N times rate of flux change, and Vp over Vs equals Np over Ns; the rest follows from energy conservation.

Why teach high-voltage transmission?

It is the I squared R application: ten times the voltage means a tenth of the current and a hundredth of the loss.

Can Lenz law be taught as direction only?

Link it to energy: pushing the magnet in takes work, so the induced field must oppose the motion.

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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.