DSE Physics: Electromagnetic Induction and Transformers

DSE Physics: Electromagnetic Induction and Transformers — Tutor Lesson Plan

2026-11-01

Tutor lesson-prep guides · Secondary physics

Students can quote Vs/Vp = Ns/Np yet answer “why high voltage?” with “because the voltage is higher”, never reaching I-squared-R.

Objectives — The student can calculate an induced emf with Faraday law and name its three variables, use Lenz law to predict current direction, and compute secondary voltage and current for a transformer.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Flux, phi = BA	Weber; B times area at right angles	Double the area, flux doubles
P = VI	An ideal transformer passes all the power	Same P, higher V means lower I
Lenz in one line	Induced current opposes the flux change	A magnet pushed in is pushed back

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Prerequisite check	Write phi = BA and ask what changes it	Names B, A, angle
8-20 min	Demo and observation	Push a magnet in and out, watch the galvanometer	Records deflection direction
20-32 min	Faraday law	200 turns, 0.004 Wb in 0.2 s gives 4 V	Reworks it with units
32-44 min	Transformers	220 V, 1100 turns, 100 turns gives 20 V, then current	Computes the ratio alone
44-54 min	Transmission	100 kW at 11 kV is 9.09 A, at 400 V it is 250 A	Explains with I-squared-R
54-60 min	Exit check	Three questions marked at once	Works alone

03 Teaching script (read it out)

Scene 1: Show the effect before naming the law

Connect a coil to a sensitive galvanometer. With the magnet held still inside the coil the needle does not move. Push it in, pull it out, move it faster, flip the pole: the needle swings, and swings the other way.
Ask the class to name the one thing every deflecting action has in common. The answer is not “a strong field”, it is the flux is changing. A magnet resting inside a coil, however strong, gives zero emf because flux is constant.
Faraday law is just that observation made countable: 200 turns, 0.004 Wb changed in 0.2 s, emf = 200 x 0.004 / 0.2 = 4 V. The unit check (webers per second, times turns) is worth saying aloud.

Ask the student: why a magnet sitting still inside a coil gives no current.

Scene 2: Lenz law is a search for the opposition

Direction questions are where marks disappear. Give them a three-step routine: which way is the flux, is it increasing or decreasing, then what field would oppose that change, then use the right-hand grip rule to get the current.
Push a north pole into a coil: flux to the right is increasing, so the near face must become a north pole to push back, so the current seen from that face is anticlockwise. Pull the magnet out and every arrow reverses.
That reversal is the self-check to teach: if a student gets the same direction for pushing in and pulling out, the reasoning is incomplete somewhere.

Ask for the sentence: the induced current always opposes the change in flux.

Scene 3: Transformers and transmission: one chain of two formulas

Ideal transformer: Vs/Vp = Ns/Np. 220 V across 1100 turns, 100 turns secondary, Vs = 220 x 100/1100 = 20 V, a 11:1 step-down.
Power is conserved: Ps = 20 x 0.5 = 10 W, so the primary current is 10/220 about 0.0455 A (45.5 mA). At 90 percent efficiency the input must be 11.11 W and the primary current rises to 0.0505 A.
Transmission: the same 100 kW at 11 kV draws 9.09 A, at 400 V it draws 250 A, a factor of 27.5. Cable loss is I-squared-R, so raising the voltage 27.5 times cuts the loss by about 27.5 squared, roughly 756 times. Step-up is not about sending more power, it is about wasting less.

Ask the student: which formula carries the answer on transmission loss (I-squared-R).

04 Question types and worked steps

Type	Example	Steps	Answer
Emf calculation	N=200, flux change 0.004 Wb, time 0.2 s	Write emf = N dPhi/dt, then substitute	4 V
Transformer calculation	220 V, 1100 and 100 turns	Vs from turns ratio, then I from P = VI	20 V, 0.0455 A
Direction question	North pole pushed into a coil	Flux direction, change, opposing field, current	Near face becomes north, anticlockwise
Explanation question	Why transmit at high voltage	Fixed P, higher V lower I, loss is I-squared-R	Both formulas must appear

Sanity check (teach this): Check the direction of the transformer first: it is a step-down only if Ns is smaller than Np. Then check power: Vs times Is should match Vp times Ip for an ideal device, so a factor-of-ten mismatch means the turns were inverted. Transmission currents stay in the tens of amps, so an answer of 25,000 A means kilowatts were never converted to watts.

05 Independent practice (3 questions)

Question	Answer	What to watch
220 V on 1100 turns, 100 turns secondary: secondary voltage?	20 V	Does the student write the ratio before substituting
Secondary current 0.5 A: ideal primary current?	0.0455 A (about 45.5 mA)	Uses P = VI rather than copying the current
100 kW at 11 kV: line current?	9.09 A	Converts kW to W first

06 Common mistakes and what to say

Mistake	Student writes	Say this
Treating a strong field as a current	Magnet held still, answers “yes, the field is strong”	“A strong field on its own does nothing; the flux has to be changing.”
Inverting the turns ratio	Writes Vs = 220 x 1100/100	“Fewer turns on the secondary means the voltage goes down, not up.”
Forgetting kW to W	Substitutes 100 for 100 kW	“A kilowatt is a thousand watts, convert before dividing by volts.”
Believing a transformer creates power	Says power rises after step-up	“A transformer trades voltage for current; the power stays the same.”

07 Homework (paper, section and question numbers)

DSE Physics paper 1 — Section A multiple choice, first 3 items: induced emf and direction
Physics chapter index — Section B structured question 1: transformer calculation, both formulas shown

08 Exit check (last 5 minutes)

Question	Answer	Decision
220 V, 1100 turns down to 100 turns: secondary voltage?	20 V	Three correct means move on to a.c. power
Steady magnet inside a coil: induced current?	Zero, the flux is constant	One wrong means reteach act one
Which formula explains lower transmission loss?	P_loss = I squared R	Two wrong means return to act three

09 Tutor FAQ

How many formulas are needed?

Three: emf = N dPhi/dt, Vs/Vp = Ns/Np, and P = VI. Everything else is derived.

Must students draw the field arrows?

Yes. Without arrows, roughly four in ten direction answers come out reversed.

Why mention efficiency?

DSE asks about real transformers, where input power exceeds output power.

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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.