DSE Physics | Alternating Current and Transformers: Tutor Lesson Plan

2026-11-03

Tutor lesson-prep guides · Secondary Physics

Students read 220 V as a fixed value, answer 220 V when asked for the peak, and apply the turns ratio to current as well. This lesson holds three rules: rms and peak differ by 1.41, voltage ratio equals turns ratio, current ratio is its reciprocal, and power conservation checks the result.

Objectives — Students can: (1) state that 220 V is rms with a peak of 311 V and a period of 0.02 s at 50 Hz; (2) use 1100 and 55 turns to get 11 V; (3) use power conservation to get 0.11 A in the primary when the secondary carries 2.2 A; (4) classify step-up and step-down transformers and explain why one ratio cannot serve both voltage and current.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Electromagnetic induction	What does a changing flux through a coil produce?	An induced emf
Power	What is the power for 220 V at 0.11 A?	24.2 W
Frequency	How many cycles per second at 50 Hz?	Fifty
rms value	Peak of a 12 V rms sine wave?	About 17 V

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Opening	Ask what the highest voltage on a 220 V mains lead is	Recognise rms and peak differ
8-18 min	The sine wave	Draw a 50 Hz wave and mark the period	Mark peak and period themselves
18-30 min	Transformer principle	Two coils on one iron core	Explain mutual induction
30-42 min	Turns ratio	1100 turns against 55 turns	Compute the secondary voltage
42-50 min	Power conservation	Secondary at 2.2 A	Back out the primary current
50-60 min	Independent practice	Three questions in eight minutes	Mark the errors and name the cause

03 Teaching script (read it out)

Scene 1: 220 V is rms, so the peak is always 1.41 times higher

Mains 220 V is a root mean square value, so the peak is 220 times 1.414, about 311 V. Students treat 220 V as the ceiling, which drags every insulation and capacitor answer low. Fix the habit: a stated mains voltage is rms, and only a question about the highest voltage invites the 1.41 factor. Compute the period once as well: 50 Hz means 0.02 s, twenty milliseconds, which gives students a physical feel for the cycle.

Ask for the peak and the period of a 220 V, 50 Hz supply.

Scene 2: Voltage follows the turns ratio, current follows its reciprocal

1100 turns against 55 turns is a ratio of 20, so 220 V falls to 11 V. Current runs the other way and rises twentyfold, so a 2.2 A secondary draws only 0.11 A from the primary. Make students justify it: power cannot be created, so a twentyfold voltage drop demands a twentyfold current rise. Every claim that a transformer saves energy dies here.

Ask why the secondary current is larger and demand the power argument.

Scene 3: Power conservation is the only check that catches a mixed ratio

Primary 220 V times 0.11 A gives 24.2 W and secondary 11 V times 2.2 A gives the same 24.2 W. Students should multiply both sides after every calculation; a mismatch means the same ratio was used for both voltage and current. Real transformers lose copper and iron losses and run near ninety percent efficiency, while school questions assume an ideal unit, so add one sentence about the heat to keep the physics honest.

Ask students to verify both powers and name the case where they disagree.

04 Question types and worked steps

Type	Example	Steps	Answer
Voltage from turns	1100 turns to 55 turns at 220 V	Ratio, multiply, classify step direction	11 V, step down
Current from power	Secondary at 2.2 A	Secondary power, divide by primary voltage, check the reciprocal	24.2 W, 0.11 A
rms and peak	Peak of 220 V rms	Multiply by 1.414, keep volts, check it is larger	311 V
Frequency and period	Period at 50 Hz	One over frequency, convert to milliseconds, mark the wave	0.02 s, 20 ms

Sanity check (teach this): Direction check: a step-down transformer must show a smaller secondary voltage and a larger secondary current, so a reversed pair means a ratio was used the wrong way. Power check: both products must match at 24.2 W. Size check: a peak of 311 V exceeds 220 V, so 156 V means the factor was divided rather than multiplied. If efficiency is written as 100 percent, label it as an ideal case.

05 Independent practice (3 questions)

Question	Answer	What to watch
500 turns to 25 turns at 220 V	11 V	Was the ratio of 20 found first
Primary current for 4 A secondary with a 1 to 20 ratio	0.20 A	Was the reciprocal used
Peak of 12 V rms	17.0 V	Was 1.414 multiplied

06 Common mistakes and what to say

Mistake	Student writes	Say this
Taking rms as the peak	“220 V is the highest voltage”	“Mains ratings are rms for power calculations; the peak is 1.41 times higher at 311 V, and insulation is chosen against the peak.”
Using the voltage ratio for current	“The secondary voltage is twenty times lower, so is the current”	“Voltage times current must hold, so a twentyfold drop in voltage needs a twentyfold rise in current. Multiply both powers and see.”
Reversing step-up and step-down	“More turns means a lower voltage”	“Each turn carries the same volts per turn, so more turns give a higher total; your version says extra winding lowers the voltage.”
Swapping period and frequency	“50 Hz means one cycle every 50 seconds”	“Frequency counts cycles per second, so the period is its reciprocal, 0.02 s.”

07 Homework (paper, section and question numbers)

Physics chapter index — Revise electromagnetic induction, then questions 1 and 2
Circuits in series and parallel — Question 3 uses P = VI as a check
Waves and refraction — Frequency and period only, to fix the reciprocal

08 Exit check (last 5 minutes)

Question	Answer	Decision
Voltage ratio for 1100 and 55 turns	20 to 1	Error: written upside down
Peak of 220 V rms	311 V	Error: written as 220 V
Primary current with 2.2 A secondary	0.11 A	Error: written as 44 A
Period at 50 Hz	0.02 s	Error: written as 50 s

09 Tutor FAQ

Does a transformer change the frequency?

No, the frequency comes from the supply; a transformer only changes voltage and current, so 50 Hz in gives 50 Hz out.

Why transmit power at high voltage?

At a fixed power a higher voltage means a smaller current, and line loss falls with the square of current, so stepping up saves a lot of waste.

How does a real transformer differ from an ideal one?

Real units have copper resistance and eddy currents, run near ninety percent efficiency and get warm, while the ideal model sets input power equal to 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.