The shape of the lesson
By the end of the lesson, learners can
- Wind the coils: Change the turns on each coil and watch the secondary voltage step up or down.
- Up to send, down to use: The grid raises the voltage for transmission and lowers it again for homes.
- Equation: Vₚ ÷ Vₛ = Nₚ ÷ Nₛ. Rearrange carefully.
- Type: step-up if the secondary has more turns; step-down if fewer.
- A.C. only: the field must keep changing to induce a voltage.
One picture every idea returns to
A transformer changes an alternating voltage using two coils on a soft iron core. The changing field from the primary induces an e.m.f. in the secondary. More turns on the secondary steps the voltage up; fewer steps it down. The grid steps voltage up for efficient transmission and down for safe use.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Changing the voltage.. Two coils sharing an iron core, never touching, yet one feeds power to the other across the gap. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Wind the coils | Change the turns on each coil and watch the secondary voltage step up or down. Project tab 5 of the lab (Transformers) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Up to send, down to use | The grid raises the voltage for transmission and lowers it again for homes. | Jigsaw |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Up to send, down to use, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| A transformer works only with alternating current, because it needs a changing magnetic field to induce an e.m.f. in the secondary. | A steady direct current would induce nothing once the field had settled. |
Transformers: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Name the three main parts of a simple transformer, and state which material the core is made from.
A transformer has 100 turns on the primary and 500 turns on the secondary. The input voltage is 12 V. Calculate the output voltage and state the type of transformer.
A charger steps the 230 V mains down to 11.5 V. The primary coil has 2000 turns. Calculate the number of turns needed on the secondary coil.
A student connects a battery (d.c.) to the primary of a transformer and measures the output voltage. State what they find, and explain why.
Explain why electricity is sent across the country at very high voltage, using a step-up transformer at the power station.
Explain briefly how an alternating voltage in the primary coil produces a voltage in the secondary coil, even though the two coils are not connected by a wire.
Answer key and marking notes
- A primary coil and a secondary coil. ✓
- Wound on a core. ✓
- The core is made of soft iron. ✓
- Vₛ = Vₚ × Nₛ ÷ Nₚ = 12 × 500 ÷ 100
Vₛ = 60 V
output larger than input, so step-up
- Nₛ = Nₚ × Vₛ ÷ Vₚ = 2000 × 11.5 ÷ 230
Nₛ = 100 turns
- The output voltage is zero (apart from a brief moment when switching on or off). ✓
- A steady d.c. gives a steady field, and a transformer needs a changing field to induce a voltage. ✓
- A higher voltage means a smaller current for the same power. ✓
- A smaller current wastes less energy as heat in the cables, so transmission is more efficient. ✓
- The alternating current in the primary makes a changing magnetic field in the soft-iron core. ✓
- This changing field passes through the secondary coil and induces an alternating voltage in it. ✓
1 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
A transformer has fewer turns on its secondary coil than on its primary coil. What does it do to the voltage, and what supply does it need?
- AIt steps the voltage down, and it needs an alternating supply.
- BIt steps the voltage up, and it needs a direct supply.
- CIt steps the voltage down, and it needs a direct supply.
- DIt leaves the voltage unchanged.
Quiz key
Fewer secondary turns gives a lower output, so it is a step-down transformer. It needs alternating current, because a changing magnetic field is required to induce a voltage in the secondary; a steady d.c. would induce nothing.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.