The shape of the lesson
By the end of the lesson, learners can
- Keep it spinning: Watch the forces turn the coil and the commutator flip the current each half turn.
- The job of the commutator: Without it the coil would stall; with it the turning continues smoothly.
- Why it turns: the two sides feel opposite forces, giving a turning effect.
- Commutator: reverses the current every half turn to keep it turning one way.
- Faster: more current, stronger magnet, more turns. Reverse current or field to change direction.
One picture every idea returns to
A d.c. motor spins a current-carrying coil in a magnetic field. The forces on opposite sides act in opposite directions and turn the coil. A split-ring commutator reverses the current every half turn so the rotation keeps going the same way.
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, Turning, and turning.. Put the motor effect on a coil and the two sides are pushed opposite ways, so it spins. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Keep it spinning | Watch the forces turn the coil and the commutator flip the current each half turn. Project tab 4 of the lab (D.c. motor) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: The job of the commutator | Without it the coil would stall; with it the turning continues smoothly. | Mix-Pair-Share |
| 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, 14 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 The job of the commutator, 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.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
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 d.c. motor needs a split-ring commutator, not slip rings. | The commutator reverses the current every half turn, which keeps the coil turning the same way instead of stalling or rocking back and forth. |
The D.C. motor: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Explain why a current-carrying coil placed in a magnetic field begins to turn.
State the job of the split-ring commutator in a d.c. motor.
A student removes the commutator and connects the coil so the current cannot reverse. Describe what the coil does now, and explain why.
State three ways to increase the speed at which a d.c. motor turns.
State two separate changes, each of which on its own would reverse the direction in which the motor turns.
A d.c. motor uses a split-ring commutator, while an a.c. generator uses slip rings. State the different job each one does.
Answer key and marking notes
- The two sides of the coil carry current in opposite directions. ✓
- So the motor effect pushes one side up and the other side down. ✓
- These opposite forces give a turning effect, so the coil rotates. ✓
- It reverses the direction of the current in the coil every half turn. ✓
- This keeps the coil turning in the same direction. ✓
- The coil only rocks back and forth and does not keep turning. ✓
- After half a turn the forces would push it back, and without the commutator the current cannot reverse to keep it going the same way. ✓
- Increase the current. ✓
- Use a stronger magnet. ✓
- Increase the number of turns on the coil. ✓
- Reverse the direction of the current (reverse the supply connections). ✓
- Reverse the magnetic field by swapping the magnet poles. ✓
- The split-ring commutator reverses the current every half turn to keep the motor turning one way. ✓
- The slip rings simply keep the spinning generator coil connected, allowing the output to alternate. ✓
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.
What is the job of the split-ring commutator in a simple d.c. motor?
- Ato reverse the current in the coil every half turn so the coil keeps spinning the same way
- Bto increase the voltage of the supply
- Cto store energy between turns
- Dto stop the coil from overheating
Quiz key
The split-ring commutator swaps the current direction in the coil every half turn, so the force on each side keeps turning the coil the same way. Without it the coil would only oscillate rather than rotate fully.
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.