The shape of the lesson
By the end of the lesson, learners can
- Turn the coil, read the output: Rotate the coil and watch the alternating e.m.f. trace out its sine shape.
- Why the output alternates: As the coil turns, the sides cut the field first one way then the other, so the e.m.f. reverses each half turn.
- Shape: a smooth wave; one full turn makes one full cycle.
- Positions: maximum when cutting field lines fastest, zero when moving along them.
- Speed: faster rotation raises both the peak and the frequency.
- Slip rings: keep the coil connected and give an alternating output.
One picture every idea returns to
An a.c. generator spins a coil in a magnetic field, inducing an alternating e.m.f. Slip rings keep the coil connected to the circuit as it turns. The e.m.f. is greatest when the coil sides cut the field fastest and zero when they move along the field, giving a sine-shaped output.
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, Spinning out electricity.. Turn a coil between the poles of a magnet and induction does the rest, pumping out a voltage that rises, falls, reverses and rises again. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Turn the coil, read the output | Rotate the coil and watch the alternating e.m.f. trace out its sine shape. Project tab 2 of the lab (A.c. generator) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Why the output alternates | As the coil turns, the sides cut the field first one way then the other, so the e.m.f. reverses each half turn. | Anticipation / Reaction Guide |
| 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 Why the output alternates, 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.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
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 |
|---|---|
| An a.c. generator uses slip rings, not a split-ring commutator. | The induced e.m.f. is greatest when the coil is moving fastest across the field lines, and falls to zero twice per turn as the coil moves along them. |
The A.C. generator: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Describe the basic structure of a simple a.c. generator, and state the job of the slip rings.
Sketch the shape of the output voltage from an a.c. generator as the coil makes one full turn, and state how many times the voltage reaches zero in that turn.
State the position of the coil when the induced e.m.f. is at its maximum, and explain why it is largest there.
At one instant the induced e.m.f. is zero, even though the coil is still spinning quickly. Explain how this is possible.
A generator coil is made to spin faster. Describe the effect on the peak voltage and on the frequency of the output, and state one other change that would increase the peak without changing the frequency.
A power station and a wind turbine both supply electricity. Explain what they have in common in terms of how the electricity is generated.
Answer key and marking notes
- A coil that rotates in a magnetic field between the poles of a magnet. ✓
- Slip rings and brushes connect the spinning coil to the external circuit. ✓
- They allow continuous rotation and give an alternating output. ✓
- A smooth wave: up to a peak, down through zero to a negative peak, then back to zero (one full cycle). ✓
- The voltage is zero twice in one full turn. ✓
- When the coil plane is parallel to the field (the sides moving straight across the field lines). ✓
- The sides cut the field lines at the fastest rate there, giving the largest e.m.f. ✓
- At that instant the coil sides are moving along the field lines, not across them. ✓
- So they cut no field lines, and the induced e.m.f. is zero even though the coil is moving fast. ✓
- The peak voltage increases. ✓
- The frequency increases (more cycles each second). ✓
- Using more turns, or a stronger magnet, raises the peak without changing the frequency. ✓
- Both spin a coil (or magnet) to produce a changing magnetic field through a coil. ✓
- This induces an alternating e.m.f. by electromagnetic induction; only the source of the spinning (steam, wind) differs. ✓
2 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 simple generator has a coil rotating in a magnetic field. What kind of current does it produce?
- Aalternating current
- Bdirect current only
- Ca steady current that never changes
- Dno current at all
In an a.c. generator, at which position is the induced e.m.f. greatest?
- Awhen the coil is parallel to the field, cutting field lines fastest
- Bwhen the coil is perpendicular to the field
- Cwhen the coil is stationary
- Dwhen the coil is exactly halfway between the poles and not moving
Quiz key
As the coil rotates, each side cuts field lines first one way then the other, so the induced e.m.f. reverses every half turn, giving alternating current. A split-ring commutator would be needed to produce d.c.
The induced e.m.f. is greatest when the coil is parallel to the field, because its sides are then moving across the field lines fastest, cutting them at the greatest rate. When the coil is perpendicular to the field, the sides move along the lines and the e.m.f. is momentarily zero.
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.