Physics With Kate
Empowering Girls in Physics
Spec-aligned revision resources, group courses, and 1:1 tutoring for GCSE and A Level Physics — built by an experienced teacher and examiner.
AQA GCSE Physics · Topic 4.7 Magnetism and Electromagnetism · physics (triple) only
The Generator Effect: Alternators, Dynamos and Microphones
In this lesson you'll learn to: explain the generator effect (force → cutting the field → induced p.d.),
explain how an alternator makes a.c. and a dynamo makes d.c., link where the coil is to a maximum or
zero induced p.d., say what turning the coil faster or slower does to the trace, and explain how a
microphone turns sound into an electrical signal.
▶
Prefer to watch? Scroll to the bottom for the video on this topic
▶
A video walkthrough is coming soon — it will appear at the bottom of this page
▶
Start here — have a play
Don't read anything yet. On tab 1, push the wire and watch the needle, then hold it still. On tab 2, follow one side of the coil: when is the trace at a peak, and when is it zero? Press Show maximum p.d. and Show zero p.d., swap to the dynamo, then try Faster and Slower. Tab 3 is the microphone.
- A force acts on the wire, so it moves
- The wire cuts the magnetic field of the magnet
- A p.d. is induced across the ends of the wire
- This is called the generator effect
Motor effect: current in → force out. Generator effect: force (movement) in → p.d. out.
Generator
Speed
Watch one side of the coil. Moving up or down, at right angles to the field lines, it cuts them fastest: maximum p.d. A quarter of a turn later it moves along the field lines and cuts none: zero p.d.
Sound
The microphone: sound → electrical signal
- Sound waves are a series of high and low pressure, which create a force on the diaphragm that keeps changing direction
- The diaphragm vibrates, so a force acts on the coil and it moves
- The coil cuts the magnetic field of the permanent magnet
- A p.d. is induced across the ends of the coil
- The coil moves back and forth, so the induced p.d. keeps changing direction
- The p.d. has the same pattern as the sound wave: sound → an electrical signal
A microphone is a loudspeaker in reverse. The loudspeaker uses the motor effect (current in → movement); the microphone uses the generator effect (movement in → p.d.).
Slowed right down so you can see it. A power-station alternator turns 50 times a second.
▲ Three tabs: the generator effect, the alternator and dynamo, then the microphone.
Seen the peaks and the zeros? Now learn the chain.
Every generator answer uses the same three links. The notes below have them, plus the alternator, the dynamo and the microphone.
✎
Revision notes
The generator effect
Held still: zero p.d. Pushed: a p.d. is induced.
- A force acts on the wire, so it moves
- The wire cuts the magnetic field of the magnet
- A p.d. is induced across the ends of the wire
- This is called the generator effect
| What goes in | What comes out | |
|---|---|---|
| Motor effect | a current | a force (movement) |
| Generator effect | a force (movement) | an induced p.d. |
⭐ The key words
force · cuts · magnetic field · p.d. is induced · across the ends of the wire (or coil). No movement means no cutting, so no p.d., even with the wire right inside the field.
The a.c. generator (alternator)
A magnet · B coil · C slip rings · D brush · E axle · F lamp
- generator effectA force acts on the coil, so it turns
- generator effectThe sides of the coil cut the magnetic field from the magnets
- generator effectA p.d. is induced across the ends of the coil
- the generatorEach side cuts the field moving up, then down: every half turn it cuts the field in the opposite direction, so the induced p.d. changes direction every half turn: a.c.
- the generatorThe slip rings and brushes connect the turning coil to the circuit, completing the circuit (and stopping the wires getting tangled), so an a.c. current flows in the lamp
Alternator or dynamo?
Alternator → a.c. Each end of the coil has its own slip ring, so the connections never swap.
Dynamo → d.c. A split-ring commutator swaps the connections every half turn.
| Alternator | Dynamo | |
|---|---|---|
| Connection | two slip rings | a split-ring commutator |
| Output | alternating p.d. (a.c.): changes direction every half turn | direct p.d. (d.c.): always in one direction |
| Trace | positive and negative | never negative: humps all on one side |
Where is the coil when the p.d. is a maximum, or zero?
Look at the left-hand side of the coil in the alternator picture above: it is moving up, at right angles to the field lines, so it is cutting them. Seen end-on:
P: coil parallel to the field. The sides move at right angles to the field lines and cut them fastest: maximum induced p.d.
Q: a quarter of a turn later, coil at right angles to the field. The sides move along (parallel to) the field lines and cut none: zero induced p.d.
Turning the coil faster or slower
Normal speed
Half the speed: lower peaks, further apart
Twice the speed: higher peaks, closer together
| Coil turns… | Induced p.d. (peaks) | Frequency | Time period |
|---|---|---|---|
| faster: cuts the field at a higher rate | bigger: higher peaks | higher: peaks closer together | shorter |
| slower: cuts the field at a smaller rate | smaller: lower peaks | lower: peaks further apart | longer |
The microphone
- start: the soundSound waves are a series of high and low pressure, which create a force on the diaphragm that keeps changing direction
- generator effectThe diaphragm vibrates, so a force acts on the coil and it moves
- generator effectThe coil cuts the magnetic field of the permanent magnet
- generator effectA p.d. is induced across the ends of the coil
- the signalThe coil moves back and forth, so the induced p.d. keeps changing direction
- the signalThe p.d. has the same pattern as the sound wave: sound → an electrical signal
⭐ Microphone and loudspeaker
They are built the same way. The loudspeaker uses the motor effect (current in → movement). The microphone uses the generator effect (movement in → p.d.).
✍
How to write it in the exam
Every generator answer starts the same way
A force acts on the wire (or coil), so it moves →
it cuts the magnetic field →
a p.d. is induced across its ends.
the forcethe cuttingthe induced p.d.
Workbook page 13 · the generator effect · 4 marks
- (a) The wire is held still between the poles. State the induced p.d. [1]: zero (no cutting).
- (b) The wire is pushed to the right. Explain why a p.d. is induced across its ends. [3]: a force acts on the wire, so it moves (1); the wire cuts the magnetic field of the magnet (1); so a p.d. is induced across the ends of the wire (1). This is the generator effect.
Workbook page 13 · explain how the generator produces a.c. · 5 marks
- A force acts on the coil, so it turns (1)
- The coil cuts the magnetic field from the magnets (1)
- A p.d. is induced across the ends of the coil (1)
- Each side of the coil cuts the field moving up, then down, so the induced p.d. changes direction every half turn (1)
- The slip rings and brushes complete the circuit, so an a.c. current flows in the lamp (1)
Workbook page 13 · explain how a microphone turns a sound into an electrical signal · 6 marks
Sound waves are a series of high and low pressure, which create a force on the diaphragm that keeps changing direction. The diaphragm vibrates, so a force acts on the coil and it moves. The coil cuts the magnetic field of the permanent magnet, so a p.d. is induced across the ends of the coil. The coil moves back and forth, so the induced p.d. keeps changing direction. The p.d. has the same pattern as the sound wave, so the sound becomes an electrical signal.
Workbook page 12 · exam-style question · 9 marks
One graph shows the induced p.d. from an alternator during one turn of its coil. The other shows the induced p.d. from a dynamo.
graph 1
graph 2
- (a) Which graph comes from the dynamo? Give a reason. [2]: graph 2 (1); the induced p.d. is never negative (never changes direction), so it is d.c. (1)
- (b) Explain how the dynamo gives this output. [2]: a split-ring commutator swaps the connections every half turn (1), so the induced p.d. in the circuit does not reverse (1)
- (c) The two points on graph 1 where the coil is in position P. [1]: A and C (both needed)
- (d) Explain why the induced p.d. is zero at B. [2]: the coil is in position Q: its sides move along the field lines (1), so they do not cut the field lines, and no p.d. is induced (1)
- (e) One turn takes 0.020 s. Calculate the frequency of the a.c. [2]: f = 1 ÷ T = 1 ÷ 0.020 = 50 Hz
📘 Now do it in your workbook
Page 12: the alternator and dynamo, draw the trace at half and twice the speed, and the 9-mark graph question. Page 13: the generator effect, label the a.c. generator and explain it, then the microphone.
AQA GCSE Physics Workbook · MAGNETISM AND ELECTROMAGNETISM · pages 12 and 13The full workbook covers the whole Magnetism and Electromagnetism topic, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
▶
Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
🎬
Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Pages 12 and 13 of the workbook have the alternator and dynamo, the traces to draw, the generator to label and the microphone, with full answers at the back.
Proud member · Accredited · Trusted partners
© 2026 Physics With Kate · All rights reserved