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Almost every joule of electrical energy you use today was made by spinning a coil in a magnetic field. Power stations, wind turbines and hydroelectric dams all do the same thing: they use a source of movement to turn a coil, and the generator effect induces a potential difference that drives current out to the grid. This lesson, part of Topic P4 (Magnetism and magnetic fields) of OCR Gateway Science A, applies the generator effect to two real machines — the a.c. generator (alternator), which produces alternating current, and the d.c. dynamo, which produces a one-directional output — and explains the output–time graphs that distinguish them. It also looks at the moving-coil microphone, a neat everyday device that uses the generator effect to turn sound into electricity.
By the end of this lesson you should be able to describe how an a.c. generator and a d.c. dynamo work, explain why one produces alternating current and the other a one-directional output, sketch and interpret their output–time graphs, and explain how a moving-coil microphone uses the generator effect.
An a.c. generator, or alternator, produces alternating current — current that repeatedly reverses direction. It is built much like a motor, but run in reverse: instead of putting current in to get movement, you put movement in to get current out.
Its parts are:
As the coil spins, its sides cut through the magnetic field lines, inducing a potential difference (the generator effect). The crucial detail is how fast the sides cut the lines as the coil rotates:
And because each side of the coil moves up through the field for half a turn and then down for the next half, the induced p.d. reverses direction every half turn — giving alternating current. The slip rings keep each end of the coil connected to the same brush throughout, so the alternating nature of the induced p.d. is passed straight out to the circuit.
Exam Tip: An a.c. generator uses slip rings (complete rings), which keep the same coil-end on the same brush, so the alternating p.d. is passed straight out. The p.d. is maximum when the coil is horizontal (cutting fastest) and zero when vertical (moving along the lines).
A d.c. dynamo is almost identical to the a.c. generator, with one change: it uses a split-ring commutator (a ring split into two halves) instead of slip rings — exactly the commutator from the motor.
The split ring swaps the coil's connections over every half turn, at just the moment the induced p.d. would reverse. This swap cancels out the reversal, so the output is always in the same direction — a one-directional (d.c.) output. The output still rises and falls (it pulses), but it never goes negative: every "hump" is on the same side.
The bicycle dynamo that powers a bike lamp works on this principle (in practice often as an alternator with rectification, but at GCSE the split-ring d.c. dynamo is the model you must know).
| Feature | a.c. generator (alternator) | d.c. dynamo |
|---|---|---|
| Connection to coil | Slip rings (complete rings) | Split-ring commutator (split ring) |
| Output direction | Alternates (reverses each half turn) | One direction (never reverses) |
| Output–time graph | Goes positive and negative | All on the same side, pulsing |
Exam Tip: The only structural difference is slip rings (a.c.) versus a split-ring commutator (d.c.). The split ring reverses the connections every half turn, flipping the would-be-negative halves back up to give a one-directional output.
The shapes of the output graphs are a favourite exam topic. The a.c. generator gives a smooth wave that swings above and below zero (positive and negative), while the d.c. dynamo gives a series of humps all on the same side of the axis.
For both graphs, spinning the coil faster has two effects: the peaks become higher (a larger induced p.d., because the field lines are cut faster) and the peaks become closer together (a higher frequency, because each rotation takes less time). Using stronger magnets or more turns raises the height of the peaks but does not change their spacing.
Exam Tip: Spinning the coil faster makes the output peaks taller and closer together (bigger p.d., higher frequency). Stronger magnets / more turns make the peaks taller only. Be ready to redraw a graph for "the coil is turned faster".
A moving-coil microphone is a clever everyday use of the generator effect — it does the reverse job of a loudspeaker. Inside it, a small coil is attached to a thin diaphragm and sits in the field of a permanent magnet.
When a sound wave hits the diaphragm, the pressure variations make the diaphragm vibrate back and forth. Because the coil is attached to the diaphragm, the coil also moves in the magnetic field, cutting field lines. By the generator effect, this induces a potential difference across the coil that varies in step with the sound wave — louder sounds move the coil more and induce a larger p.d.; the frequency of the sound sets the frequency of the induced p.d. The microphone therefore converts the pressure variations of a sound wave into an electrical signal that mirrors the sound.
graph LR
A[Sound wave] --> B[Diaphragm vibrates]
B --> C[Coil moves in magnetic field]
C --> D[Field lines cut - generator effect]
D --> E[Varying p.d. induced = electrical signal]
Exam Tip: A moving-coil microphone uses the generator effect: sound vibrates a diaphragm, which moves a coil in a magnetic field, inducing a varying p.d. that matches the sound. (Compare with the loudspeaker in lesson 7, which uses the motor effect to do the opposite.)
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