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Edexcel GCSE Physics · 1PH0
Edexcel 1PH0 · 13.3P Check the specification (PDF) (opens in a new tab)
A generator uses electromagnetic induction to transfer energy from movement to an electrical circuit. In a simple school model, a coil is turned between the poles of a magnet. As the coil rotates, its sides cut through the magnetic field, inducing a potential difference across the coil. If the circuit is complete, this drives a current. Neither an nor a needs an electrical supply to start that current: the coil is turned by an external source of mechanical energy.
Each side of the coil moves through the magnetic field in the opposite direction after half a turn. The induced potential difference therefore reverses, and so does the current in the external circuit. This is alternating current (a.c.).
The two ends of the rotating coil connect to separate, continuous slip rings. Stationary brushes touch the rings and carry the current into the external circuit. Because each brush stays connected to the same end of the coil, the rings do not undo the reversal caused by rotation. A generator arranged this way is an alternator.
An alternator’s slip rings maintain contact with the rotating coil without changing its connections; the induced output reverses every half-turn.
A dynamo also uses a rotating coil and electromagnetic induction. Its coil’s induced potential difference still reverses every half-turn, but a split-ring commutator replaces the two continuous slip rings. Each half of the commutator is connected to one end of the coil. As the coil completes each half-turn, the halves exchange contact with the stationary brushes. This swaps the coil’s connections to the external circuit at the same time as the induced potential difference reverses. The two reversals cancel at the output, so current in the external circuit flows in one direction: direct current (d.c.). Its size can still rise and fall during each turn; d.c. does not necessarily mean a steady current.
In a dynamo, the split-ring commutator swaps the coil’s external connections every half-turn, producing a one-direction, varying output.
The key difference is therefore not how the current is induced. Both devices use relative movement between a conductor and a magnetic field. It is how the rotating coil connects to the external circuit that determines whether the output reverses direction.
Rotating a coil in a magnetic field induces a potential difference. A complete circuit allows current to flow. The coil’s induced potential difference reverses every half-turn.
For an alternator, link the reversing motion of the rotating coil to an induced current that changes direction every half-turn.
For a dynamo, explain what the split-ring commutator does; do not describe its d.c. output as necessarily constant.
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Last updated
Electromagnetic induction
The production of a potential difference when a conductor moves through a magnetic field or the magnetic field through it changes.
Alternator
A generator that produces a current which repeatedly reverses direction.
Dynamo
A generator that produces a current flowing in one direction in the external circuit.
Slip rings
Two continuous rings that maintain electrical contact between a rotating coil and stationary brushes.
Split-ring commutator
A ring divided into two halves that swaps a rotating coil’s connections to the external circuit every half-turn.
Put your knowledge into practice — try past paper questions for Physics
Electromagnetic induction
The production of a potential difference when a conductor moves through a magnetic field or the magnetic field through it changes.
Alternator
A generator that produces a current which repeatedly reverses direction.
Dynamo
A generator that produces a current flowing in one direction in the external circuit.
Slip rings
Two continuous rings that maintain electrical contact between a rotating coil and stationary brushes.
Split-ring commutator
A ring divided into two halves that swaps a rotating coil’s connections to the external circuit every half-turn.