Alternators and Dynamos
Revise Alternators and Dynamos for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) physics topic: an alternator uses slip rings to generate alternating current, while a dynamo uses a split-ring commutator to generate direct current.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
This is a Higher tier only topic, so there's no Foundation paper — only the Higher worksheet and mark scheme below.
Topic overview
An alternator produces alternating current and a dynamo produces direct current. Both use the generator effect, and the difference between them is one component. This is a Triple-only and Higher-only topic.
An alternator uses slip rings, which maintain a constant connection to each end of the coil. As the coil rotates the induced potential difference reverses direction every half turn, so the output is alternating current.
A dynamo uses a split-ring commutator instead. This swaps the connections every half turn, so although the induced potential difference reverses within the coil, the output always has the same polarity — giving direct current. On an oscilloscope the alternator gives a wave crossing the zero line; the dynamo gives a series of positive humps.
Revision notes
The alternator
An alternator uses slip rings, which maintain a constant connection to each end of the rotating coil.
As the coil turns, the induced potential difference reverses direction every half turn, so the output is an alternating current.
The dynamo
A dynamo uses a split-ring commutator instead of slip rings.
The commutator swaps the connections every half turn, so although the potential difference reverses within the coil, the output always has the same polarity. The result is direct current.
The output traces
On an oscilloscope, the alternator produces a wave that crosses the zero line, going alternately positive and negative.
The dynamo produces a series of humps all on the same side of the zero line. Recognising these two traces is frequently examined.
Key points
- An alternator produces alternating current.
- A dynamo produces direct current.
- An alternator uses slip rings.
- A dynamo uses a split-ring commutator.
- The commutator swaps connections every half turn.
- The alternator trace crosses the zero line.
Worked examples
Example 1
State the difference between an alternator and a dynamo. [2 marks]
Model answer
An alternator uses slip rings and produces alternating current (1 mark). A dynamo uses a split-ring commutator and produces direct current (1 mark).
Example 2
Explain how a split-ring commutator produces direct current in a dynamo. [3 marks]
Model answer
The commutator swaps the connections to the coil every half turn (1 mark). Although the induced potential difference reverses direction within the coil (1 mark), the output connections are swapped at the same moment, so the output always has the same polarity (1 mark).
Example 3
Describe how the oscilloscope traces of an alternator and a dynamo differ. [2 marks]
Model answer
The alternator produces a wave that crosses the zero line, going alternately positive and negative (1 mark). The dynamo produces a series of humps all on the same side of the zero line (1 mark).
Common mistakes
Confusing slip rings with a commutator.
Slip rings give ac; a split-ring commutator gives dc.
Saying a dynamo produces alternating current.
The commutator ensures the output is direct current.
Saying the potential difference does not reverse in a dynamo.
It does reverse within the coil; the commutator swaps the connections to compensate.
Mixing up the two oscilloscope traces.
Crossing the zero line means ac; humps on one side mean dc.
Exam tips
- Link slip rings to ac and commutator to dc.
- Explain the commutator as swapping connections, not changing the coil.
- Learn both oscilloscope traces.
- Remember this is Triple-only AND Higher-only.
Key terms
- Alternator
- A generator producing alternating current using slip rings.
- Dynamo
- A generator producing direct current using a commutator.
- Slip ring
- A ring maintaining a constant connection to the coil.
- Polarity
- Which output terminal is positive.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.