AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
The generator effect is used in an alternator to generate ac and in a…
Full specification wording
Uses of the generator effect (HT only): The generator effect is used in an alternator to generate ac and in a dynamo to generate dc.
Both machines spin a coil in a magnetic field, and the only real difference is how the coil is connected to the outside circuit.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 1 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Both machines spin a coil in a magnetic field, and the only real difference is how the coil is connected to the outside circuit. An alternator uses two slip rings, one joined to each end of the coil, with a brush pressing on each. Each end keeps its own ring all the way round, so as the coil turns and the induced potential difference reverses, the output reverses with it: alternating current. A dynamo uses a single split-ring commutator, a ring cut into two halves. Every half turn the halves change brushes, which swaps the connections at exactly the moment the induced potential difference in the coil reverses. The two reversals cancel in the external circuit, so the output never changes sign: direct current, although it still rises and falls in size.
What earns the marks
- The alternator having slip rings and producing alternating current.
- The dynamo having a split-ring commutator and producing direct current.
- Explaining that the commutator swaps the connections each half turn so the output does not reverse.
Common mistakes and how to put them right
Swapping the two devices over and giving slip rings to the dynamo.
Saying a dynamo produces a steady direct current, when the output rises and falls even though it never reverses.
Claiming the commutator stops the potential difference in the coil reversing, when the coil's own potential difference still reverses.
Describing a split ring as two separate rings.
Exam tips
- Count the rings in the diagram: two rings means alternating current, one ring in two halves means direct current.
- Write out alternating current and direct current once, then use ac and dc.
- Use the phrase external circuit when you explain what the commutator puts right.
Try the lesson
The start of the Uses of the generator effect (HT only): The generator effect is used in an alternator to generate ac and in a dynamo to generate dc. lesson, made from checked teaching notes for this specification point.
Have a go first
A student is comparing an alternator and a dynamo. Both devices use the generator effect. Which statement correctly describes the type of current they generate?
Listen
Both machines spin a coil in a magnetic field, and the only real difference is how the coil is connected to the outside circuit. An alternator uses two slip rings, one joined to each end of the coil, with a brush pressing on each. Each end keeps its own ring all the way round, so as the coil turns and the induced potential difference reverses, the output reverses with it: alternating current. A dynamo uses a single split-ring commutator, a ring cut into two halves. Every half turn the halves change brushes, which swaps the connections at exactly the moment the induced potential difference in the coil reverses. The two reversals cancel in the external circuit, so the output never changes sign: direct current, although it still rises and falls in size. A strong answer earns marks for these points. the alternator having slip rings and producing alternating current. the dynamo having a split-ring commutator and producing direct current. explaining that the commutator swaps the connections each half turn so the output does not reverse. Exam tip: Count the rings in the diagram: two rings means alternating current, one ring in two halves means direct current.
Key words
Say what each word means before you turn the card.
alternator
Try answering aloud or in your head before turning the card.
dynamo
Try answering aloud or in your head before turning the card.
slip rings
Try answering aloud or in your head before turning the card.
split-ring commutator
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See every definition
- alternator
- A machine that uses the generator effect to generate ac, spinning a coil in a magnetic field with two slip rings, one joined to each end of the coil.
- dynamo
- A machine that uses the generator effect to generate dc, spinning a coil in a magnetic field with a single split-ring commutator.
- slip rings
- Two rings, one joined to each end of the coil, each with a brush pressing on it, so each end keeps its own ring all the way round.
- split-ring commutator
- A ring cut into two halves, so that every half turn the halves change brushes and swap the connections at exactly the moment the induced potential difference in the coil reverses.
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