Uses of the generator effect (HT only) — AQA GCSE Physics
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Uses of the generator effect (HT only) explained
Both machines spin a coil in a magnetic field, and the only real difference is how the coil is connected to the outside circuit.
Read the full explanation
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.
Students should be able to: explain how the generator effect is used in an alternator to generate ac and in a dynamo to generate dc draw/interpret graphs of potential difference generated in the coil against time.
A generator's output is never steady, because the rate at which the sides of the coil cut field lines changes as the coil turns. When the coil lies along the field its sides sweep straight across the field lines and the induced potential difference peaks; a quarter turn later those sides are moving along the lines, cutting none, and the output is zero. An alternator therefore traces a smooth curve that crosses the time axis twice per revolution, with one peak above the axis and one below. A dynamo traces the same humps, but the commutator flips the negative ones over, so every hump sits on the same side of the axis and the trace touches zero twice per revolution without crossing. Spin either faster and the peaks grow taller while the period gets shorter.
Your focus
- Identify an alternator and a dynamo from a diagram of their rings and brushes.
- Explain how a split-ring commutator turns a reversing induced potential difference into a one-way output.
- Decide from a potential difference against time graph whether the output came from an alternator or a dynamo, and give one use that needs each type of current.
Show all 6 objectives
- Sketch the potential difference against time output of an alternator and of a dynamo over two revolutions.
- Explain, using the position of the coil, why the output passes through zero twice in every revolution.
- Redraw a given output graph for a generator that is turned twice as fast.
Uses of the generator effect (HT only) exam tips
Marking Points
- one mark for both devices using a coil rotating in a magnetic field
- one mark for the alternator having slip rings and producing alternating current
- one mark for the dynamo having a split-ring commutator and producing direct current
- one mark for explaining that the commutator swaps the connections each half turn so the output does not reverse
- one mark for a smooth curve rather than straight lines or a square wave
- one mark for the alternator trace crossing the axis, with one positive and one negative peak per revolution
- one mark for the dynamo trace staying on one side of the axis and touching zero twice per revolution
- one mark for a faster rotation drawn as both taller peaks and a shorter period
- one mark for linking the zero output to the moment the coil sides move along the field lines rather than across them
Examiner 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.
- 💡Mark the revolutions on the time axis first, so that you draw the right number of peaks.
- 💡Sketch means a labelled shape rather than plotted points, but it must still start and finish in the right places.
- 💡If the question doubles the speed, change two things on the graph, the height and the period.
Common Mistakes
- 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
- drawing a dynamo output as a flat horizontal line, as though it were a battery
- drawing taller peaks for a faster rotation but keeping the period the same
- drawing a square wave, which suggests the potential difference switches instantly
- putting the peaks where the coil is at right angles to the field, which is where the output is zero
- giving the alternator an output that never goes below the axis