Uses of the generator effect (HT only) — AQA GCSE Physics
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Uses of the generator effect (HT only) explained
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.
Read the full explanation
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
- 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 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
- 💡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
- 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