Direct and alternating potential difference — AQA GCSE Physics
Test yourself on Direct and alternating potential difference with AQA GCSE practice questions.
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Direct and alternating potential difference explained
Mains electricity is alternating current: the potential difference of the supply repeatedly reverses, so charges in the wire move backwards and forwards rather than drifting one way.
Read the full explanation
A frequency of 50 Hz means the supply completes 50 full cycles every second, so one cycle lasts 1 ÷ 50 = 0.02 s and the current reverses direction 100 times a second. The UK domestic supply is about 230 V, the figure to quote whenever a question involves a plug, a socket or a household appliance. On an oscilloscope the trace is a repeating wave crossing zero twice per cycle, unlike the flat horizontal line from a cell. These two numbers, 50 Hz and 230 V, are the only mains values you are expected to recall.
Your focus
- State from memory the frequency and potential difference of the UK domestic mains supply.
- Calculate the time for one mains cycle and state how many times each second the current reverses.
- Recognise a mains supply on an oscilloscope trace from its repeated crossings of the zero line.
Direct and alternating potential difference exam tips
Marking Points
- one mark for naming the mains supply as alternating current, ac
- one mark for a frequency of 50 Hz
- one mark for a potential difference of about 230 V
- one mark for the time for one cycle found from 1 ÷ frequency, giving 0.02 s
Examiner Tips
- 💡Learn 50 Hz and 230 V as a pair; they appear as one-mark recall questions almost every series.
- 💡Convert between frequency and period with T = 1 ÷ f whenever an oscilloscope trace is supplied.
- 💡Write 'alternating' rather than 'changing', because that is the word that pairs with ac.
Common Mistakes
- quoting 240 V or 110 V as the UK mains potential difference
- giving the frequency in volts or the potential difference in hertz because units were not checked
- saying the mains current is constant because the appliance runs steadily
- counting one reversal per cycle rather than two when asked how often the current changes direction