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.
Students should be able to explain the difference between direct and alternating potential difference.
Direct potential difference keeps the same sign: one terminal stays positive, so charge in the circuit is pushed one way only. Cells, batteries and solar cells are dc sources, and on an oscilloscope a dc supply gives a horizontal line displaced from the zero line. Alternating potential difference reverses regularly: the terminals swap over 100 times a second, because the supply completes 50 full cycles each second, so the charge oscillates back and forth with no net drift. Its oscilloscope trace is a wave spending equal time above and below zero. The distinction matters because generators produce ac and transformers work only with ac, while chargers and electronic circuits need dc, which is why a laptop carries a power brick that converts one into the other.
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.
Show all 6 objectives
- Describe the difference between dc and ac in terms of whether the direction of the potential difference reverses.
- Sketch and label oscilloscope traces for a cell and for the mains, marking the zero line on each.
- Classify supplies such as a battery, a dynamo and a wall socket as dc or ac and justify each choice.
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
- one mark for direct potential difference acting in one direction only, with unchanging polarity
- one mark for alternating potential difference repeatedly changing direction or reversing polarity
- one mark for naming a source of each, such as a cell for dc and the mains or a generator for ac
- one mark for a dc oscilloscope trace as a horizontal line and an ac trace as a repeating wave
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.
- 💡Answer in terms of direction, not size; the mark scheme wants 'reverses' or 'changes direction'.
- 💡If an oscilloscope screen is given, mark the zero line first and then decide whether the trace crosses it.
- 💡Name a real source of each type, a cell and the mains, to secure the example mark.
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
- saying ac 'goes up and down' without stating that the direction of the potential difference reverses
- claiming dc is always smaller than ac, confusing size with direction
- drawing an ac trace entirely above the zero line so that it never reverses
- describing a battery as ac because its current can be switched on and off