Direct and alternating potential difference — AQA GCSE Combined Science
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Direct and alternating potential difference explained
Mains electricity in the United Kingdom is an alternating current supply.
Read the full explanation
In an ac supply the potential difference repeatedly reverses direction, so the current changes direction many times each second. The frequency of 50 Hz means the supply completes 50 full cycles every second, reversing direction 100 times per second. The stated value of about 230 V is the root mean square potential difference, which is the value that transfers the same average power as a 230 V direct supply; the peak value is higher, about 325 V. By contrast, a direct supply such as a battery keeps potential difference and current in one direction. You should be able to describe the difference between direct and alternating potential difference, read the frequency and voltage values from a UK mains label, and relate frequency to the number of cycles per second.
Students should be able to explain the difference between direct and alternating potential difference.
Potential difference (p.d.) drives current and is measured in volts (V). In a direct p.d., the polarity stays fixed, so current flows one way around the circuit; a cell or battery gives direct p.d., and its voltage is steady, as shown by a horizontal line on a voltage–time graph. In an alternating p.d., the polarity swaps repeatedly, so current reverses direction; the mains supply in the UK alternates at about 50 Hz, completing 50 cycles each second, and its voltage–time graph is a wave crossing zero. The peak value of the UK mains is about 325 V, while the quoted value of 230 V is an average effect, not the peak. To explain the difference, state the direction of current, the shape of the voltage–time graph and a named source for each type.
Your focus
- State the frequency and approximate potential difference of the UK domestic electricity supply.
- Describe the difference between direct and alternating potential difference in terms of direction of current.
- Relate a frequency of 50 Hz to the number of cycles and direction reversals occurring each second.
Show all 6 objectives
- State that direct p.d. keeps a constant polarity while alternating p.d. reverses polarity repeatedly.
- Interpret voltage–time graphs to identify direct and alternating supplies.
- Relate the UK mains frequency of about 50 Hz and quoted 230 V to the alternating p.d. from the mains.
Direct and alternating potential difference exam tips
Marking Points
- State that mains electricity is an alternating supply in which the potential difference and current repeatedly reverse direction.
- Recall that the UK domestic supply has a frequency of 50 Hz, meaning 50 cycles each second.
- Recall that the UK domestic supply potential difference is about 230 V.
- Describe direct potential difference as one that keeps the current in a single direction, as from a battery.
- Interpret a simple graph of potential difference against time for an ac supply by identifying the frequency from the cycle length.
- Distinguish between the stated 230 V value and the higher peak value of the alternating waveform.
- Direct p.d. has constant polarity, so current flows in one direction only; a cell or battery is a common source.
- Alternating p.d. repeatedly reverses polarity, so current changes direction; the mains supply is a common source.
- A voltage–time graph for direct p.d. is a horizontal line, whereas for alternating p.d. it is a wave that crosses the zero line.
- UK mains alternating p.d. has a frequency of about 50 Hz, meaning 50 cycles per second.
- The quoted 230 V for UK mains is not the peak value; the peak is about 325 V.
- Explanations should link the p.d. pattern to the resulting current direction rather than merely naming the two types.
Examiner Tips
- 💡Learn the two UK mains figures together as a pair: 50 Hz and about 230 V, and be ready to state both.
- 💡When comparing ac and dc, always mention direction of current or potential difference rather than vague descriptions.
- 💡If a graph is given, count the cycles and read the time for one cycle to find frequency using frequency = 1 ÷ time period.
- 💡Use the phrase 'about 230 V' where appropriate, since the supply value is quoted as approximate.
- 💡Sketch both voltage–time graphs and label the axes with potential difference in V and time in s to make the contrast visible.
- 💡Use the phrase 'reverses direction' for alternating and 'one direction' for direct, then name a source for each.
- 💡If asked to compare, give one similarity (both measured in volts) and then two differences to secure the explanation.
Common Mistakes
- Error: saying the current in an ac circuit flows only one way. Correction: in an ac supply the current reverses direction repeatedly, completing 50 cycles per second at 50 Hz.
- Error: confusing frequency with the number of reversals per second. Correction: 50 Hz means 50 complete cycles per second, which is 100 direction reversals per second.
- Error: treating 230 V as the peak potential difference. Correction: 230 V is the value quoted for the UK supply and represents the effective value; the peak is higher, about 325 V.
- Error: describing direct current as simply 'safer' or 'steadier' without reference to direction. Correction: define direct supply by the fact that current flows in one direction only.
- Saying alternating current flows both ways at once; correct this by stating that the direction reverses repeatedly over time, so at any instant it flows one way.
- Treating 230 V as the peak mains voltage; correct this by noting that 230 V is an average value and the peak is about 325 V.
- Describing a battery as alternating because it can be recharged; correct this by stating that batteries supply direct p.d. and only the charging supply may be alternating.