A student connects three identical lamps in a series circuit. The student then reconnects the same three lamps in a parallel circuit, using the same battery. Describe how the potential difference across each lamp and the current through each lamp change when moving from the series circuit to the parallel circuit.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
In the series circuit the battery voltage is divided between the three lamps, so each lamp has a lower potential difference than the supply voltage.
In the parallel circuit each lamp is directly across the battery, so each lamp has the full supply potential difference.
In series the same current flows through all three lamps, so the current through each lamp equals the total circuit current.
In parallel the total current from the battery splits between the three branches, so the current through each lamp is less than the total current.
Because the lamps are identical and each receives a higher potential difference in parallel, the current through each lamp is greater in the parallel arrangement than in the series arrangement.
In the parallel circuit each lamp is directly across the battery, so each lamp has the full supply potential difference.
In series the same current flows through all three lamps, so the current through each lamp equals the total circuit current.
In parallel the total current from the battery splits between the three branches, so the current through each lamp is less than the total current.
Because the lamps are identical and each receives a higher potential difference in parallel, the current through each lamp is greater in the parallel arrangement than in the series arrangement.
Examiner tips
- Use the word "potential difference" and "current" explicitly; show the change for each lamp.
- Explain that voltage divides in series but is the same in parallel.
- Mention that current is the same in series but splits in parallel.
- Relate the higher voltage in parallel to the higher current through each lamp.
Common mistakes
- Saying the current is the same in both circuits; forgetting that it splits in parallel.
- Confusing voltage across the whole circuit with voltage across each lamp.
- Not specifying that the lamps are identical, so the current change is due to voltage change.
Mark scheme (4 marks)
- In series, the potential difference is shared between the lamps; in parallel, each lamp has the full (supply) potential difference across it.
- So the potential difference across each lamp is greater in the parallel circuit than in the series circuit.
- In a series circuit the same current passes through every lamp; in a parallel circuit the total current splits between the branches so each lamp carries less than the total current.
- Because the potential difference across each lamp is greater in parallel (and the lamps are identical), the current through each individual lamp is greater in the parallel circuit than in the series circuit.
Key terms in this question
potential difference · current · series · parallel
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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