A student has a series circuit containing a battery and two bulbs. The student reconnects the circuit so that the two bulbs are in parallel with each other. Describe how the current and potential difference change for each bulb when the circuit is changed from series to parallel.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
In a series circuit the two bulbs share the battery’s potential difference, so each bulb has only part of the total voltage across it. The same current flows through both bulbs.
When the bulbs are moved to parallel, each bulb is connected directly to the battery, so each one has the full supply voltage across it. The current from the battery splits, with a larger current flowing through each branch than the single current that passed through each bulb in series.
Because the parallel arrangement has a lower total resistance than the series arrangement, the battery supplies a greater total current. Consequently, each bulb in parallel receives a higher current than it did in series, making it brighter.
Thus, in parallel each bulb has the full potential difference and a larger current, whereas in series each bulb has only part of the voltage and the same smaller current.
When the bulbs are moved to parallel, each bulb is connected directly to the battery, so each one has the full supply voltage across it. The current from the battery splits, with a larger current flowing through each branch than the single current that passed through each bulb in series.
Because the parallel arrangement has a lower total resistance than the series arrangement, the battery supplies a greater total current. Consequently, each bulb in parallel receives a higher current than it did in series, making it brighter.
Thus, in parallel each bulb has the full potential difference and a larger current, whereas in series each bulb has only part of the voltage and the same smaller current.
Examiner tips
- Use the term "potential difference" for voltage; mention current splitting; note total resistance change; link higher current to brightness.
Common mistakes
- Saying the current is the same in parallel; confusing series and parallel voltage drops; ignoring that total current increases in parallel.
Mark scheme (4 marks)
- In a series circuit the potential difference is shared between the two bulbs, but in a parallel circuit each bulb has the full potential difference (of the supply) across it.
- In a series circuit the same current passes through both bulbs, whereas in a parallel circuit the current splits between the two branches.
- The total resistance of the parallel circuit is less than in the series circuit, so the total current drawn from the battery is greater in parallel.
- Each individual bulb in the parallel circuit is brighter than in the series circuit because each has the full supply potential difference across it (whereas in series the pd was shared).
Key terms in this question
series · parallel · potential difference · current
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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