A student connects two identical lamps in parallel across a battery. Explain how the current and voltage in this parallel circuit compare to a circuit where only one of the same lamps is connected across the same battery.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
In a parallel circuit the voltage across each lamp is the same as the voltage across the single lamp – it equals the battery’s e.m.f. Because the voltage on each lamp is unchanged, each lamp carries the same current as it did when alone. However, the total current drawn from the battery is greater in the parallel arrangement because there are two separate paths for the current to flow, allowing more current to flow overall.
Examiner tips
- State the voltage equality first, then the current per lamp, finally the increased total current.
- Use the word ‘therefore’ to link unchanged voltage to unchanged lamp current.
- Mention the two separate paths to justify the higher total current.
Common mistakes
- Confusing the voltage across the battery with the voltage across each lamp.
- Saying the current in each lamp is higher instead of the same.
- Failing to explain why the total current increases (missing the two paths).
Mark scheme (4 marks)
- The voltage across each lamp in the parallel circuit is the same as the voltage across the single lamp (equal to the battery voltage / e.m.f.)
- Each lamp therefore carries the same current as the single lamp did (because the voltage across each lamp is unchanged)
- The total current drawn from the battery is greater in the parallel circuit than with a single lamp
- This is because there are now two separate paths for the current to flow, so more current can flow overall
Key terms in this question
parallel circuit · current · voltage
Related
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