Explain why adding a second lamp in parallel with the first lamp in a circuit causes the current drawn from the battery to increase, whilst the voltage across each lamp remains the same.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A torch is modified so that a second identical lamp is connected in parallel with the original lamp, both connected to the same battery.
Model answer (4 marks)
In a parallel circuit each lamp is connected directly across the battery, so each lamp receives the full battery voltage.
Adding a second lamp in parallel provides an additional path for current to flow.
The overall/total resistance of the circuit decreases when a second lamp is added in parallel.
Because the battery voltage stays the same but the total resistance decreases, the total current from the battery increases.
Adding a second lamp in parallel provides an additional path for current to flow.
The overall/total resistance of the circuit decreases when a second lamp is added in parallel.
Because the battery voltage stays the same but the total resistance decreases, the total current from the battery increases.
Examiner tips
- Use the word ‘parallel’ to show you understand the circuit layout.
- Show that each lamp sees the same voltage as the battery.
- Explain that adding a path reduces total resistance.
- Link the reduced resistance to increased current via Ohm’s law.
Common mistakes
- Saying the voltage across the lamps changes.
- Confusing series and parallel concepts.
- Failing to mention that the battery voltage remains constant.
Mark scheme (4 marks)
- In a parallel circuit, each lamp is connected directly across the battery, so each lamp receives the full battery voltage
- Adding a second lamp in parallel provides an additional path for current to flow
- The overall/total resistance of the circuit decreases when a second lamp is added in parallel
- Because the battery voltage stays the same but the total resistance decreases, the total current from the battery increases
Key terms in this question
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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