Explain why the lamps in a parallel circuit remain at the same brightness when one lamp is removed from the circuit, but the lamps in a series circuit all go out when one lamp is removed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student is comparing how lamps behave in series and parallel circuits. She notices that switching off one lamp in a parallel circuit does not affect the other lamps, but removing one lamp from a series circuit causes all the other lamps to go out.
Model answer (4 marks)
In a parallel circuit each lamp is connected across its own branch, so each receives the full supply voltage. Removing one lamp only opens that branch; the other branches remain closed and current can still flow, so the remaining lamps keep the same brightness.
In a series circuit all lamps share a single path for current. Removing one lamp breaks that path, so no current can flow through the circuit and all the lamps go out.
In a series circuit all lamps share a single path for current. Removing one lamp breaks that path, so no current can flow through the circuit and all the lamps go out.
Examiner tips
- Use the phrase ‘full supply voltage’ for parallel lamps
- Explain that a broken branch in parallel does not affect other branches
- State that a single path is required for current in series
- Show that breaking the path stops all current
Common mistakes
- Saying the lamps in parallel get the same current instead of the same voltage
- Claiming the series circuit still has voltage across the remaining lamps
- Forgetting to mention that the path is broken in series
Mark scheme (4 marks)
- In a parallel circuit, each lamp is connected across its own separate branch / the full potential difference of the supply
- Removing one lamp in a parallel circuit breaks only that branch, so current can still flow through the remaining lamps
- In a series circuit, all components share a single path / loop for current
- Removing one lamp in a series circuit breaks the single path, so no current can flow anywhere in the circuit and all lamps go out
Key terms in this question
series circuit · parallel circuit
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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