A student has a battery connected to three identical lamps. She first connects all three lamps in series. She then reconnects them so that all three lamps are in parallel. Compare the brightness of the lamps in the two arrangements and explain, in terms of voltage and current, why the brightness differs.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The lamps are brighter in the parallel arrangement.
1. In series the supply voltage is divided between the three lamps, so each lamp receives only 1/3 of the battery voltage.
2. In parallel each lamp receives the full battery voltage.
3. The current through each lamp is greater in parallel because the total resistance is lower and each lamp has the same resistance as in series.
4. Greater voltage and current in parallel means each lamp receives more power (P=VI), so the lamps are brighter.
1. In series the supply voltage is divided between the three lamps, so each lamp receives only 1/3 of the battery voltage.
2. In parallel each lamp receives the full battery voltage.
3. The current through each lamp is greater in parallel because the total resistance is lower and each lamp has the same resistance as in series.
4. Greater voltage and current in parallel means each lamp receives more power (P=VI), so the lamps are brighter.
Examiner tips
- State the brightness comparison first. Explain voltage division in series and full voltage in parallel. Mention current and power for each lamp.
- common_mistakes
- :
- Claiming the series lamps are brighter. Forgetting that current through each lamp is higher in parallel. Using incorrect formula for power or omitting units.
Mark scheme (5 marks)
- The lamps in the parallel arrangement are brighter than in the series arrangement (or: lamps in series are dimmer than in parallel)
- In the series circuit, the supply voltage is shared (divided) between the three lamps
- In the parallel circuit, each lamp receives the full supply voltage
- The current through each lamp is greater in the parallel arrangement than in the series arrangement
- Greater voltage (and current) through each lamp in parallel means more power is transferred, so the lamps are brighter
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Compare" question →
- Decode the mark scheme abbreviations →
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