# Explain why the brightness of a lamp decreases when a second identical lamp is connected in series with it, compared to when the first lamp operates alone across the same power supply of negligible internal resistance.

> IB DP Physics Standard Level (2023 syllabus) — B.5 Current and circuits · Explain · 4 marks

## Mark scheme (4 marks)

1. Connecting a second lamp in series increases the total resistance of the circuit.
2. Because the supply voltage is fixed (negligible internal resistance), the increased resistance causes the current through the circuit to decrease.
3. The voltage is now shared/divided between the two lamps, so the potential difference across the original lamp is less than the full supply voltage.
4. The power dissipated in the original lamp is reduced (P = IV or P = V²/R), so it is less bright.

## Key terms

- [resistance](https://www.gradenine.co.uk/glossary/resistance)
- [power](https://www.gradenine.co.uk/glossary/power)

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-brightness-of-a-44c01920) · Published by Druglandscape Ltd.