# Explain why the brightness of a filament lamp decreases when a second identical filament lamp is connected in series with it in a circuit containing a battery of fixed electromotive force (EMF) and negligible internal resistance.

> IB DP Physics Higher 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. Since the EMF is fixed and resistance increases, the current in the circuit decreases (by Ohm's law / I = V/R).
3. In a series circuit the same (reduced) current flows through the original lamp.
4. Lower current through the original lamp means less power dissipated in it (P = I²R), so it is less bright.

## Key terms

- [electromotive force (EMF)](https://www.gradenine.co.uk/glossary/electromotive-force-emf)
- [resistance](https://www.gradenine.co.uk/glossary/resistance)

## Related

- [Revision notes for IB DP Physics Higher 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-5d7ff004) · Published by Druglandscape Ltd.