# A parallel circuit contains two branches. One branch contains a filament lamp and the other branch contains a fixed resistor. Explain what happens to the total resistance of the circuit and the current through the fixed resistor when the filament lamp is disconnected from the circuit.

> Edexcel A-Level Physics (9PH0) — 10.1 Stars, life cycles and cosmology · Explain · 5 marks

> A parallel circuit contains two branches connected to a 6 V battery. One branch contains a filament lamp with a resistance of 12 Ω and the other branch contains a fixed resistor with a resistance of 6 Ω.

## Mark scheme (5 marks)

1. When the lamp is disconnected, the circuit changes from parallel to series (or: only the fixed resistor branch remains).
2. The total resistance of the circuit increases.
3. Because in a parallel circuit the total resistance is less than the smallest individual resistance, so removing a branch always increases total resistance.
4. The potential difference across the fixed resistor remains 6 V (equal to the supply voltage).
5. Therefore the current through the fixed resistor does not change (stays the same), because the potential difference across it and its resistance are both unchanged.

## Key terms

- [parallel](https://www.gradenine.co.uk/glossary/parallel)
- [total resistance](https://www.gradenine.co.uk/glossary/total-resistance)
- [current](https://www.gradenine.co.uk/glossary/current)
- [fixed resistor](https://www.gradenine.co.uk/glossary/fixed-resistor)

## Related

- [Revision notes for Edexcel A-Level Physics (9PH0)](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/a-parallel-circuit-contains-two-branches-c74df3ba) · Published by Druglandscape Ltd.