# Explain why the terminal potential difference of a battery decreases as the external resistance in a circuit is decreased.

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

> A battery of electromotive force (emf) ε and internal resistance r is connected to an external resistor. The external resistance is gradually reduced.

## Mark scheme (4 marks)

1. The current in the circuit increases as external resistance decreases (from I = ε / (R + r)).
2. The internal resistance causes a voltage drop (lost volts) equal to Ir.
3. As current increases, the voltage drop across the internal resistance (Ir) increases.
4. Since terminal pd = ε − Ir, a larger Ir means the terminal pd is smaller.

## Key terms

- [terminal potential difference](https://www.gradenine.co.uk/glossary/terminal-potential-difference)

## 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)
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