# Explain why the terminal potential difference of a battery decreases when the current drawn from the battery increases.

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

> A battery has an electromotive force (EMF) of 9.0 V and a fixed internal resistance. When the battery is connected to an external circuit, the measured terminal potential difference is found to be less than 9.0 V, and this difference becomes larger as more current is drawn.

## Mark scheme (4 marks)

1. EMF is the total energy supplied per unit charge by the battery (or: the open-circuit potential difference across the battery terminals).
2. The battery has an internal resistance, so when current flows there is a potential difference (voltage drop) across the internal resistance.
3. The terminal potential difference equals EMF minus the voltage drop across the internal resistance (terminal pd = EMF − Ir).
4. As current increases, the voltage drop across the internal resistance (Ir) increases, so the terminal potential difference decreases further.

## Key terms

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

## 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)
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