# Explain why a straight conducting rod moving at constant velocity through a uniform magnetic field develops an electromotive force (emf) across its ends, and why this emf remains constant as long as the velocity and field are unchanged.

> IB DP Physics Higher Level (2023 syllabus) — D.4 Induction (HL only) · Explain · 4 marks

> A straight conducting rod of length L is oriented horizontally and moved horizontally at constant velocity v through a uniform magnetic field directed vertically downward. The rod's motion is perpendicular to both its length and the field direction.

## Mark scheme (4 marks)

1. The free (conduction) electrons in the rod experience a magnetic force because they are moving with the rod through the magnetic field.
2. This magnetic force causes electrons to accumulate at one end of the rod, creating a separation of charge (a potential difference) across the rod's ends.
3. An equilibrium is reached when the electric force due to the accumulated charge exactly opposes the magnetic force on further charge carriers, at which point the emf equals BLv.
4. Because v, B, and L are all constant, the equilibrium condition and hence the emf (= BLv) remain unchanged over time.

## Key terms

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

## 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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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-a-straight-conducting-rod-01bd6be6) · Published by Druglandscape Ltd.