# A positive ion moves horizontally to the right with constant velocity through a region where both a uniform electric field and a uniform magnetic field are present. The electric field acts vertically downward and the magnetic field acts perpendicularly into the page. Explain how the magnitudes and directions of the two forces acting on the ion combine to allow it to travel in a straight line at constant velocity, and state what happens to the ion's path if it then enters a region where only the magnetic field remains.

> IB DP Physics Standard Level (2023 syllabus) — D.3 Motion in electromagnetic fields · Explain · 4 marks

## Mark scheme (4 marks)

1. The electric field exerts a downward force on the positive ion (F = qE, directed downward).
2. The magnetic force on the ion moving right in a field into the page acts vertically upward (by Fleming's left-hand rule / F = qv × B).
3. The two forces are equal in magnitude and opposite in direction, so the net force is zero and the ion travels in a straight line at constant velocity (velocity selector condition: qE = qvB, so v = E/B).
4. In the region with only the magnetic field, the ion follows a circular (curved) path because the magnetic force is always perpendicular to the velocity, providing a centripetal force but doing no work.

## 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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