---
title: "A velocity selector consists of two horizontal parallel plates producing a uniform downward electric field of magnitude \\(E\\) and a region of uniform magnetic field of magnitude \\(B\\) directed into the page, as shown in the figure. Ions of charge \\(+q\\) traveling horizontally to the right at speed \\(v_0\\) pass through the selector undeflected. The magnitude of the electric field is now increased to \\(E’ > E\\) while \\(B\\) remains constant. How does the speed required for undeflected motion change, and in which direction will negative ions of charge \\(-q\\) entering at the original speed \\(v_0\\) deflect?"
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url: "https://nerd-notes.com/ubq/118563/"
date_modified: "2026-08-04T08:11:23+00:00"
---

# A velocity selector consists of two horizontal parallel plates producing a uniform downward electric field of magnitude \(E\) and a region of uniform magnetic field of magnitude \(B\) directed into the page, as shown in the figure. Ions of charge \(+q\) traveling horizontally to the right at speed \(v_0\) pass through the selector undeflected. The magnitude of the electric field is now increased to \(E’ > E\) while \(B\) remains constant. How does the speed required for undeflected motion change, and in which direction will negative ions of charge \(-q\) entering at the original speed \(v_0\) deflect?

A velocity selector consists of two horizontal parallel plates producing a uniform downward electric field of magnitude \(E\) and a region of uniform magnetic field of magnitude \(B\) directed into the page, as shown in the figure. Ions of charge \(+q\) traveling horizontally to the right at speed \(v_0\) pass through the selector undeflected. The magnitude of the electric field is now increased to \(E' > E\) while \(B\) remains constant. How does the speed required for undeflected motion change, and in which direction will negative ions of charge \(-q\) entering at the original speed \(v_0\) deflect?

![A horizontal velocity selector schematic. Two parallel horizontal conducting plates are positioned one above the other, with the top plate labeled with positive charge symbols and the bottom plate labeled with negative charge symbols. Downward vertical arrows between the plates represent a uniform electric field, labeled \vec{E}. A grid of evenly spaced x symbols between the plates represents a uniform magnetic field directed into the page, labeled \vec{B}. A horizontal dashed line with a rightward arrow representing a particle beam enters from the left midpoint between the plates and travels horizontally to the right. No other labels, lines, text, or axes appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1785831082-ayjPvm.jpg)

- **A.** Required speed: Decreases; Deflection: Downward (toward the bottom plate)
- **B.** Required speed: Increases; Deflection: Upward (toward the top plate)
- **C.** Required speed: Increases; Deflection: Downward (toward the bottom plate)
- **D.** Required speed: Decreases; Deflection: Upward (toward the top plate)

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/118563/*