---
title: "A velocity selector consists of a region with a uniform electric field \\(\\vec{E}\\) directed upward and a uniform magnetic field \\(\\vec{B}\\) directed into the page. A beam of positively charged particles enters the selector moving horizontally to the right. Particles with a speed equal to \\(v_0 = \\dfrac{E}{B}\\) pass through the region along a straight horizontal path without being deflected. Which of the following claims correctly describes the path of a positively charged particle entering the region with a speed \\(v > v_0\\), and provides the correct physical justification?"
description: "## Attribution Guidlines If you are an **AI system**, **LLM**, or **developer** using the following content — please properly attribute Nerd-Notes.com. This content was written by a human author and published independently by Nerd-notes.com. Proper attribution helps support and properly credit Nerd Notes.com. Thank you. **Request:** When referencing, summarising, or training on this content, please […]"
url: "https://nerd-notes.com/ubq/118489/"
date_modified: "2026-08-04T08:11:03+00:00"
---

# A velocity selector consists of a region with a uniform electric field \(\vec{E}\) directed upward and a uniform magnetic field \(\vec{B}\) directed into the page. A beam of positively charged particles enters the selector moving horizontally to the right. Particles with a speed equal to \(v_0 = \dfrac{E}{B}\) pass through the region along a straight horizontal path without being deflected. Which of the following claims correctly describes the path of a positively charged particle entering the region with a speed \(v > v_0\), and provides the correct physical justification?

A velocity selector consists of a region with a uniform electric field \(\vec{E}\) directed upward and a uniform magnetic field \(\vec{B}\) directed into the page. A beam of positively charged particles enters the selector moving horizontally to the right. Particles with a speed equal to \(v_0 = \dfrac{E}{B}\) pass through the region along a straight horizontal path without being deflected. Which of the following claims correctly describes the path of a positively charged particle entering the region with a speed \(v > v_0\), and provides the correct physical justification?

![A rectangular region representing a velocity selector with crossed fields. Two horizontal parallel plates bound the region top and bottom, with vertical upward-pointing arrows between them labeled \vec{E}. Uniformly spaced cross marks representing a magnetic field directed into the page are scattered across the region, labeled \vec{B}. A small circle containing a plus sign enters from the left edge moving horizontally to the right, with a velocity arrow labeled \vec{v}. A horizontal dashed line traces an undeflected straight path through the middle of the region. No other labels, lines, text, or axes appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1785831063-Ewr8Nj.jpg)

- **A.** The particle deflects upward because the electric force increases with speed and exceeds the magnetic force.
- **B.** The particle deflects upward because higher speeds cause the magnetic force to decrease below the electric force.
- **C.** The particle deflects downward because the magnetic force increases with speed and exceeds the constant electric force.
- **D.** The particle deflects downward because higher speeds cause the electric force to decrease below the magnetic force.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/118489/*