---
title: "A positron (a particle with the same mass as an electron but a positive charge \\(+e\\)) travels with constant velocity in the \\(+y\\)-direction. It enters a region of uniform magnetic field directed in the \\(-z\\)-direction (perpendicularly into the page), as shown. Which of the following diagrams best represents the direction of the initial magnetic force \\(\\vec{F}_B\\) exerted on the positron at the moment it enters the magnetic field?"
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url: "https://nerd-notes.com/ubq/123414/"
date_modified: "2026-09-28T11:58:27+00:00"
---

# A positron (a particle with the same mass as an electron but a positive charge \(+e\)) travels with constant velocity in the \(+y\)-direction. It enters a region of uniform magnetic field directed in the \(-z\)-direction (perpendicularly into the page), as shown. Which of the following diagrams best represents the direction of the initial magnetic force \(\vec{F}_B\) exerted on the positron at the moment it enters the magnetic field?

A positron (a particle with the same mass as an electron but a positive charge \(+e\)) travels with constant velocity in the \(+y\)-direction. It enters a region of uniform magnetic field directed in the \(-z\)-direction (perpendicularly into the page), as shown. Which of the following diagrams best represents the direction of the initial magnetic force \(\vec{F}_B\) exerted on the positron at the moment it enters the magnetic field?

![A coordinate system in the xy-plane where the x-axis points horizontally to the right and the y-axis points vertically upward, meeting at an origin. In the upper half-plane (y > 0), a rectangular bounded region contains a regular 3 by 3 grid of nine 'x' symbols, with a label \(\vec{B}\) in the upper-right corner. Below the region, on the y-axis at y < 0, a small circle labeled \(+e\) has a solid arrow pointing vertically upward along the y-axis toward the magnetic region, labeled \(\vec{v}\). No other labels, lines, text, or axes appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1790596706-n0hh2J.jpg)

- **A.** A vector pointing in the \(+x\)-direction
- **B.** A vector pointing in the \(-x\)-direction
- **C.** A vector pointing in the \(+z\)-direction
- **D.** A vector pointing in the \(+y\)-direction

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123414/*