---
title: "A long, straight, solid cylindrical wire of radius \\(R\\) carries a total current \\(I\\) that is uniformly distributed across its cross-sectional area. Which of the following expressions gives the magnitude of the magnetic field \\(B\\) at a distance \\(r\\) from the central axis of the wire, for \\(r < R\\)?"
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url: "https://nerd-notes.com/ubq/118499/"
date_modified: "2026-08-04T08:11:05+00:00"
---

# A long, straight, solid cylindrical wire of radius \(R\) carries a total current \(I\) that is uniformly distributed across its cross-sectional area. Which of the following expressions gives the magnitude of the magnetic field \(B\) at a distance \(r\) from the central axis of the wire, for \(r < R\)?

A long, straight, solid cylindrical wire of radius \(R\) carries a total current \(I\) that is uniformly distributed across its cross-sectional area. Which of the following expressions gives the magnitude of the magnetic field \(B\) at a distance \(r\) from the central axis of the wire, for \(r < R\)?

![A perspective view of a solid gray cylinder of radius R extending horizontally. A dashed circular loop of radius r, centered on the main longitudinal axis of the cylinder, is drawn inside the circular cross section at one end. A dashed line runs horizontally through the center of the cylinder. An arrow labeled I points horizontally along the length of the cylinder. A radial arrow extending from the central axis to the outer edge of the circular end is labeled R. A smaller radial arrow extending from the central axis to the dashed inner circle is labeled r. No other labels, lines, text, or axes appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1785831064-i1kCCY.jpg)

- **A.** \(\dfrac{\mu_0 I R}{2\pi r^2}\)
- **B.** \(\dfrac{\mu_0 I}{2\pi R}\)
- **C.** \(\dfrac{\mu_0 I}{2\pi r}\)
- **D.** \(\dfrac{\mu_0 I r}{2\pi R^2}\)

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/118499/*