---
title: "A very long, straight wire carries a constant current \\(I\\). At a radial distance \\(r\\) from the central axis of the wire, the magnitude of the magnetic field is \\(B\\). If the current in the wire is increased to \\(4I\\) and the radial distance from the axis is increased to \\(2r\\), what is the magnitude of the new magnetic field in terms of \\(B\\)?"
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url: "https://nerd-notes.com/ubq/116001/"
date_modified: "2026-08-03T07:27:47+00:00"
---

# A very long, straight wire carries a constant current \(I\). At a radial distance \(r\) from the central axis of the wire, the magnitude of the magnetic field is \(B\). If the current in the wire is increased to \(4I\) and the radial distance from the axis is increased to \(2r\), what is the magnitude of the new magnetic field in terms of \(B\)?

A very long, straight wire carries a constant current \(I\). At a radial distance \(r\) from the central axis of the wire, the magnitude of the magnetic field is \(B\). If the current in the wire is increased to \(4I\) and the radial distance from the axis is increased to \(2r\), what is the magnitude of the new magnetic field in terms of \(B\)?

![A cross-section of a long wire with current I directed out of the page, indicated by a dot in a circle. An Amperian loop of radius r is drawn as a dashed circle centered on the wire. A magnetic field vector B is shown tangent to the dashed circle at a point a distance r from the center.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-setup-1785742067-DCUE5p.jpg)

- **A.** \(0.5 B\)
- **B.** \(B\)
- **C.** \(2 B\)
- **D.** \(8 B\)

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/116001/*