---
title: "Magnetic Flux Density and the Force on a Current-Carrying Wire"
description: "The strength of a magnetic field is defined by the force it produces: F = BIL when the wire is at right angles to the field, and BIL sin theta otherwise. One tesla is one newton per amp per metre."
canonical: https://lightmysky.com/learn/science/magnetic-flux-density-and-the-force-on-a-current-carrying-wire-mt_zuqc4gjBVh
source: https://lightmysky.com/learn/science/magnetic-flux-density-and-the-force-on-a-current-carrying-wire-mt_zuqc4gjBVh.md
retrieved: 2026-09-12
---

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# Magnetic Flux Density and the Force on a Current-Carrying Wire

The strength of a magnetic field is defined by the force it produces: F = BIL when the wire is at right angles to the field, and BIL sin theta otherwise. One tesla is one newton per amp per metre.

Subject: Science · Area: Forces & Motion · Ages 16 to 18
Page: https://lightmysky.com/learn/science/magnetic-flux-density-and-the-force-on-a-current-carrying-wire-mt_zuqc4gjBVh

## Ready when they can

- Uses the left hand rule to give the direction of the force on a current-carrying wire
- Calculates the force for a wire set at an angle to the field and says when it becomes zero
- Defines the tesla from the defining equation rather than quoting it from memory

## Lesson: Pushing wires with magnetism

A wire carrying current in a magnetic field feels a force of B I L sin theta, where theta sits between the wire and the field. The force is largest at ninety degrees and zero when the wire runs parallel.

**Example.** A 5 cm wire carrying 20 A across a 1.5 T field feels 1.5 N at right angles. This push is strong enough to move the wire, and motors are built from loops that exploit it.

The direction comes from the left hand rule: field, current, then thumb for force. Use conventional current, since electron flow points the other way and would flip the answer.

**Tip.** Read the tesla from the defining equation: one tesla is one newton per amp per metre. Always check the angle first, since sin zero is zero.

**Recap.** Size comes from B I L sin theta, and direction from the left hand rule.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Motors & the Motor Effect](https://lightmysky.com/learn/science/motors-and-the-motor-effect-mt__7BVYUN180)
- [Discharging a Capacitor and the Time Constant](https://lightmysky.com/learn/science/discharging-a-capacitor-and-the-time-constant-mt_5sjDPudyad)

## Opens up

- [Charged Particles Moving in a Magnetic Field](https://lightmysky.com/learn/science/charged-particles-moving-in-a-magnetic-field-mt_evuhZ6cSfI)
- [The Biot-Savart Law and the Field of a Current Element](https://lightmysky.com/learn/science/the-biot-savart-law-and-the-field-of-a-current-element-mt_RRsHUaXrFq)