--- 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: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # 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 Use F = B I L sin theta to calculate the motor effect. F is force, B field strength, I current and L wire length in the field. Theta is the wire-field angle. In degree mode, sin 0 = 0 and sin 90 = 1. Multiply these factors. With B, I and L fixed, force is largest at 90 degrees and zero when parallel. **Example.** A 20 cm wire carrying 4 A at right angles to a 0.25 T field feels a force of 0.25 times 4 times 0.2, which is 0.2 N. Convert centimetres to metres first. Motors use loops of wire so that this push turns a coil. The direction comes from Fleming's left hand rule. Hold the thumb, first finger and second finger at right angles. Point the first finger along the field and the second finger along the conventional current, and the thumb gives the force. Use conventional current, since electron flow points the other way and would flip the answer. At right angles, F = B I L, so B = F/(I L): divide force by current times length. One tesla gives one newton on one metre carrying one amp, or one newton per amp per metre. Current is charge per second, not charge alone. Keep both I and L in the denominator. **Recap.** Size comes from B I L sin theta, and direction from the left hand rule. *Original wire-force lesson and questions written for Marble.* ## 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) ## Optional connections - [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)