--- title: "Induced Electric Fields and Motional EMF" description: "A changing magnetic flux produces a real electric field that pushes charge round a loop, whether the flux changes because the field changes or because the circuit moves. Both readings give the same EM" canonical: https://lightmysky.com/learn/science/induced-electric-fields-and-motional-emf-mt_LpmHnz46tu source: https://lightmysky.com/learn/science/induced-electric-fields-and-motional-emf-mt_LpmHnz46tu.md retrieved: 2026-09-12 --- > **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`. # Induced Electric Fields and Motional EMF A changing magnetic flux produces a real electric field that pushes charge round a loop, whether the flux changes because the field changes or because the circuit moves. Both readings give the same EMF for a rod sliding on rails. Subject: Science · Area: Electricity & Magnetism · Ages 19 to 21 Page: https://lightmysky.com/learn/science/induced-electric-fields-and-motional-emf-mt_LpmHnz46tu ## Ready when they can - Computes the EMF of a rod moving on rails in two ways and shows they agree - Describes the induced electric field around a region of changing flux, with no conductor needed - Explains eddy current braking as induction plus Lenz's law ## Lesson: Motion, flux and induced fields Slide a rod of length l at speed v across a uniform field B and the charges feel a magnetic push that piles them at the ends. The EMF is B l v. The flux view agrees: the loop area l x grows, so the flux change per second is also B l v. Two readings, one EMF. **Example.** A 1.0 m rod moving at 3.0 m/s through Earth's 5.0e-5 T field makes only 150 microvolts. Scale up to a 20.0 km tether flying at 7.80 km/s and the same rule gives up to 7.80 kV when the motion is square to the field. With rails and resistance R the current is B l v over R, and the pushing power exactly matches the heat I squared R in the resistor. A changing flux also makes a real swirling electric field, with no wire needed at all. Its loop integral equals the EMF, which is nonzero, unlike the electrostatic field whose loop integral is zero. Eddy currents are this effect inside solid metal: loops of current whose fields fight the change, so a moving sheet or falling magnet brakes and its motion turns to heat. **Tip.** Only the perpendicular part counts: the max B l v needs motion square to the field, and tilted motion brings a sine factor. For direction, ask what fights the change. If flux into the page grows, the induced current circles to send field back out of the page. **Recap.** Moving rods and changing flux give the same EMF, induced fields need no wire, and eddy currents turn motion into heat. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Lenz's Law and the Alternating-Current Generator](https://lightmysky.com/learn/science/lenzs-law-and-the-alternating-current-generator-mt_69u-fTOSku) - [Ampere's Law: Wires, Solenoids and Toroids](https://lightmysky.com/learn/science/amperes-law-wires-solenoids-and-toroids-mt_uTzRcSiUQ-) ## Opens up - [Self-Inductance and Energy Stored in a Magnetic Field](https://lightmysky.com/learn/science/self-inductance-and-energy-stored-in-a-magnetic-field-mt_BLyLSJT-Fr) - [Displacement Current and Maxwell's Equations](https://lightmysky.com/learn/science/displacement-current-and-maxwells-equations-mt_YR8m8TNfyy)