---
title: "Lenz's Law and the Alternating-Current Generator"
description: "An induced current always flows so as to oppose the change that produced it, which is conservation of energy written out for induction. A coil turning steadily in a field therefore gives an emf that v"
canonical: https://lightmysky.com/learn/science/lenzs-law-and-the-alternating-current-generator-mt_69u-fTOSku
source: https://lightmysky.com/learn/science/lenzs-law-and-the-alternating-current-generator-mt_69u-fTOSku.md
retrieved: 2026-09-12
---

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# Lenz's Law and the Alternating-Current Generator

An induced current always flows so as to oppose the change that produced it, which is conservation of energy written out for induction. A coil turning steadily in a field therefore gives an emf that varies as a sine curve.

Subject: Science · Area: Energy · Ages 17 to 18
Page: https://lightmysky.com/learn/science/lenzs-law-and-the-alternating-current-generator-mt_69u-fTOSku

## Ready when they can

- Predicts the direction of an induced current and names the change it opposes
- Argues that the opposite sign would let a machine make energy from nothing
- Relates the peak emf and the frequency of a generator output to how the coil is turning

## Lesson: Fighting change, spinning power

Lenz fixes the direction: the induced current flows so its own field fights the change that made it. Push a north pole in and the near face turns north to repel it. Pull out and it flips south to hold back. No change means no current at all.

The minus sign in Faraday's law is this fight in symbols. Flip the sign and a magnet would be sucked in and spat out while lighting bulbs: free energy from nothing. Since that cannot happen, Lenz is an energy argument, not just a rule.

A coil turning steadily in a field gives output that swings as a sine curve. Spin rate sets the frequency, and turns, area, field, and spin rate together set the peak emf. Double the spin and both the frequency and the peak double.

**Recap.** Fight the change for direction, bill it to energy, and spin faster for taller faster output.

## Practice

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

## Needs first

- [Transformers and the National Grid](https://lightmysky.com/learn/science/transformers-and-the-national-grid-mt_ExV4Kc_7_P)
- [Magnetic Flux, Flux Linkage and Faraday's Law](https://lightmysky.com/learn/science/magnetic-flux-flux-linkage-and-faradays-law-mt_ZoN5JePCpt)

## Opens up

- [Induced Electric Fields and Motional EMF](https://lightmysky.com/learn/science/induced-electric-fields-and-motional-emf-mt_LpmHnz46tu)