# Electromagnetic induction

> CIE A-Level Physics · 9702
> Source: https://www.owlsprep.com/study/cie-9702-u24-overview/
> Weight: n/a

This unit explores how changing magnetic fields generate electric current and e.m.f., the core principle behind generators, transformers, and most modern electrical technology.

**Prerequisites:** [Magnetic fields and forces on moving charges](https://www.owlsprep.com/study/cie-9702-u23-magnetic-fields-overview/)

## Learning objectives

- Define magnetic flux and flux linkage, and apply these quantities to induction problems
- State and correctly apply Faraday's law and Lenz's law of electromagnetic induction
- Calculate magnitude and direction of induced e.m.f. in a range of practical arrangements
- Explain self-inductance and calculate its effects in DC and changing current circuits

## Unit at a glance

This unit reverses the relationship you learned in the previous unit: instead of moving charges creating magnetic fields, we explore how changing magnetic fields create electric effects. We build from foundational definitions up to core laws and practical applications of induction, with each sub-topic relying on the concepts from the previous step.

We cover the following core sub-topics in this unit:
- [Magnetic flux](https://www.owlsprep.com/study/cie-9702-u24-magnetic-flux/) — Defines magnetic flux and flux linkage, the foundational quantities for all induction calculations.
- [Faraday's law and Lenz's law](https://www.owlsprep.com/study/cie-9702-u24-faraday-s-law-and-lenz/) — Covers the two core laws that describe the magnitude and direction of induced effects.
- [Induced e.m.f.](https://www.owlsprep.com/study/cie-9702-u24-induced-e-m-f/) — Walks through calculation of induced e.m.f. in rotating coils, moving conductors, and other common scenarios.
- [Self-inductance](https://www.owlsprep.com/study/cie-9702-u24-self-inductance/) — Introduces inductance as a circuit property and explores its effects when current changes.

## Common pitfalls

- **Wrong:** Assuming a large magnetic flux always produces an induced e.m.f.
  - Why it fails: Induced e.m.f. depends on change in flux over time, not the absolute magnitude of flux.
  - Correct: Always confirm that flux is changing before solving an induction problem.
- **Wrong:** Misapplying Lenz's law by opposing the original magnetic field instead of the change.
  - Why it fails: This leads to incorrect direction for the induced current and e.m.f.
  - Correct: Focus on opposing the change in flux, not the original magnetic field itself.

## Cheatsheet

| Concept | Key Formula/Rule |
| --- | --- |
| Magnetic flux | $\Phi = BA\cos\theta$ |
| Flux linkage (N turns) | $N\Phi = BAN\cos\theta$ |
| Faraday's Law | $\varepsilon = - \frac{d(N\Phi)}{dt}$ |
| Lenz's Law | Induced current opposes the change that produced it |
| Self-inductance definition | $\varepsilon = - L \frac{dI}{dt}$ |

## What's next

Begin this unit with the first sub-topic on magnetic flux to build the foundational understanding you need for all subsequent topics. Once you complete all sub-topics in this unit, move on to the next unit on alternating currents to apply your knowledge of induction to AC circuit concepts.

- [Magnetic flux](https://www.owlsprep.com/study/cie-9702-u24-magnetic-flux/)
- [Faraday's law and Lenz's law](https://www.owlsprep.com/study/cie-9702-u24-faraday-s-law-and-lenz/)
- [Induced e.m.f.](https://www.owlsprep.com/study/cie-9702-u24-induced-e-m-f/)

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