Physics HL · Chapter 20: Electromagnetic Induction
How to Read This Electromagnetic Induction Chapter
Set up a flux-first workflow so every induction problem starts from geometry and change, not formula hunting.
Estimated time: 14 minutes
Why Induction Is the Energy-Conversion Core of Physics
Chapter 20 answers a central engineering question: how do we convert mechanical motion into electrical energy and vice versa? Induction is the bridge. A changing magnetic flux through a conductor creates an emf, and that single idea underpins power stations, alternators, transformers, motors, induction cookers, and many sensing technologies.
Students usually struggle here when they memorize disconnected formulas. The reliable method is to track three linked quantities in order: flux, flux linkage, and rate of change of flux linkage. Once those are explicit, induced emf magnitude and direction become predictable, and later AC and transformer relations stop feeling like separate topics.
Learning Targets
By the End of Chapter 20 You Should Be Able To
- Derive and apply motional emf relations from Lorentz-force and flux-change viewpoints.
- Distinguish magnetic flux from magnetic flux linkage and apply Faraday's law with correct signs.
- Use Lenz's law to predict induced-current direction in moving-loop, moving-magnet, and changing-current setups.
- Model self-induction in RL circuits and interpret time constants, back emf, and magnetic energy storage.
- Derive AC generator sinusoidal output and connect peak values with RMS and average power.
- Apply ideal transformer turn-ratio relations and reason about power transfer limits in real systems.
Recommended Problem-Solving Workflow
Step 1: draw field direction and conductor orientation before equations. Step 2: write the flux expression Phi = BA cos(theta) for the region that matters. Step 3: identify what is changing with time (B, A, or theta, or any combination). Step 4: apply Faraday's law for magnitude, then Lenz's law for direction. Step 5: run a final energy check: where is energy coming from and where is it going?
No simulation is embedded in this orientation section because this stage is strategy setup rather than quantitative modeling. Interactivity begins in Section 20.1 where geometry-to-emf links can be manipulated directly.