Fields in motion. Magnetic fields exert forces on moving charges and current-carrying wires, currents create their own magnetic fields, and changing magnetic flux induces an EMF — the principle behind generators, transformers, and electromagnetic induction.
Six ways to master Unit 4 — pick whichever fits how you like to study.
Four topics from the College Board CED, in order.
Unit 4 extends the field concept from Unit 2 into magnetism. You'll learn how magnetic fields exert forces on moving charges and current-carrying wires — always perpendicular to both the field and the motion, governed by the right-hand rule — and how, in reverse, moving charges and currents create their own magnetic fields.
The unit closes with electromagnetic induction: Faraday's law and Lenz's law, which describe how a changing magnetic flux through a loop induces an EMF and current. This unit makes up roughly 12–15% of the AP Physics 2 exam, with induction usually the most challenging topic for students.
Note: the College Board's revised Course and Exam Description numbers the AP Physics 2 units 9–15, continuing from AP Physics 1's units 1–8 — so this unit appears there as Unit 12: Magnetism and Electromagnetism, with topics 12.1–12.4. We number it Unit 4 since it's the fourth unit of the Physics 2 course.