12–15% of the AP exam. 10 key topics. 4 common errors analyzed.
Unit 12 (Magnetism and Electromagnetism) extends the AP Physics 2 curriculum. Emphasize conceptual justification: FRQ scoring rubrics award points for physical reasoning stated in precise sentences.
These are the most frequently penalized mistakes on AP exam FRQs and MCQs. Review each before your next practice session.
Inconsistent or incorrect right-hand rule application — three distinct RHR variants (force on moving charge, force on current-carrying wire, field curl around a wire) are each required; students who learn only one variant misapply it to the others, especially under 3D visualization pressure in MCQ or FRQ diagrams
Lenz's law direction errors — students identify that an EMF is induced but assign the wrong direction to the induced current; the most frequent mistake is forgetting that the induced current opposes the change in flux (not the flux itself), leading to reversed current direction for decreasing vs. increasing flux scenarios
Confusing magnetic force with electric force: magnetic force on a moving charge is always perpendicular to velocity (does no work, cannot change speed), while electric force is parallel to field and does work; students incorrectly calculate kinetic energy changes from magnetic forces
Flux calculation errors — dropping the cosθ factor when the field is not perpendicular to the surface, or failing to recognize that flux can be zero even in a non-zero field (when B is parallel to the surface plane)
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