What it covers: Fields in motion — magnetic fields, magnetic force on moving charges and current-carrying wires, magnetic fields created by currents, and electromagnetic induction (Faraday's and Lenz's laws).
Exam weight: About 12–15% of the AP Physics 2 exam.
The big question: How do magnetic fields exert forces on moving charge, and how does a changing magnetic flux induce an EMF in the reverse direction?
In the College Board CED: Unit 12: Magnetism and Electromagnetism (Topics 12.1–12.4) — the revised CED numbers Physics 2 units 9–15.
Key topics at a glance
Magnetic Fields
Field lines run from north to south pole outside a magnet. Magnetic field is a separate field from the electric field, exerting force only on moving charges.
Force on Moving Charges
F = qvB sinθ. Direction found with the right-hand rule; force is always perpendicular to both velocity and field. Produces uniform circular motion for charges moving perpendicular to B.
Force on Current-Carrying Wires
F = BIL sinθ. Same right-hand-rule logic, applied to current direction instead of a single charge's velocity.
Fields Created by Currents
A current-carrying wire creates a magnetic field that circles around it. Field strength increases with current, decreases with distance from the wire.
Magnetic Flux
Φ = B·A·cosθ. Measures how much field "passes through" a loop — depends on field strength, area, and orientation.
Faraday's Law
EMF = −ΔΦ/Δt. A changing magnetic flux through a loop induces an EMF, and therefore a current if the loop is part of a closed circuit.
Lenz's Law
The induced current always opposes the change in flux that caused it — a direct consequence of energy conservation.
Generators
Rotating a loop in a magnetic field continuously changes flux, inducing an alternating EMF — the basic principle behind electric generators.
The key terms you must know
Magnetic force (F = qvB sinθ) — force on a moving charge in a magnetic field, perpendicular to both velocity and field.
Right-hand rule — the technique for determining the direction of magnetic force or magnetic field.
Magnetic flux (Φ = B·A·cosθ) — the amount of magnetic field passing through an area.
Faraday's law (EMF = −ΔΦ/Δt) — a changing magnetic flux induces an EMF.
Lenz's law — the induced current opposes the change in flux that created it.
Electromagnetic induction — the general phenomenon of generating EMF/current from changing magnetic flux.
Key themes to remember
Magnetic force is fundamentally different from electric force. It only acts on MOVING charges, and it's always perpendicular to the charge's velocity — so it never speeds up or slows down a charge, only changes its direction.
Currents and magnetic fields are two sides of the same coin. Moving charges create magnetic fields, and magnetic fields exert force on moving charges — magnetism is fundamentally an electric phenomenon.
Induction is about CHANGE, not the field itself. A steady magnetic field through a stationary loop induces nothing — only a changing flux (from changing field, area, or orientation) induces an EMF.
Lenz's law is energy conservation wearing a magnetic disguise. If induced currents reinforced their own cause, you'd get free energy — physically impossible.
The right-hand rule shows up everywhere in this unit. Master it once and apply it consistently to force on charges, force on wires, and field direction from current.
Common exam traps
Magnetic force does no work. Since it's always perpendicular to velocity, magnetic force can never change a particle's speed or kinetic energy — only its direction.
Don't forget the angle in F = qvB sinθ or F = BIL sinθ. Force is maximum when velocity/current is perpendicular to the field (θ = 90°) and zero when parallel (θ = 0°).
For negative charges, reverse the right-hand-rule direction. The right-hand rule assumes positive charge; flip the resulting direction for electrons or other negative charges.
A loop sitting still in a constant magnetic field has zero induced EMF — students often assume any magnetic field nearby induces current, but only a CHANGING flux does.
Lenz's law gives direction, Faraday's law gives magnitude. Don't try to get both pieces of information from a single equation — use Faraday's law (with the negative sign as a reminder) for magnitude, then reason through Lenz's law separately for direction.
Increasing flux and decreasing flux induce currents in opposite directions — always re-derive the direction from Lenz's law rather than memorizing a single "clockwise" or "counterclockwise" answer.