Inside This Unit: The Full Breakdown
This unit covers magnetism and electromagnetic induction: magnetic fields, forces on moving charges and currents, magnetic flux, and Faraday’s and Lenz’s laws.
Why it matters
Magnetism connects electricity and motion and is central to generators and motors. Induction problems reward careful reasoning about changing flux.
Key concepts
- A magnetic field exerts F = qvB sinθ on a moving charge, perpendicular to v and B.
- A current-carrying wire feels F = BIL sinθ in a magnetic field.
- Magnetic flux Φ = BA cosθ; a changing flux induces EMF (Faraday’s law).
- Lenz’s law: induced current opposes the change in flux that created it.
Magnetic Forces
Moving charges and currents experience magnetic forces given by F = qvB sinθ and F = BIL sinθ, with direction from the right-hand rule. A charge moving perpendicular to a uniform field follows a circular path, since the magnetic force acts as a centripetal force.
Electromagnetic Induction
Magnetic flux is the field passing through an area. Faraday’s law says a changing flux induces an EMF proportional to the rate of change, and Lenz’s law fixes the induced current’s direction to oppose the change. Motional EMF (BLv) and generators apply these ideas.
AP exam tip
For induction, always identify what is changing the flux (B, A, or angle) first, then apply Faraday for magnitude and Lenz for direction.
Connections to other units
- Unit 3: Moving charges respond to and create fields.
- Unit 4: Induced EMF drives currents in circuits.
