Unit 5: Magnetism and Electromagnetic Induction

AP Physics 2: 51 practice questions with detailed explanations.

Unit Study Guide

Executive Summary

Moving charges feel magnetic forces; changing magnetic flux creates electric fields — magnetism and electricity are two faces of one phenomenon.

Magnetic forces

$F = qvB\sin\theta$ on moving charges; $F = ILB\sin\theta$ on current-carrying wires. Right-hand rules give directions. Charges circle in uniform fields ( $r = \frac{mv}{qB}$ ) — the physics of mass spectrometers and auroras.

Fields from currents

Straight wires make concentric fields ( $B = \frac{\mu_0 I}{2\pi r}$ ); solenoids make uniform interior fields. Parallel currents attract, antiparallel repel.

Induction

Faraday's law: changing magnetic flux induces an EMF ( $\mathcal{E} = -N\frac{\Delta\Phi}{\Delta t}$ ). Lenz's law fixes the direction: induced effects oppose the change causing them. Generators, transformers, and induction cooktops all run on this.

Exam traps

Magnetic forces do NO work — they change direction, not speed. Use the RIGHT-hand rule for moving positive charges; flip for electrons. Lenz's law is about opposing the CHANGE, not the field itself. A constant field induces nothing — only change does.

Top 5 Concepts to Master

1Compute magnetic forces and their directions.
2Find fields from wires and solenoids.
3Apply Faraday’s law to changing flux.
4Use Lenz’s law for induced current direction.
5Analyze transformers and generators.

Key Terms & Definitions

Practice with Flashcards

Magnetic force

F = qvB sinθ (moving charge); F = ILB sinθ (wire).

Right-hand rule

Direction convention for magnetic forces and fields.

Magnetic flux

Φ = BA cosθ.

Faraday’s law

ε = −NΔΦ/Δt.

Lenz’s law

Induced effects oppose the flux change.

Solenoid

Coil producing a uniform interior field.

Transformer

Device changing AC voltage via turns ratio.

Common Misconceptions: Exam Traps

Magnetic forces change a particle’s speed.

Correct: They are perpendicular to motion and do zero work.

A constant magnetic field through a loop induces current.

Correct: Only a CHANGING flux induces EMF.

Lenz’s law opposes the magnetic field.

Correct: It opposes the change in flux.

The left-hand rule works for electrons.

Correct: Use the right-hand rule and flip direction for negative charge.

Question Bank Breakdown

By difficulty

easy 20medium 21hard 10

By topic

Magnetic Fields and Forces 16Magnetic Force on Moving Charges 9Faraday's Law and Lenz's Law 8Electromagnetic Induction 7Inductance and LC Circuits 2