Covers: Magnetic forces, Biot–Savart and Ampère’s law. Quiz one topic at a time or take the whole unit.
The idea each topic turns on, and the mistake students make most often.
Key ideaF = qv × B is perpendicular to both v and B, so it changes the direction of motion but not the speed, and does no work.
Common mistakeUsing the right-hand rule for a negative charge without reversing the result.
Key ideaF = IL × B. Parallel currents attract and antiparallel currents repel.
Common mistakePointing the force along the field instead of perpendicular to it.
Key ideadB = (μ₀/4π) I dl × r̂ / r². Every current element contributes, and the cross product sets the direction.
Common mistakeForgetting that segments pointing straight toward or away from the point contribute zero (dl × r̂ = 0).
Key idea∮B · dl = μ₀Ienc. Choose a loop where B is constant and tangent to it, using the symmetry of the current.
Common mistakeUsing the total current when the loop encloses only part of it.
This is one of the 19 questions in the Unit 12 quiz.
A particle of mass and charge moves at speed perpendicular to a uniform magnetic field . What is the radius of its circular path?
The magnetic force supplies the centripetal force. Set them equal and solve for .
Answer: A.
The magnetic force provides the centripetal force: , so .
AP-style FRQs for this unit. Students score themselves point by point against a rubric and see a full solution.
