Magnetism · AP Physics 2 · 6 min
Magnetic-force questions are vector questions first and magnitude questions second.
01 The trap
Wrong path
Students treat magnetic force like electric force and point it along the field, or forget to reverse direction for negative charge.
Why it feels right
Electric fields push charges along or against the field, so it is tempting to reuse that mental model for magnetic fields.
02 Correct model
For a moving charge, F = q v x B. The force is perpendicular to both velocity and magnetic field, has magnitude |q|vB sin θ, and is zero when v is parallel to B.
03 Mini-example
Prompt
A proton moves right through a magnetic field into the page. Which way is the magnetic force?
Common wrong answer
Into the page, along the field.
Correct reasoning
Up. For a positive charge, right cross into-page points upward.
Diagnostic cue
If a magnetic-force answer points along the magnetic field, it is almost certainly using the wrong model.
04 Guided practice
Warm-up isolates the principle. Bridge changes the context. Contest-style requires a complete setup on less familiar geometry.
A proton moves to the right through a uniform magnetic field into the page. Which way is the magnetic force?
Right cross into-page points upward.
The magnetic force is upward. For a proton, F follows v x B directly: right crossed with into the page gives up.
Trap: Pointing the force into the page because that is the direction of the magnetic field.
An electron moves upward through a magnetic field out of the page. Which way is the magnetic force on the electron?
For positive charge, up cross out-of-page points right.
For a positive charge, v × B would point right. An electron is negative, so the magnetic force points left.
Trap: Forgetting to reverse the right-hand-rule result for negative charge.
A charged particle enters a uniform magnetic field perpendicular to its velocity and moves in a circular path. If its speed doubles while q, m, and B stay fixed, what happens to the path radius?
|q|vB = mv²/r.
From |q|vB = mv²/r, the radius is r = mv/(|q|B). If speed doubles, the radius doubles.
Trap: Saying the radius halves because the magnetic force is larger, while missing that centripetal force also contains v²/r.
An electron moves upward on the page through a magnetic field directed into the page. What direction is the magnetic force on the electron?
For a positive charge, upward × into the page points left.
The force on a positive charge would point left, so the electron's force points right.
Trap: Using the positive-charge right-hand-rule result without reversing for a negative charge.