ACT SSC Physics — Unit 2
Magnetic Force — Flashcards & Quiz
Magnetic force acts on moving charges and current-carrying conductors in magnetic fields, and ACT SSC Physics Year 12 Unit 2 tests both the force on a single charged particle (F = qvB sinθ) and the force on a current-carrying wire (F = BIL sinθ). The right-hand rule determines force direction, and the circular-motion application to charged particles in uniform fields is a standard exam context.
Key Points
- Force on a moving charge: F = qvB sinθ, where q is charge, v is speed, B is field strength, θ is angle between v and B.
- Force on current-carrying wire: F = BIL sinθ, where I is current, L is wire length in the field.
- Right-hand rule: fingers point along v (or I), curl toward B, thumb gives direction of F on a positive charge.
- For negative charges, reverse the direction (or use the left hand).
- In a uniform magnetic field, a charged particle with velocity perpendicular to B moves in a circle with r = mv/(qB).
- Parallel current-carrying wires attract if currents are parallel, repel if antiparallel.
Common Mistakes to Avoid
- Forgetting the sinθ factor — force is zero when motion is parallel to B.
- Using the wrong hand for negative charges — right hand is for positive.
- Confusing magnetic force (always perpendicular to v) with electric force (along field lines).
- Claiming magnetic force does work on a charged particle — it doesn't, because F ⊥ v means F·v = 0.
- Missing that parallel currents attract — opposite of what you might guess from static charges.
Exam Strategy
BSSS Unit 2 magnetic force questions ask you to calculate force, radius of circular motion, or force direction. Method: (1) identify charge, velocity, current, or wire length, (2) apply F = qvB sinθ or F = BIL sinθ, (3) use the right-hand rule for direction, (4) for circular motion, set magnetic force = centripetal force to find r.
Revision Tip
Right-hand rule direction questions are spatial recall — drill a Revizi deck with 8+ scenarios until you can assign force direction without hesitation.
Last updated: March 2026