ConceptElectromagnetism
← All conceptsMagnetic field & forces
magnetic induction B, field lines/right-hand rule, Ampere force F=BIL sin(θ), Lorentz force f=qvB sin(θ) (does no work, perpendicular to v), charged-particle circular motion qvB=mv2/r so r=mv/qB and T=2·π·m/(qB) (independent of speed), field of a straight wire and superposition
14 ways this goes wrong. Each one is a named misconception the questions are built to catch.
- Charge sign ignored for forceGives the same deflection direction for a positive and a negative charge moving identically through the field.→
- Circular period assumed speed-dependentThinks a faster charge takes longer (or shorter) per revolution, missing that T=2·π·m/(qB) is independent of speed and radius.→
- F=BIL ignores angleOmits the sin(θ) factor, so a wire parallel to the field is wrongly assigned a nonzero force.→
- Field circles wire wrong wayReverses the circular field direction around a straight current-carrying wire.→
- Flux density vs flux confusedUses magnetic flux density B and magnetic flux Φ interchangeably, ignoring the area factor.→
- Force on stationary chargeAssigns a magnetic force to a charge sitting at rest in a magnetic field.→
- Force parallel to B or IDirects the magnetic force along the field or along the current instead of perpendicular to both.→
- Magnetic force does workBelieves the Lorentz force speeds up a charge, when it is perpendicular to velocity and does no work (only changes direction).→
- Parallel currents repelBelieves parallel currents in the same direction repel (they attract) and antiparallel ones attract.→
- Right-hand rule misappliedGets the force or field direction wrong by using the wrong hand/fingers or swapping the roles of current and field.→
- Straight path in uniform BPredicts a straight or parabolic path for a charge moving perpendicular to a uniform B field, where the path is circular.→
- Wire fields added as scalarsAdds the magnetic fields from two current-carrying wires by magnitude without accounting for their directions at the point.→
- f=qvB ignores angleOmits sin(θ), so a charge moving along the field is given a nonzero magnetic force.→
- r=mv/qB invertedInverts the radius relation, e.g. writes r=qB/(mv), mismatching how the radius scales with speed, mass, and field.→
