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Concept · Electromagnetism
Magnetic 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
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Named traps
14 ways this goes wrong.
Each is a named misconception the questions are built to catch. Open one for the move that tempts, and the fix.
- 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.→
Key formulas
No formula sheet is matched to this concept yet. Work it from first principles, and watch the traps.
Shaky underneath?
If the concept underneath still feels loose, patch it here before coming back to CSCA questions.
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