I keep getting “Circular and helical motion” questions wrong in Moving Charges & Magnetism. What's the trap?
using the total speed to find the circular radius when motion is helical.
The usual trap: using the total speed to find the circular radius when motion is helical. How to avoid it: v_perp sets radius; v_parallel sets pitch. Go back to the definition — A velocity component perpendicular to B produces circular motion, while a parallel component remains unchanged. Together they make a helix whose pitch depends on the parallel speed.
Key point
Circular and helical motion
Common mistake
using the total speed to find the circular radius when motion is helical.
Memory tip
v_perp sets radius; v_parallel sets pitch.
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