alleryStart learning →Why do charged particles often travel along magnetic field lines rather than cut across them?
AThe force is zero along the field
BThe force is largest along the field
CThe field drags charges along itself
DThe field speeds charges up along it
Answer & Solution
Correct answer: A. The force is zero along the field
1. The magnetic force carries a factor sin(theta), where theta is the angle between v and B.
2. For motion parallel to B that angle is zero, so sin(theta) is zero and the force vanishes.
3. Motion along a field line is therefore unopposed, while motion across the field is bent.
4. That is why charged particles often follow magnetic field lines rather than cross them.
5. A field that dragged charges along itself would do work on them, which a magnetic field never does.
_Source: OpenStax College Physics (CC BY 4.0), Ch 22 "Magnetism", section 22.4 Magnetic Field Strength: Force on a Moving Charge in a Magnetic Field_
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