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A positive charge moves with speed v parallel to a uniform magnetic field B. The magnetic force on the charge is:
AqvB, perpendicular to v
BqvB, directed along v
CZero, as v is parallel to B
DMaximum, along B
Answer & Solution
Correct answer: C. Zero, as v is parallel to B
1. Magnetic force magnitude: $F = qvB\sin\theta$.
2. When $\mathbf{v}$ is parallel to $\mathbf{B}$, the angle $\theta = 0^\circ$.
3. Then $\sin 0^\circ = 0$, so $F = 0$.
4. The vector product vanishes when velocity and field are parallel.
_Source: NCERT Class 12 Physics Ch 4 "Moving Charges and Magnetism", p.135_
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