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A charged particle enters a uniform magnetic field with its velocity exactly perpendicular to the field. The path of the particle is:
AA helix about the field axis
BA straight line along B
CA circle in a plane normal to B
DA parabola opening along B
Answer & Solution
Correct answer: C. A circle in a plane normal to B
1. The magnetic force $q\mathbf{v} \times \mathbf{B}$ is always perpendicular to the velocity.
2. A constant-magnitude force perpendicular to velocity acts as a centripetal force.
3. With no velocity component along B, the motion stays in the plane normal to B.
4. Hence the particle moves in a circle (a helix would require a parallel velocity component).
_Source: NCERT Class 12 Physics Ch 4 "Moving Charges and Magnetism", p.138_
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