Concept:If a charge enters a magnetic field parallel to the field lines, the cross product of its velocity and the magnetic field is zero, resulting in no force.
Formula:$$ F = qvB \sin(\theta) $$
Solution:- "Parallel" implies the angle \( \theta \) between velocity \( \mathbf{v} \) and magnetic field \( \mathbf{B} \) is \( 0^\circ \) (or \( 180^\circ \)).
- Substitute this into the formula: \( \sin(0^\circ) = 0 \).
- The calculated magnetic force is therefore zero.
- According to Newton's First Law, with no net force, the particle continues moving in a straight line at constant speed.
Why other options are incorrect:Options A, B, and D describe deflections, which can only happen if a non-zero magnetic force acts on the particle (requiring a non-zero angle).
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