A positive charge moves with velocity $\vec{v}$ PARALLEL to a uniform magnetic field $\vec{B}$. The magnetic force on it is

1. The magnetic force magnitude is $F = qvB\sin\theta$, where $\theta$ is the angle between $\vec{v}$ and $\vec{B}$. 2. When $\vec{v}\parallel\vec{B}$, $\theta = 0$ and $\sin 0 = 0$. 3. So $F = 0$. The charge experiences no magnetic force when its velocity is parallel (or antiparallel) to the field. 4. Consequence: a charged particle moving along magnetic field lines is unaffected by the field — explains why charged particles streaming along Earth's magnetic field lines can reach the poles (aurora). 5. Option A is for $\theta = 90^\circ$. Options B and C correspond to specific intermediate angles ($30^\circ$, $45^\circ$). _Source: syllabus Physics Part 1, Ch 4, §4.2 (Magnetic Force — perpendicular dependence), p. 2–3._