A charged particle enters a uniform magnetic field with its velocity exactly perpendicular to the field. The path of the particle is:

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_