Video narration
Watch a charged particle enter a region filled with a uniform magnetic field—straight, evenly spaced field lines. If the particle’s velocity points exactly along the field, nothing bends. But if it enters at an angle, split its velocity into two parts: one parallel to the field lines, and one perpendicular. The magnetic force—the Lorentz force—always acts at right angles to both the particle’s velocity and the magnetic field. That means it can’t push the particle forward or backward along the field. So the parallel component stays unchanged. But the perpendicular component feels a sideways force that keeps turning it. A constant sideways turn produces circular motion in the plane perpendicular to the field. Now combine the two motions: steady forward motion along the field plus circular motion around it. The result is a helix, spiraling around the magnetic field lines at a constant pitch.