Question

what can be caused of helical motion of a charged particle

Answer 1

Helical motion of charged particle in external magnetic field. When an electron moves in uniform external magnetic field, with velocity not perpendicular, I understand that the magnitude of force is only due to the perpendicular component of velocity and is perpendicular to both components of velocity

Answer 2

when we give the velocity to charge which is neither perpendicular and nor parallel to magnetic field , then there are two component of velocities first component vcosθ which is parallel to magnetic field ( this component trying to move the charge along the direction of magnetic field) . another component vsinθ, which is perpendicular to magnetic field ( this component trying to move the charge in circular path). Due to these to component of velocities charge moving in helical path.