A charge particle is moving on a helical path in a uniform magnetic field. The correct statement describing the velocity of this charge particle, is:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → The particle has non-zero components, of velocity along the parallel as well as perpendicular directions of magnetic field
The magnetic force \( q\,\vec{v}\times\vec{B} \) acts only on the velocity component perpendicular to the field, turning it in a circle. The component parallel to the field feels no force and stays constant, carrying the particle along the field. A circle combined with a steady drift along the field is a helix.
Option (2) The particle velocity is in a direction parallel to the direction of magnetic field — Incorrect. With velocity parallel to the field there is no force, and the particle moves in a straight line.
Option (3) The particle velocity is in a direction perpendicular to the direction of magnetic field — Incorrect. With velocity perpendicular to the field the particle moves in a circle, with no drift along the field.
Option (4) The particle is initially at rest while switching the magnetic field on — Incorrect. A charge at rest feels no magnetic force and stays at rest.
NCERT (2026-27):
Motion in a Magnetic Field – “… If velocity has a component along B, this component remains unchanged as the motion along the magnetic field will not be affected by the magnetic field. The motion in a plane perpendicular to B is as before a circular one, thereby producing a helical motion (Fig. 4.6).”