If charge is moving parallel to uniform magnetic field, its path will be:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → straight line
The magnetic (Lorentz) force on a moving charge depends on the angle \( \theta \) between the velocity and the field.
\( \displaystyle F = q v B \sin\theta \)
For motion parallel to the field, \( \theta = 0^\circ \), so \( F = 0 \). With no force, the charge keeps its velocity and moves in a straight line.
Why the other options are incorrect
Option (2) circular — Incorrect. A circular path needs the velocity perpendicular to the field (\( \theta = 90^\circ \)).
Option (3) helical — Incorrect. A helical path needs the velocity at an angle between \( 0^\circ \) and \( 90^\circ \) to the field, so that one component is parallel and one perpendicular.
Option (4) elliptical — Incorrect. A uniform magnetic field never gives an elliptical path; the force is always perpendicular to the velocity and of constant size, which gives a circle or a helix.
NCERT (2026-27):
Magnetic Field, Lorentz Force – “… (ii) The magnetic force q [ v × B ] includes a vector product of velocity and magnetic field. The vector product makes the force due to magnetic field vanish (become zero) if velocity and magnetic field are parallel or anti-parallel.”