In a region, steady and uniform electric and magnetic fields are present, parallel to each other. A charged particle is released from rest in this region. The path of the particle will be a
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → straight line
The force on the particle is the Lorentz force \( q\,(\vec{E}+\vec{v}\times\vec{B}) \). Its magnetic part is zero when the velocity is parallel or anti-parallel to \( \vec{B} \), and also when the particle is at rest.
Option (1) helix — Incorrect. A helix needs a velocity component perpendicular to \( \vec{B} \) for the circular part of the motion. Here the velocity never has such a component.
Option (2) straight line — Correct. At release the particle is at rest, so only the electric force \( q\vec{E} \) acts and accelerates it along \( \vec{E} \), that is, along \( \vec{B} \). Its velocity then stays parallel to \( \vec{B} \), so \( \vec{v}\times\vec{B}=0 \) at every instant. The constant force \( q\vec{E} \) then moves it along a straight line with uniform acceleration.
Option (3) ellipse — Incorrect. An ellipse needs a force that keeps turning the velocity. The only force acting is the constant \( q\vec{E} \) along the line of motion.
Option (4) circle — Incorrect. Circular motion arises when \( \vec{v} \) is perpendicular to \( \vec{B} \) and \( q\,\vec{v}\times\vec{B} \) acts as the centripetal force. Here the magnetic force stays zero.
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.”
Magnetic Field, Lorentz Force – “(iii) The magnetic force is zero if charge is not moving (as then |v|= 0). Only a moving charge feels the magnetic force.”