A charge moving with velocity \( \vec{v} \) along the X-direction enters a uniform magnetic field directed along the negative X-direction. As a result, the charge will
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → remain unaffected
The magnetic force on a moving charge is the Lorentz force \( \vec{F}=q\,(\vec{v}\times\vec{B}) \). Here \( \vec{v} \) is along +X and \( \vec{B} \) is along −X. So the two are anti-parallel and \( \vec{v}\times\vec{B}=0 \).
Option (1) remain unaffected — Correct. With \( \vec{v} \) anti-parallel to \( \vec{B} \), the magnetic force is zero, so the charge goes on moving along the X-direction with the same velocity.
Option (2) start moving in the YZ-plane — Incorrect. To move into the YZ-plane, the charge needs a force along Y or Z. Here the force on it is zero.
Option (3) accelerate along the X-axis — Incorrect. The magnetic force is always perpendicular to the velocity. So it can never speed up a charge along its direction of motion. Here the force is zero as well.
Option (4) move in a circular path around the X-axis — Incorrect. Circular motion about the field needs a velocity component perpendicular to \( \vec{B} \). This charge moves only along the field direction.
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.”