Positively charged particles are projected into a magnetic field. If the direction of magnetic field is along the direction of motion of the charged particles, the particles will experience
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → no change in velocity
The magnetic force on a moving charge is \( \vec{F} = q\,\vec{v}\times\vec{B} \), with magnitude \( F = qvB\sin\theta \). When the field is along the direction of motion, \( \theta = 0^\circ \) and \( \sin\theta = 0 \), so the force is zero.
With no force the particle has no acceleration, so its velocity does not change.
NCERT (2026-27):
Magnetic Field, Lorentz Force – “… The vector product makes the force due to magnetic field vanish (become zero) if velocity and magnetic field are parallel or anti-parallel.”