A positive charge enters a magnetic field and travels parallel to the magnetic field. The charge experiences
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → no force
The magnetic force on a moving charge is the Lorentz force
\( \displaystyle \vec{F} = q\,(\vec{v} \times \vec{B}), \quad F = qvB\sin\theta \)
For motion parallel to the field, \( \theta = 0^\circ \) and \( \sin 0^\circ = 0 \), so the charge feels no force and continues in a straight line with the same speed.
NCERT (2026-27):
Magnetic Field, Lorentz Force – “… (ii) The magnetic force \( q[\mathbf{v} \times \mathbf{B}] \) 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.”