Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): When a charged particle moves in a uniform magnetic field, with no other force acting on it, its kinetic energy does not change.
Reason (R): A charged particle moving parallel to a uniform magnetic field experiences no magnetic force.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
Assertion (A) When a charged particle moves in a uniform magnetic field, with no other force acting on it, its kinetic energy does not change. — True. The magnetic force can bend the path of the particle. But it does no work on it, so the speed and the kinetic energy stay the same.
Reason (R) A charged particle moving parallel to a uniform magnetic field experiences no magnetic force. — True. The force \( q\,(\vec{v}\times\vec{B}) \) contains a vector product, which is zero when \( \vec{v} \) and \( \vec{B} \) are parallel.
Why R does not explain A?
The Reason covers only one special direction of motion. The Assertion holds for a particle moving in any direction, including at right angles to the field, where the force is largest.
The kinetic energy stays constant because the force is always perpendicular to the velocity, so no work is done on the particle.
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. The force acts in a (sideways) direction perpendicular to both the velocity and the magnetic field. …”
Motion in a Magnetic Field – “… In the case of motion of a charge in a magnetic field, the magnetic force is perpendicular to the velocity of the particle. So no work is done and no change in the magnitude of the velocity is produced (though the direction of momentum may be changed).”