Identify the incorrect statements from the following.
(A) A charged particle of charge e is moving through a magnetic field, work will be done by the magnetic field on the charge.
(B) A conductor with free electrons is placed in a magnetic field. No current is flowing through the conductor. No magnetic force acts on the conductor.
(C) A charged particle is moving perpendicular to the magnetic field. The magnetic force does not change the speed of the particle but changes its direction.
(D) A charged particle is moving in a straight direction without any change of direction. It shows that no magnetic field is there.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) and (D) only
The magnetic force on a moving charge, \( \vec{F} = q\,\vec{v}\times\vec{B} \), is perpendicular to \( \vec{v} \) and vanishes when \( \vec{v} \) is parallel to \( \vec{B} \). The question asks for the incorrect ones.
(A) A charged particle of charge e is moving through a magnetic field, work will be done by the magnetic field on the charge. — Incorrect. The force is perpendicular to the velocity, so it does no work.
(B) A conductor with free electrons is placed in a magnetic field. No current is flowing through the conductor. No magnetic force acts on the conductor. — Correct. With no net drift of charge there is no net force.
(C) A charged particle is moving perpendicular to the magnetic field. The magnetic force does not change the speed of the particle but changes its direction. — Correct. The particle moves on a circle at constant speed.
(D) A charged particle is moving in a straight direction without any change of direction. It shows that no magnetic field is there. — Incorrect. A charge moving parallel to the field also moves straight.
The incorrect statements are (A) and (D), which is Option (4).
NCERT (2026-27):
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).”
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.”