Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): It is misleading to call magnetic field lines the lines of force on a moving charge.
Reason (R): The magnetic force on a moving charge, \(q\mathbf{v}\times\mathbf{B}\), is always normal to \(\mathbf{B}\).
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are correct and (R) is the correct explanation of (A).
Assertion (A) It is misleading to call magnetic field lines the lines of force on a moving charge. — True. A field line shows the direction of \( \mathbf{B} \) at each point. A moving charge is not pushed along this line.
Reason (R) The magnetic force on a moving charge, \(q\mathbf{v}\times\mathbf{B}\), is always normal to \(\mathbf{B}\). — True. The force \( q\mathbf{v}\times\mathbf{B} \) is perpendicular to both \( \mathbf{v} \) and \( \mathbf{B} \).
Why Reason (R) explains Assertion (A)?
A line of force would show the direction of the force at each point.
The force on a moving charge is normal to the field, so it never acts along a field line. Calling these lines “lines of force” would give the wrong picture.
NCERT (2026-27):
The magnetic field lines – “In some textbooks the magnetic field lines are called magnetic lines of force. This nomenclature is avoided since it can be confusing. Unlike electrostatics the field lines in magnetism do not indicate the direction of the force on a (moving) charge.”
Example 5.4 – “(a) No. The magnetic force is always normal to B (remember magnetic force = qv × B). It is misleading to call magnetic field lines as lines of force.”