Consider the following statements about bar magnet and magnetic field lines and identify the correct statements only.
A. Every magnetic configuration always has a north pole and a south pole.
B. If a bar magnet is cut into two pieces along its length, new pieces do not behave like magnets.
C. The larger the number of field lines crossing per unit area, the stronger is the magnitude of the magnetic field.
D. Magnetic field lines due to a solenoid at its ends and outside cannot be completely straight and confined.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → C and D only
(A) Every magnetic configuration always has a north pole and a south pole. — Incorrect. A toroid is a magnetic configuration with no poles: its field lines are closed circles entirely inside the winding.
(B) If a bar magnet is cut into two pieces along its length, new pieces do not behave like magnets. — Incorrect. Each piece is a complete magnet with its own north and south poles; magnetic poles always occur in pairs and an isolated magnetic pole does not exist.
(C) The larger the number of field lines crossing per unit area, the stronger is the magnitude of the magnetic field. — Correct. The density of field lines (number crossing unit area held normal to them) is a measure of the magnitude of \( \vec{B} \).
(D) Magnetic field lines due to a solenoid at its ends and outside cannot be completely straight and confined. — Correct. Magnetic field lines always form closed loops. At the ends of a real (finite) solenoid the lines spread out and return through the outside; perfectly straight, confined lines are possible only for an ideal, infinitely long solenoid.
NCERT (2026-27):
The Magnetic Field Lines – “… The larger the number of field lines crossing per unit area, the stronger is the magnitude of the magnetic field \( \mathbf{B} \).”
Magnetism and Gauss’s Law – “… Field lines due to a solenoid at its ends and outside cannot be so completely straight and confined; such a thing violates Ampere’s law. The lines should curve out at both ends, and meet eventually to form closed loops.”