A bar magnet is cut into two equal halves along a plane parallel to the magnetic axis. The one parameter which remains unchanged is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → intensity of magnetisation
The magnetic moment of a bar magnet is \( M = m\,l \) (pole strength times length) and its intensity of magnetisation is the magnetic moment per unit volume, \( I = M/V \).
Cutting the magnet parallel to its axis halves the area of each pole face, so the pole strength becomes \( m/2 \), while the length \( l \) is unchanged. Each half also has half the volume.
\( \displaystyle M' = \frac{m}{2}\,l = \frac{M}{2}, \qquad V' = \frac{V}{2} \;\Rightarrow\; I' = \frac{M/2}{V/2} = \frac{M}{V} = I \)
Option (1) intensity of magnetisation — Correct. The magnetic moment and the volume both halve, so their ratio stays the same.
NCERT (2026-27):
Magnetisation and Magnetic Intensity – “… We define magnetisation M of a sample to be equal to its net magnetic moment per unit volume:
\( \displaystyle \mathbf{M} = \frac{\mathbf{m}_{\mathrm{net}}}{V} \qquad (5.7) \)”