A bar magnet of magnetic moment \( m \) is cut into two equal parts. The magnetic moment of each part is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \( m/2 \)
Magnetisation \( M \) is the magnetic moment per unit volume of a sample. A uniformly magnetised bar has the same \( M \) in every part, so its magnetic moment is \( m=MV \). Each part keeps the same magnetisation but has half the volume.
\( \begin{aligned} m' &= M\times\frac{V}{2} \\ &= \frac{m}{2} \end{aligned} \)
The result is the same for a cut across the length and for a cut along it. Each part is a complete magnet with its own north and south poles, but with half the magnetic moment.
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:”
\( \displaystyle M=\frac{m_{net}}{V} \qquad (5.7) \)
Introduction – “(iv) We cannot isolate the north, or south pole of a magnet. If a bar magnet is broken into two halves, we get two similar bar magnets with somewhat weaker properties. …”