Magnetic moment of a thin bar magnet is 'M'. If it is bent into a semicircular form, its new magnetic moment will be _______________.
Fill in the blank with the correct answer from the options given below.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \( 2M/\pi \)
The magnetic moment of a magnet is the pole strength times the straight-line distance between the poles. Bending the magnet does not change its pole strength \( m \), but it brings the poles closer.
\( \displaystyle M = m\,l \)
Bent into a semicircle, the length \( l \) becomes the arc, so \( l = \pi R \) and \( R = l/\pi \). The poles are now at the two ends of a diameter.
\( \displaystyle \text{distance between the poles} = 2R = \frac{2l}{\pi} \)
\( \displaystyle M' = m \times \frac{2l}{\pi} = \frac{2}{\pi}\,(m l) = \frac{2M}{\pi} \)
Note: A similar question was also asked in CUET (UG) 2025 on 30-05-2025 (Slot 2).