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A bar magnet of length 14 cm is placed in magnetic meridian with its north pole pointing towards the geographic north pole. A neutral point is obtained at a distance of 18 cm from the center of the magnet. If B_H = 0.4 G, the magnetic moment of the magnet is: (1 G = 10⁻⁴ T)

1. 2.880 × 10³ J T⁻¹
2. 2.880 × 10² J T⁻¹
3. 2.880 J T⁻¹
4. 28.80 J T⁻¹

Correct answer: 2.880 J T⁻¹

Solution

With N pole toward geographic north the neutral points lie on the equatorial line. So 0.4e-4 = 1e-7 * M/(0.18^2+0.07^2)^{3/2}. (0.0373)^{3/2}=7.21e-3, giving M = 4e-5*7.21e-3/1e-7 = 2.88 J/T.

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