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Two small bar magnets, each 1 cm long, have magnetic moments of 1.20 A m² and 1.00 A m². They are kept on a level table side by side, parallel to one another, with their north poles directed toward the south. Their magnetic equators coincide, and the distance between their centres is 20.0 cm. At the midpoint O of the segment joining the centres, the resultant horizontal magnetic induction is nearest to (take the horizontal component of Earth’s magnetic field as 3.6 × 10⁻⁵ Wb/m²).

1. 3.6 × 10⁻⁵ Wb/m²
2. 2.56 × 10⁻⁴ Wb/m²
3. 3.50 × 10⁻⁴ Wb/m²
4. 5.80 × 10⁻⁴ Wb/m²

Correct answer: 2.56 × 10⁻⁴ Wb/m²

Solution

O lies on the equatorial line of both magnets at d=0.1 m. B = (mu0/4pi)M/d^3 gives 1.2e-4 and 1.0e-4 T, both directed north (opposite to the southward moments), adding to Earth's 0.36e-4 T: total ~ 2.56e-4 Wb/m^2.

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