Practice question

**Magnetic field of bar magnet** follows inverse cube law B ∝ m/r³, unlike inverse square for electric dipole. Given B at distance r, moment m = B r³/(μ₀/4π) for equatorial, m = B r³/(2·μ₀/4π) for axial, enabling moment extraction from measured field. B = (μ₀/4π) (2m/r³) . Given: m = 3.0 A m² , r = 0.4 m , (μ₀/4π) = 10⁻⁷ . B = 10⁻⁷ × (2 × 3.0/(0.4)³) = 10⁻⁷ × (6.0/0.064) = 9.375 × 10⁻⁶ T ≈ 9.38 × 10⁻⁶ T . Substituting values gives 9.38 × 10⁻⁶ T, which matches expected magnitude for this magnetic configuration, confirming dipole field dependence on