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) (m/r³) . Given: m = 1.5 A m² , r = 0.5 m , (μ₀/4π) = 10⁻⁷ . B = 10⁻⁷ × (1.5/(0.5)³) = 10⁻⁷ × (1.5/0.125) = 1.2 × 10⁻⁶ T . Substituting values gives 1.2 × 10⁻⁶ T, which matches expected magnitude for this magnetic configuration, confirming dipole field dependence on m/r³ and torque relation τ = m B sinθ.