Practice question

**Ferromagnetism** shows large positive χ ≈ 10³ to 10⁵, strong attraction, domain structure with spontaneous magnetization, hysteresis, retentivity. Distinction based on sign and magnitude of χ and behaviour in non-uniform field, explaining attraction versus repulsion. B = (μ₀/4π) (2m/r³) . Given: m = 0.9 A m² , r = 0.2 m , (μ₀/4π) = 10⁻⁷ . B = 10⁻⁷ × (2 × 0.9/(0.2)³) = 10⁻⁷ × (1.8/0.008) = 2.25 × 10⁻⁵ T . Substituting values gives 2.25 × 10⁻⁵ T, which matches expected magnitude for this magnetic configuration, confirming dipole field dependence on m/r³ and torque relation τ = m B sinθ.