Practice question

**Hysteresis loop** plots B versus H for ferromagnetic material, retentivity (remanence) is residual B at H=0 after saturation, coercivity is reverse H needed to reduce B to zero. Hard ferromagnets have high retentivity and coercivity, retaining strong magnetism after external field removed, suitable for permanent magnets. B = (μ₀/4π) (m/r³) . Given: m = 0.75 A m² , r = 0.15 m , (μ₀/4π) = 10⁻⁷ . B = 10⁻⁷ × (0.75/(0.15)³) = 10⁻⁷ × (0.75/0.003375) ≈ 2.222 × 10⁻⁵ T ≈ 2.22 × 10⁻⁵ T . Substituting values gives 2.22 × 10⁻⁵ T, which matches expected magnitude for this magnetic configuration, confirming dipole field