Practice question

**Magnetic energy density** u = B²/(2μ₀), for B=0.5 T, u=0.25/(2×4π×10⁻⁷)=0.25/(2.513×10⁻⁶)=99471 J/m³, large, but volume small, total energy moderate. Inductor stores energy in field, released when current interrupted causing spark. ω = 2π × (36/60) = 1.2π rad/s . ε = (1/2) B ω R² = (1/2) × 0.6 × 1.2π × (0.45)² = 0.383 V ≈ 0.38 V . Using Φ = B A cosθ, e = -N dΦ/dt = -N A dB/dt = B l v = N B A ω sinωt, L = μ₀ N²A/l, M = e/(dI/dt) and U = ½ L I², result 0.38 V follows, reflecting Faraday's law and Lenz's opposition.