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. ε = L (Δ I/Δ t) . Δ I = 2 - 5 = -3 A , Δ t = 0.15 s . ε = 1.0 × (-3/0.15) = 1.0 × (-20) = -20 V , magnitude = 20 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