Practice question

**Energy stored in inductor** U =½ L I² (J), L inductance (H), I current (A), resides in magnetic field, energy density u = B²/(2μ₀) (J/m³), B field (T), μ₀=4π×10⁻⁷ H/m. For solenoid B=μ₀ n I, volume V= A l, U = (B²/2μ₀) V =½ (μ₀ n² A l) I² =½ L I², consistent. Δ Φ = B A = 0.1 × 0.07 = 0.007 Wb . ε = N (Δ Φ/Δ t) = 170 × (0.007/0.5) = 170 × 0.014 = 2.38 V . Using Φ = B A cosθ, e = -N dΦ/dt = -N A dB/dt = B l v = N B