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. ω = 2π × (30/60) = π rad/s . ε = (1/2) B ω R² = (1/2) × 0.7 × π × (0.4)² = 0.1759 V ≈ 0.176 V . Using Φ = B A cosθ, e = -N dΦ/dt = -N A dB/dt = B l v = N B A