Practice question

**Energy density** in electric field u = ½ ε E², ε = K ε₀, E = V/d, total energy U = u·volume = ½ ε E²·A d =½ ε A d·(V/d)²=½ ε A V²/d=½ C V², consistent. For parallel plate, E = V/d ≈10⁶ V/m for 400 V across 0.4 mm, u≈½×8.85×10⁻¹²×10¹²≈4.4 J/m³. Distance to midpoint = 0.04 m. V = 9 × 10⁹ ( (16 × 10⁻⁶/0.04) + (-4 × 10⁻⁶/0.04) ) = 9 × 10⁹ × (12 × 10⁻⁶/0.04) . V = 9 × 10⁹ × (12 × 10⁻⁶/0.04) = 2.7 × 10⁶ V . Using V = kQ/r, U = k q₁q₂/r,