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. In series, the charge Q on each capacitor is the same. Energy stored in a capacitor is U = (Q²/2C) . For a smaller capacitance C , the denominator 2C is smaller, so U is larger compared to a capacitor with larger C . Alternatively, since V = (Q/C) , the smaller