In an isolated system of two capacitors initially charged and then connected in parallel with opposite polarities, what
**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. When two capacitors are connected in parallel with opposite polarities, charges redistribute such that the net charge on the positive plates (and negative plates) adjusts to a new equilibrium. The initial energy stored in the capacitors ( U_i = (1/2) C₁ V₁² + (1/2) C₂ V₂² ) is greater than the final
Ref: NCERT > Physics Book > Electrostatic Potential and Capacitance > Energy Stored in Capacitor and Energy Density