Charging a capacitor stores energy in its field: U = ½CV² (equivalently an energy density u = ½ε₀E² fills the gap). Two capacitors can be combined: in SERIES the charge is common and the voltages add, giving 1/Cₛ = 1/C₁ + 1/C₂ (Cₛ is smaller than either); in PARALLEL the voltage is common and the charges add, giving Cₚ = C₁ + C₂ (larger than either). The walkthrough steps through single → series → parallel; slide the source voltage V and the two capacitances.
A single capacitor C₁ charged to V stores energy U = ½C₁V² in its field — the brown bar on the right grows with U.
U = ½CV² · u = ½ε₀E² · series 1/Cₛ = 1/C₁+1/C₂ · parallel Cₚ = C₁+C₂
V = — V · C₁ = — · C₂ = — µF · C₋ = — µF · U = — µJ