Two identical air-filled parallel-plate capacitors, designated 1 and 2, each have capacitance \(C_0\) and are initially charged to a potential difference \(V_0\). Capacitor 1 remains connected to its battery while a dielectric slab of dielectric constant \(\kappa\) (where \(\kappa > 1\)) is fully inserted between its plates. Capacitor 2 is disconnected from its battery, isolating the charge on its plates, before an identical dielectric slab is fully inserted between its plates. What is the ratio of the final electrostatic energy stored in capacitor 1 to the final electrostatic energy stored in capacitor 2, \(U_1/U_2\)?