Answer:
the energy comes from the increase in the electric field
Explanation:
The capacitance is
C = ε₀ A / d
The electric charge on the condenser plates
Q = C ΔV
The stored electrical energy is
U = ½ C ΔV²
ΔV = E d
U = ½ (ε₀ A / d) (E d)²
U = ½ ε₀ A d E²
We see that the stored energy is proportional to the square of the electric field, so the capacitor can increase its energy with increasing voltage
In short, the energy comes from the increase in the electric field