Energy stored in capacitor w/ dielectric
Suppose a (parallel plate) capacitor of capacitance C is charged to a potential difference V and then disconnected and isolated. Energy stored E1= Q2/2C.
Now if a material of appropriate dimensions and dielectric constant K is fully inserted between the plates, energy stored E2= Q2/2KC.
E2 < E1.
My question is: where did the "missing" energy go?