The energy stored in a capacitor is given by:

U

=

Q DV 2

=

C DV2 2

=

Q2 2C

Consider a capacitor with nothing between the plates. The capacitor is charged, and isolated so the charge on the plates is constant.

Inserting a dielectric increases the capacitance, reducing the energy stored in the capacitor. Where does the energy go?

Adding to the mystery, if we remove the dielectric the energy goes back up again to its original value. Where does this extra energy come from?

The answer is that the dielectric wants to be inside the capacitor because the charges on its surface are attracted to the plates of the capacitor. The capacitor actually does work to pull the dielectric in between the plates, reducing the stored energy. We have to do work to pull the dielectric back out again, and this work shows up as an increase in the energy stored in the capacitor.