see that because V doesn’t change, neither will E. Finally, using the equation U =
QV, we conclude that since V doesn’t change and Q increases by a factor of k, the
stored electric potential energy increases by a factor of k. An important point to
notice is that V doesn’t change because the battery will transfer additional charge to
the capacitor plates. This increase in Q offsets any momentary decrease in the
electric field strength when the dielectric is inserted (because the molecules of the
dielectric are polarized, as above) and brings the electric field strength back to its
original value. Furthermore, as more charge is transferred to the plates, more
electric potential energy is stored.
The following figures summarize the effects on the properties of a capacitor with
the insertion of a dielectric in the two cases: