Work done to charge a capacitor is stored as potential energy in the electric field.
Energy When Dielectric is Inserted
When a dielectric of constant \( K \) is inserted into a capacitor:
Isolated (constant Q): \( C \) increases by \( K \), so \( U = Q^2/(2C) \)
decreases by factor \( K \). The electric field and voltage both decrease. Energy is released — it
goes into the dielectric (polarisation).
Battery-connected (constant V): \( C \) increases by \( K \), so \( U =
\frac{1}{2}CV^2 \) increases by factor \( K \). Extra charge flows from the battery — the battery
does more work than the energy stored (the other half goes into the dielectric and heat).