Hi, I have trouble on the following problem: Given a parallel plate capacitor, fixed area A, and fixed separation d. Find the energy stored, before and after insertion of a slab of dielectric, which completely fills the space between plates, for each of the two cases: a) Plates are connected to a battery which maintains constant potential difference b) Plates are charged with fixed charges +Q and -Q, and battery disconnected. Explain differences in the two cases. For part a, I think the answers would be just: Before insertion of dielectric: U = (1/2)CV^2 where C = A(epsilon)/d and after insertion of dielectric: U = (1/2)KCV^2 where C = A(epsilon)/d and K is the dielectric constant. But I'm not sure about part b. Thanks.