A parallel-plate capacitor has rectangular plates of length L = 19 cm and width W = 3 cm . The region between the plates is filled with a dielectric slab of dielectric constant K= 4 which can slide along the length of the capacitor. Initially, the slab completely fills the rectangular region, and the capacitor holds a charge of 0.1 µC. How far should the dielectric slab be pulled so that the stored energy is double its initial value? http://www.webassign.net/tipler4/25-33.gif Ok. I know there are two capacitance that are parallel with each other. One with a dielectric (1) with length L-X and the other without (2) with length X. Q1+Q2=Qo V1=V2=Vo The doubling of stored energy confuses me, and I really dont know where to begin.