A parallel plate capacitor has plate of length
'l', width ‘w’ and separation of plates is ‘d’. It
is connected to a battery of emf V. A dielectric
slab of the same thickness ‘d’ and of dielectric
constant k = 4 is being inserted between the
plates of the capacitor. At what length of the
slab inside plates, will the energy stored in the
capacitor be two times the initial energy
stored?