Two capacitors are identical, except that one is empty and the other is filled with a dielectric (k = 4.5). The empty capacitor is connected to a 15 -V battery. What must be the potential difference across the plates of the capacitor filled with a dielectric so that it stores the same amount of electrical energy as the empty capacitor?

Answer 1

energy=1/2 C v^2

so v=sqrt(1/k)

check that.