Two capacitors are identical, except that one is empty and the other is filled with a dielectric (k = 3.6). 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

C₂=kC₁
Energy = CV²/2
C₁V₁²/2=C₂V₂²/2
V₂=V₁/√(k)=15/√(3.6) = 7.9 V