A vacuumfilled cylindrical capacitor connected to a battery

A vacuum-filled, cylindrical capacitor connected to a battery stores 6.45 J when it is fully charged. Polyester is inserted between the plates without removing the capacitor from the battery. How much energy is stored in the polyester-filled capacitor? Your response differs from the correct answer by more than 10%. Double check your calculations.

Solution

C = k C\'

for polyster, (dielectric constant) k = 2.60

Ui = C V^2 / 2 = 6.45 J

V will be constant only c will change.

Uf = C\' V^2 /2 = 2.60 (C V^2 / 2) = 2.60 x 6.45

= 16.77 J