(9) A plastic dielectric is placed between the capacitor plates (k=3). Holding the voltage constant, how much potential energy is now stored in the capacitor?
This dielectric increases the capacitance:
The potential energy is now given by
Since the voltage is held constant but the capacitance changes, the charge on the capacitor must change as well. Therefore, if either of the other two equations for potential energy are used, the new charge must first be calculated:
