Posted by Jim on Monday, September 3, 2007 at 11:02am.
I am also having problems understanding how to complete this problem, any help would be greatly appreciated.
Two capacitors are identical, except that one is empty and the other is filled with a dielectric ( = 4.20). The empty capacitor is connected to a 18.0 V battery. What must be the potential difference across the plates of the capacitor filled with a dielectric such that it stores the same amount of electrical energy as the empty capacitor?
_________ V

Physics  drwls, Monday, September 3, 2007 at 3:50pm
The capacitor with the dielectric (#2) will have 4.2 times the Cvalue of the airgap capacitor (#1). Let's call the capacitances C1 and C2 = 4.2 C1.
Since the energies (E) are equal,
E1 = (1/2) C1 V^2 = E2 = (1/2) C2 V2^2
C1 * 18^2 = (4.2 C1) * V2^2
V2^2 = (1/4.2) * 324
Solve for V2