Variable Capacitance of a Radio Dial

[matthew1991]

Homework Statement

The capacitance of the variable capacitor of a radio can be changed from 100 to 350 pF by turning the dial from 0° to 180°. With the dial set at 180°, the capacitor is connected to a 130 V battery. After charging, the capacitor is disconnected from the battery and the dial is turned to 0°. What is the charge on the capacitor now?
K air= 1.000
K polystyrene= 2.3

Homework Equations

Q=C/V
V/V(sub)0=K
C=kCsub0=k*Epsilon naught*(A/D)
E=Q^2/2C=V^2*C/2=Q*V/2

The Attempt at a Solution

I tried using conservation of energy to find charge by using V^2*C/2=Q6^2/2C, but it was't the right answer.
there is a hint given that says think about what quantity is conserved. I thought energy was always conserved?

 

[gneill]

I tried using conservation of energy to find charge by using V^2*C/2=Q6^2/2C, but it was't the right answer.
there is a hint given that says think about what quantity is conserved. I thought energy was always conserved?

Energy is always conserved, but did you account for the work you had to do in order to turn the dial?

The hint was a good one though. Question: where can the charge go when you turn the dial if the capacitor is disconnected?

 

[matthew1991]

I made this way more complicated than it needed to be. Charge is always conserved, so Q=CV at 180 deg is the same Q still at 0 deg. I had pF as E-9 instead of E-12