Parallel-Plate Vacuum Capacitor

[LostinPhysics]

If a parallel-plate vacuum capacitor, with its plates separated by a distance of x1 has an amount of energy equal to U stored in it. The separation is then decreased to x2 .

What is the energy now stored if the capacitor was disconnected from the potential source before the separation of the plates was changed? I know that I am suppose to use the formula C= Q/V but how do I put U in terms of x1 and x2?

[mukundpa]

As the battery is disconnected, the charge on the capacitor plates becomes constant. Now by changing capacitance the potential difference will change and hence it is batter to use the formula Q^2 /2C of the energy.

Capacitance in the two situations are to be calculated and Q is calculated by first situation only.