Walker, Halliday & Resnick — Capacitance: Problem 17

17 The parallel plates in a capacitor, with a plate area of $8.50\text{ cm}^2$ and an air-filled separation of $8.00\text{ mm}$, are charged by a $16.0\text{ V}$ battery. They are then disconnected from the battery and pushed together (without discharge) to a separation of $3.00\text{ mm}$. Neglecting fringing, find (a) the potential difference between the plates, (b) the initial stored energy, (c) the final stored energy, and (d) the (negative) work in pushing them together.