A parallel-plate capacitor has plates of area $400\ \text{cm}^2$, $1.0\ \text{mm}$ apart, with air between them. It is charged to $600\ \text{V}$ and then taken off the battery, so its charge is now fixed. One plate is held still. The other is pulled slowly away by hand, staying parallel and facing the first one squarely, until the gap is $3.0\ \text{mm}$. The gap stays small next to the size of the plates, so ignore the effects at the edges. Take $\varepsilon_0 = 8.85 \times 10^{-12}\ \text{C}^2\,\text{N}^{-1}\,\text{m}^{-2}$. Which of the following statements are correct?