posted by K .
Air will break down as a dielectric when subjected to applied fields in excess of 3 * 10^6 N/C. a parallel plate capacitor using air as a dielectric has a capacitance of 1.0nF and a separation b/t plates of 1.0micrometer. how much energy U was stored in this capacitor at the moment air breaks down?
Stored energy = (1/2) C V^2
For V, use the breakdown field strength, in N/C (which is the same as Volts/meter). They have already given you the value of that number.
For C, use 10^-9 Farads.
Turn the crank and out will pop the answer.
Isn't physics fun?