Chapter 25 - Capacitance - Problems - Page 742: 30

$U=9.96\times10^{-8}J$

We know that $U=uV$, where $u$ is the energy density and $V$ is the volume of the air. We also know that $u=\frac{1}{2}\epsilon_{\circ}E^2=\frac{1}{2}\times8.85\times10^{-12}(150)^2=9.96\times10^{-8}\frac{J}{m^3}$ putting values in eq(1), we get $U=9.96\times10^{-8}\times1$ $U=9.96\times10^{-8}J$