The rms value of the electric field of a plane electromagnetic wave is 314 V/m. The average energy density of electric field and the average energy density are

$\begin{array}{rcl}{\mathrm{E}}_{\mathrm{rms}}& =& 314 {\mathrm{Vm}}^{-1} ; \\ {\mathrm{E}}_0& =& {\mathrm{E}}_{\mathrm{rms}}\sqrt{2}\\ & =& 314 \mathrm{x} \sqrt{2}=444 {\mathrm{Vm}}^{-1}\end{array}$

Average energy density of electric field

$\begin{array}{rcl}\left({\mathrm{u}}_{\mathrm{E}}\right)& =& \frac{1}{4}{\in }_0{{\mathrm{E}}_0}^2\\ & =& \frac{1}{4} \mathrm{x} 8.8 \mathrm{x} {10}^{-12}{\left(444\right)}^2\\ & =& 433699 \mathrm{x} {10}^{-12}\\ & =& 4.3 \mathrm{x} {10}^{-7} {\mathrm{Jm}}^{-3}\end{array}$

$\therefore$ Average energy density 

= 2u_E = 2 x 4.3 x10^-7 = 8.6 x 10^-7 Jm^-3