The electric field in an electromagnetic wave is given by $\mathrm{E}=\left(50 \mathrm{N} {\mathrm{C}}^{-1}\right)\sin \mathrm{\omega }\left(\mathrm{t}-\mathrm{x}/\mathrm{c}\right)$. The energy contained in a cylinder of cross-section $10 {\mathrm{cm}}^2$ and length $50 \mathrm{cm}$ along the $x-$axis is:

The energy density is given by:

${\mathrm{u}}_{\mathrm{av}}=\frac{1}{2}{\mathrm{\epsilon }}_0{{\mathrm{E}}_0}^2=\frac{1}{2}\times \left(8.85\times {10}^{-12}\right)\times {\left(50\right)}^2=1.1\times {10}^{-8} \mathrm{J} {\mathrm{m}}^{-3}$

The volume of the cylinder is given by:

 $\mathrm{V}=\mathrm{AL} = 10 \times {10}^{-4} \times 0.5 = 5\times {10}^{-4} {\mathrm{m}}^3$

The energy contained in cylinder will be:

$\mathrm{U}=\left(1.1\times {10}^{-8} \mathrm{J} {\mathrm{m}}^{-3}\right)\times \left(5\times {10}^{-4} {\mathrm{m}}^3\right)=5.5\times {10}^{-12} \mathrm{J}$