In an electromagnetic wave, the average energy density is

The magnetic field of a plane-polarized electromagnetic wave moving along $z$-direction is given by,

$B=1.2\times {10}^{-6}\sin \left[2\mathrm{\pi }\left(\frac{z}{240}-\frac{t\times {10}^7}{8}\right)\right] \mathrm{T}$. The maximum electric field is