A cube of edge a has its edges parallel to x, y and z-axis of rectangular coordinate system. A uniform electric field $\vec{E}$ is parallel to y-axis and a uniform magnetic field is $\vec{E}$ parallel to x-axis. The rate at which flows through each face of the cube is

$\frac{a^2 EB}{\mu_0}$ parallel to x - y plane and zero in others

Energy flowing per sec per unit area from a face is $ = \frac{1}{\mu_0} [\vec{E}× \vec{B}]$. It will be in the negative z - direction.

It shows that the energy will be flowing infaces parallel to x − y plane and is zero in all other faces. Total energy flowing per second from a face in x - y plane $=\frac{1}{\mu_0} (EB sin 90°) a^2 = \frac{EBa^2}{\mu_0}$