Helmholtz equation for the magnetic field is

$$∇^2 \mathbf{H} + k^2 \mathbf{H} = \mathbf{0}$$

Assuming time-harmonic waves Ampere's law can be rearranged

$$\mathbf{E} = \frac{∇\times\mathbf{H}}{σ+jωε}$$

Thus the time averaged power (derived from the Poynting vector) is

$$P_{\text{avg}} = \frac{1}{2}\,\operatorname{Re}\, (\mathbf{E}\times\mathbf{H}^*)$$

$$P_{\text{avg}} = \frac{1}{2}\,\operatorname{Re}\,(\frac{∇\times\mathbf{H}}{σ+jωε}\times\mathbf{H}^*)$$

For the sake of simplicity let $σ=0$

$$P_{\text{avg}} = \frac{1}{2ωε}\,\operatorname{Im}\,(\mathbf{H}\times(∇\times\mathbf{H})^*)$$

If am em wave travels from free space into a region that has a different permittivity, this formula seems to say that magnitude of power will change. Where did the power go? Is the Poynting vector continuous?

  • 1
    $\begingroup$ energy conservation is taken up with"material +em wave" $\endgroup$ – anna v Jul 21 at 5:01
  • $\begingroup$ Can you explain why you think the last equation indicates the average Poynting vector will be different on the two sides of a boundary? $\endgroup$ – Puk Jul 21 at 10:29
  • $\begingroup$ Good question Puk. I do not know. The h-field could compensate for the change in the permittivity. I tried to ask the question better in a new post link. $\endgroup$ – electroMan Jul 23 at 5:23

No. It is the same reason why the electric field component perpendicular to an interface appears to decrease by a factor $1/\epsilon$ when going from free space to the material with different permittivity. Remember, it is the perpendicular electric displacement field which is continuous, not the perpendicular electric field. You may think of this as an "electric field screening" effect.

  • $\begingroup$ Actually the tangential component of $\vec{E}$ is continuous across a boundary, not that of $\vec{D}$ in general. $\endgroup$ – Puk Jul 21 at 10:22

The Poynting vector is the directional energy flux of the EM field.

It has a normal and tangential component.

S = E x H

Now at the edge of the media, it is the normal component that is continuous.

Why is the tangential component not continuous?

The Poynting vector appears in Poynting's theorem (see that article for the derivation), an energy-conservation law:

$${\frac {\partial u}{\partial t}}=-\mathbf {\nabla } \cdot \mathbf {S} -\mathbf {J_{\mathrm {f} }} \cdot \mathbf {E}$$

The first term in the right-hand side represents the electromagnetic energy flow into a small volume, while the second term subtracts the work done by the field on free electrical currents, which thereby exits from electromagnetic energy as dissipation, heat, etc.


The rate of energy transfer (per unit volume) from a region of space equals the rate of work done on a charge distribution plus the energy flux leaving that region.


At the edge of the two media, some of the energy is reflected or inelastically scattered, or absorbed.

Energy must be conserved, so some of the photons will be reflected (returning to the original medium), some will be inelastically scattered (giving some of their energy to the atoms in the new medium), and some will be absorbed (giving all their energy to the new media).


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