Ratio of the Electric field to the Magnetic field

[goodphy]

Dear all.

I would like to know the general expression of the ratio of the electric field strength E to the magnetic flux density B. I know E/B = c, where c is the speed of light, for a vacuum, but I want to know if this is stil valid for any material where the electric and magnetic fields are present.

Dose E/B = c hold for any cases, or the general expression is E/B = v,where v is the speed of the electromagnetic wave through the medium where E and B fields are present?

 

[Dale]

E/B=c is not a general relationship. It only applies in some specific circumstances. For example, if you have two laser beams that intersect then it holds in the individual beams, but not in the intersection.

 

[jasonRF]

Dose E/B = c hold for any cases, or the general expression is E/B = v,where v is the speed of the electromagnetic wave through the medium where E and B fields are present?

Nope, but if you are looking at monochromatic plane waves of the form $e^{i\omega t - i\mathbf{k\cdot r}}$ then
$\nabla \times \mathbf{E} = - \partial \mathbf{B}/\partial t$ always implies $\mathbf{k \times E} = \omega \mathbf{B}$. In general $\omega/|\mathbf{k}|$ is of course the phase velocity of the wave. However, in some media, such as a plasma, there are wave modes where $\mathbf{k}\parallel \mathbf{E}$ so $\mathbf{k \times E} = 0 = \omega \mathbf{B}$ even though the wave speed is non-zero.