# EM waves in conductors

1. Aug 4, 2011

### cragar

When an EM wave goes in a conductor it says that the B field component lags the E field component, What causes this? I looked in Griffiths and I couldn't find the answer. Does it have something to do with the fact that when the EM wave enters the conductor it is moving the free electrons in the material?
Any input will be appreciated.

2. Aug 4, 2011

### y33t

Exactly. Magnetic field will tend to rearrange/polarize the free electrons in the conductor so that they will align to such a position where B will be seamlessly fitting to new free electron distribution. As the conductor gets bigger in volume, this lag will go bigger as well.

3. Aug 4, 2011

### cragar

can we use the Lorentz force to figure out which way it will move the electrons in the material.

4. Aug 4, 2011

### y33t

Yes Lorentz force can provide the 'directions' electrons will move to, but in a volume conductor it is not sufficient to determine the overall behavior of electrons due to applied magnetic field. It is a law which essentially applies to individual charges but in this case it will be sufficient to determine the flow direction.

5. Aug 4, 2011

### clem

Letting $$j=\sigma E$$ in Maxwell's curl B equation gives the effective wave number a positive imaginary part, so B will lag E. This will be true for any mechanism causing the conductivity.

6. Aug 4, 2011

### cragar

Could we ever make E lag B?

7. Aug 4, 2011

### y33t

Yes you can. Exposing to a non-linear optical crystal will hold E and make E lag B. Refractive index change of the crystal originates from this. This originates from polarization as well. Also there will be a delay when exposed to anisotropic material where E will try to change the permittivity tensor (shift it with the clock of oscillation).