# Magnetic field in TEM mode

1. Feb 9, 2012

### erasmooth

As for transmission line(which operates on TEM mode),
electric field is created by the opposite polarity of surface charge on both conductors,
and magnetic field is by the electric current in both conductors.
These two fields are perpendicular to each other, which makes "the" TEM mode.
Then, what happened to the magnetic field by the electric field between two conductors.
(turning on the line, electric field is created, which means changing field, and induce magnetic field loop centered by electric field)
The magnetic field is different from the other one by electric current in conductors, and
has longitudinal component toward the propagation direction, which doubtfully break "the" TEM mode.

Would you help me with the truth of the magnetic field by electric field

2. Feb 9, 2012

### yungman

I think you got it up side down. It is the TEM field that travel down the transmission line. The surface charge and the surface current are the consequence of the EM wave from the boundary condition.

It is a miss conception that the current and voltage that travel down the line. Current need movement of electrons. If you look at the velocity of electrons travel in good conductor, they are very very slow. You can inject an electron into one end of a wire by a potential that is a few feet long, go get a cup of coffee and come back and wait for that electron to come out from the other end!!! The reason you see voltage or current travel down the tx line in close to light speed is not the movement of electrons, it is the EM wave that propagate down the tx line. The voltage and current you see is the result of the boundary condition of the EM wave with the surface of the conductors of the tx lines.

Problem is EM books for EE skip a lot of the physics and only devote a page in this important concept. They quickly go into the voltage and current phasors as if the voltage and current actually travel down the line. No!!! It's the EM wave that travel down the line. You see the voltage and current at the other end of the line ONLY because of the boundary condition.

Go online, read P429 to 431 of 2nd edition of "Field and Wave Electromagnetics" by David K Cheng and you can see the drawing and explanation of this. This is important stuff. The explanation is not that good, it even skipped me the first go around.

Last edited: Feb 9, 2012