EM waves and conducting surfaces

[froggy2]

Firstly, I'm a bit confused about EM wave propagation. Take the picture you see everywhere illustrating the perpendicularity of E and B in a traveling EM wave (like this http://web.onetel.net.uk/~gdsexyboy/em_wave.jpg) -- does that actually illustrate the magnitudes of E and B at a particular time (except actually across a whole plane)? Also, after a short period of time, is that wave "longer"/would you draw another phase of E and B attached to the wavefront, or would the wave completely shift a phase in direction ExB by "gaining" a phase at the front and "losing" a phase from the back end? If you create an EM wave by "wiggling" a charged particle, does the number of "humps" in the EM wave depend on how many times you wiggle the particle?

Also, while an EM wave is traveling, what happens when E and B are both 0 at the wavefront? Where does the energy go? Previously I'd always thought that traveling waves look like standing waves, where E and B are of equal magnitude and pi/2 apart in phase so that energy oscillates between E and B, but that's apparently not the case in a traveling wave.

Last question -- what happens when an EM wave encounters a conducting surface at the instant when there's nonzero E field? Then there would be an E field parallel to the conducting surface -- I know this is impossible but I can't figure out why in this case.

Thank you!

 

[Bill_K]

... does that actually illustrate the magnitudes of E and B at a particular time (except actually across a whole plane)?

Yes indeed

Also, after a short period of time, is that wave "longer"/would you draw another phase of E and B attached to the wavefront, or would the wave completely shift a phase in direction ExB by "gaining" a phase at the front and "losing" a phase from the back end?

I think he intends this to be an infinitely long wave, not a wave packet, so the entire pattern will keep shifting to the right.

If you create an EM wave by "wiggling" a charged particle, does the number of "humps" in the EM wave depend on how many times you wiggle the particle?

Yes, equal

Also, while an EM wave is traveling, what happens when E and B are both 0 at the wavefront? Where does the energy go?

There isn't any at that point. The energy carried by the wave is not uniformly spread out, so the value we usually quote is the time average.

what happens when an EM wave encounters a conducting surface at the instant when there's nonzero E field?

It will get reflected. There will be a reflected wave created, traveling in the opposite direction, whose E vector exactly cancels our E vector at the conducting surface.