It is worth to mention that with AC current electrons are not moving at all but rather vibrate. Given electron does not change position but electric charge does.
Even if your piece of equipment is floating there is still return - most likely capacitive. Cell phone is getting this return thru your body (just a few pF). Radio signals are in fact induced in in the cable (since shield most likely does nothing to stop it) but because of skin effect they travel on the outside of the cable - shield (field inside cancels). As long as this shield is related to system ground shield will protect against electromagnetic and capacitive pick-up.
Al - agreed. I objected only to term "Flow of electrons" since it has nothing to do with fast flow of charge. I stated also that with AC electrons are standing practically still - hardly a "flow of electrons". We can say that electricity (electrons) moves very slow but electric current (charge) moves very fast.
Once again - Electric current is a flow of charge ALONE. At 1kHz electrons are practically standing still (vibrating +/-0.0001mm) NEVER MOVING ALONG THE CABLE while charge is reaching destination with almost speed of light (0.6-0.7).
Electrons might be carriers and you can calculate numbers from amperage but they don't "Flow" - charge does.
Herman - for that reason never apply DC to expensive audio cable or you will slowly loose good electrons you paid for (just kidding).
Herman - In addition people believe that since charge flows back and forth (AC) energy does the same and directionality of the cable doesn't exist. Energy flows one direction from source to destination. Some explanation here:
Companies like Audioquest mark direction on their speaker wires (sold per ft). They claim audible difference since wire was drawn one direction and there is certain "grain" of crystals. I would not listen to such claims but I remember that copper oxide is a semiconductor. I believe they know more than I do.
Current does not flow - current IS. Same as current in the river is a flow of water (current does not flow - water flows) current in the wire is a flow of electric charge. Wire creates electromagnetic field but transports charges and not the energy. Energy is transferred thru electromagnetic field from the source to the load. Direction of the transfer is determined by the Poynting Vector.
The load has some voltage drop in it, hence electric field. Together with magnetic field, this brings the energy in. Same way, the source generates voltage and, together with magnetic field, this brings the energy out.
In coax wire all energy is transferred as electromagnetic field thru dielectric between wire and the shield. More info in "Poynting Field".