Electronic – In which direction do electric signals flow

electromagneticelectromagnetismelectronRFsignal

We know that electrons move from the negative to positive terminal, and that holes flow in the conventional direction of current – from the positive to negative terminal.

I've always assumed that this means that electric signals travel in the conventional current direction and that it's the travel of holes that gets close to the speed of light. Upon further research I come across mentions of electric signals being the electromagnetic waves traveling through the medium by the excitation of the electrons.

This begs the question. If electric signals are the propagation of EM waves at close to c in which direction do these waves travel? In the conventional direction? In the direction of electron flow? In both directions?

I can easily imagine that the signal is directionless and propagates from the point of contact/source of signaling in all directions like light in a dark room. But I'm not sure.

Clarification

Note: This merely presents an experiment to perhaps clarify the intent of the question above. I'd accept answers that don't address this part of the question. Note that you can treat this as a thought experiment but I think it's possible to physically set up this circuit. The only problem is we'd have to deal with resistance in the real world.

Consider a circuit that has 3km of conductors. We start with a DC source attached to 1km of conductor. For simplicity you can assume that the conductor has zero resistance. We attach an LED at the end of the conductor. Then we complete the circuit with 2km of conductor back to the DC source:

                1km of wire
        ┌─────────────────────────────────[resistor]─(LED)───┐
        │                                                    │
         ╱ switch                                            │
        │                                                    │
[DC voltage source]                                          │
        └────────────────────────────────────────────────────┘
                      2km of wire

Assume the wires are spooled so that physically the light bulb and the switch are next to each other.

If I close the switch would I expect the LED to turn on roughly 3.34 us later (the time it takes light to travel 1km) or 6.68 us later or somewhere in between?

Does it make a difference if the 1km line is attached to the positive or negative terminal?

Does it make a difference if the wires are not spooled but are physically laid out over the 1.5km distance? Does the physical geometry make a difference eg. a circle vs a straight line and back?

Best Answer

If electric signals are the propagation of EM waves at close to c in which direction do these waves travel? In the conventional direction? In the direction of electron flow? In both directions?

"Both" directions, as well as an omnidirectional component.

The best example might be an Ethernet cable. A bit is represented as a pulse. To transmit this pulse, the driving chip puts one output low and another high briefly. If it left them like that, then a conventional circuit would be formed with current flowing in a loop between them through the termination resistor on the other end. However, it does not, it quickly (let's say one nanosecond) moves the drivers back to equal voltages.

This launches a rising edge on the positive transmit wire and a falling edge in the negative transmit wire. If you have a fast enough oscilloscope clipped to the wire, these will appear as voltages, but really they're a small EM pulse travelling down the wire.

Each pulse also radiates an EM wave outside the conductor. Because the wires are twisted together at a constant distance, the "positive" wave emitted almost completely cancels the "negative" wave emitted. If this wasn't done, you wouldn't be able to use your radio in the same room due to the EM interference.

The cable can be modelled as a chain of tiny inductors along the wire, with a ladder of tiny capacitors between the two conductors. Each inductor stores the energy of the incoming current change in a magnetic field, before re-emitting it forwards along the wire.

The general subject is called a "transmission line". The effective velocity is below the speed of light, but not much (see "velocity factor").

travel of holes that gets close to the speed of light

It's a bit more complicated than that. (I've not bothered to fully understand that myself!) However, I believe this is why NPN transistors (hole mobility in the P region) are preferred to PNP transistors (electron mobility in the N region).

One of these days I'm going to write a canonical article on why electrons are a distraction for learning about electricity. Electricity is fairly straightforward and follows some mathematical laws that can be analysed with basic calculus. Electrons are not little pingpong balls in a tube, they are weird quantum objects that are capable of appearing, disappearing, tunneling through solid objects, and are much harder to model properly.

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