There are very few wiring situations where Ohm’s Law, and its cousins Kirchoff’s Laws, are not sufficient for all practical purposes.
What you said is more or less accurate, but I think is missing some pieces.
When the switch is engaged, there’s a wavefront that travels along the wire at about the speed of light. Ahead of this wavefront is undisturbed, because of causality and all that. Behind is a region with a higher electron density, because the battery is pumping the electrons across itself.
That moving wavefront means electrons are accelerating–not very far actually, but they rearrange slightly as the wavefront passes, like a water wave. Or more specifically, something like a tidal bore.
Only accelerating charges produce electromagnetic waves. So as the wavefront passes, it produces EM waves, which can jump quickly to the return wire and induce a current in basically the same way, but reversed.
But the EM waves radiate out in all directions, so this isn’t an efficient way of transmitting power. And it only keeps going until that wavefront loops all the way around (in the case of the closed loop), or bounces a couple times off the ends (for the cut loop).
In the closed loop case, once the transient effects die down, you’re just left with electrons moving at a constant velocity (sorta). No acceleration, so no EM wave. No EM wave means the energy is just being transmitted via the electrons pushing on their neighbors.
Arguably, if you’re in a wiring situation in which Kirkhoff and Ohm don’t apply, you’re doing it wrong!
Maybe. I do not automatically assume I know anything about wiring high-frequency microwave RF circuits.
OK, thanks, that helps a lot. As electricity starts to “flow”, it makes EM waves which jump directly across to the lightbulb. This jump is still at the speed of light, and it is detectable as a small change in voltage across the lightbulb.
If the distance between the switch and lightbulb was the same as the length of wire between the switch and lightbulb, then the small change in voltage would arrive at essentially the same time as the large change in voltage.
So, the speed of light is different in different mediums (air, water, vacuum, etc). Is electrical propagation the same speed even in wires of different materials? Not voltage drop and stuff, because the resistance of the wires will be different. So let’s say I have wires made of silver, iron, and say a superconducting ceramic…
That’s a little beyond my knowledge level, but I believe that it’s about the speed of light in all conductors.
However, what can slow you down are transmission line effects, where capacitive and inductive effects play a role. For example, in a standard coaxial cable, the signal speed is about 2/3 of the speed of light. This is largely because the gap between outer and inner conductors acts as a capacitor, with some plastic there as a dielectric.
In those cases it’s largely the non-conductor materials that play a role in affecting the signal speed. Change the dielectric constant of the gap material and the speed will change. And of course the geometry as well–placing the conductors closer or farther apart will change the result.
With bare conductors a meter apart, I think these effects are going to be pretty small, though I’m sure they could be measured with a sensitive experiment.
One point of clarity that I meant to bring up earlier: you can have an EM field without EM waves. Only waves can transmit power. A circuit like this, in steady-state, will still have a static EM field. It’s a loop of wire with a current, so it’s an electromagnet. And there will be an electric field up close, though not farther away. But only accelerating charges create waves, and only waves transmit power, so it’s only this initial transient that gives you the “instant” current.
AC would be a different story, since the charges there are always accelerating. That could transmit power continuously, though it would still only be a small amount.
I find it interesting that AlphaPhoenix’s measurements show that this small current is constant up until the wavefront goes all the way around. But I think it makes sense. The wavefront induces current in the piece of wire close to it. It keeps inducing this current as it goes farther toward the end. Although the waves are themselves moving, it just kinda stretches out the induced current in a constant way, like dropping items on a treadmill next to you. They’ll still be spaced equally (but farther apart) if you walk past the treadmill.
So Veritasium has a new video up on the matter, and I’m a little torn about it. He’s doing this whole song and dance that essentially recapitulates the result Alpha Phoenix got, but then spins it as, sorry guys for not explaining it clearly enough in the first place, this was what I meant all along, and if you use an LED bulb in a pretty dark room, you actually do get some light from the initial current, so there.
Which I think is a bit unfortunate, because that’s just not what most people mean by ‘the light turns on’—we think of the state of a circuit mostly as what happens after any initial dynamic effects have died down, and so this really becomes a debate of semantics about what one means by saying ‘the light turns on’. It’s kind of like saying, you can fire a gun without any propellant, using just the momentum imparted by the hammer, if the barrel’s real smooth and by ‘fire’ you mean ‘the projectile exits the barrel at any velocity whatever’.
Which overshadows all of the cool electrical engineering, most of which was new to me, and fascinating, and better served if he’d just gone out to say, OK, what I said last time was incomplete, here’s the full story.
I agree. He did sum up reasonably honestly admitting the first video was flawed. But he never admitted that he was wrong. He glossed over a lot of what he said, and was defensive about what he said. His tone of voice was getting pretty defensive as well. Overall I think it was a good second video. I give it a B+.
He does still not quite get what is occurring inside a wire. Permissivity inside a conductor is a very strange thing. Indeed there is a whole layer of QED not mentioned. So it is still a classical approximation.
There were still a couple of odd moments. Saying it was the electric field and not the magnetic field involved was a trifle odd.