Comments on How long does it take for energy to propagate in a circuit?
Parent
How long does it take for energy to propagate in a circuit?
The premise
In a recent video by the pop-sci channel Veritasium, the concept of the flow of electricity and energy transmission in a circuit was discussed. In that video a thought experiment is presented:
The video concludes that, after the switch is flipped, the lightbulb will turn on after 1/c seconds (1 meter divided by the speed of light), since the lamp and the battery are 1 meter apart. This is explained by showing that, in an electrical circuit, energy is transmitted through an EM field, which propagates at the speed of light, and not through movement of particles in a conductor.
This explanation did not sit right with me, for mainly two reasons:
- Clearly, the conductor plays a role in the transmission of energy. This is actually pointed out in the video as well, by saying that after a connection in a circuit is made, the EM field will propagate along the conductor at the speed of light. To me, this would mean that the field would take 1s to propagate through the cables in the thought experiment, and the lamp will turn on after 1s.
- For the lamp to turn on, current must flow through it, heating the filament. Simply being in an EM field generated by the battery would not work, there needs to be some potential difference at the lamp's terminals. The signal doesn't "know" what happens at the load until it reaches it. This is a big part of reflection in signal and transmission lines, yet seems to be ignored here. Therefore, the voltage will take 1s to arrive at the lamp in this case.
The question
Is the answer of 1/c seconds correct, or should the answer be something else? If the answer is not 1/c, where does the mistake in the video's reasoning lie?
Try this out for size: - Image alt text Hope it's clear. Maybe a little simulation of 10 km 600 Ω line might h …
3y ago
Is the answer of 1/c seconds correct It can't possibly be. The question is looking for a time value. "C" is a speed …
3y ago
I add my answer here because I think the other answers are incorrect or irrelevant: namely, the answer of Andy is nice a …
3y ago
This is someone trying to rehash a Light experiment with circuits, however I do not think it holds up. Inside a medium, …
3y ago
So here's the thing. Energy doesn't "propagate" as the video suggests. Energy is used. A voltage potential may propagate …
3y ago
Post
I add my answer here because I think the other answers are incorrect or irrelevant: namely, the answer of Andy is nice after he has redrawn the schematic and changed the problem, answering to another (actually more interesting) question.
The answer of Olin seems to me a bit misleading: even if the two lines were very close, inducing a strong coupling, the lamp will certainly not glow continuously, but very briefly, until a continuous regime is reached. If the power supply were not a battery, as indicated by the schematic, but an AC supply of sufficient frequency, things would be different and far more complex, as we would have to take into account both the direct coupling and the continuous regime. This could be a very interesting high level problem.
As for the original question, the answer is: the lamp could have a very brief almost instantaneous glowing, if the current provided by the battery is huge, because of the tiny inductive and capacitive coupling of the wires (assuming they are not coaxial wires). But only after 1s, the time needed by the speed of light to reach the lamp via the wire, will the lamp glow more or less continuously, until a constant regime is reached after several reflections; then it will glow uniformly.
Actually, that's not right: the electric wave do not propagate at the speed of light in electrical wires, even not in coaxial one; this is obvious for ordinary wires which own some inductance, but is also true for coaxial wires where the inductance is locally canceled by the capacitance at high frequencies. The telegrapher equation says that the velocity of the electric signal is a fraction of the speed of light (e.g. 90%). So, the lamp will begin to glow continuously somewhat after 1 s.
My impression is that this question was created to illustrate one aspect of the electrical propagation, and was very badly thought and asked.
1 comment thread