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Q&A

Comments on How long does it take for energy to propagate in a circuit?

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How long does it take for energy to propagate in a circuit?

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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:

Thought experiment

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:

  1. 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.
  2. 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?

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1 comment thread

A really poor thought experiment (1 comment)
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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.

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2 comment threads

Have you seen this video? https://youtu.be/2Vrhk5OjBP8 (7 comments)
He has furthermore assumed the line is coaxial (2 comments)
Have you seen this video? https://youtu.be/2Vrhk5OjBP8
a concerned citizen‭ wrote almost 3 years ago

Have you seen this video? https://youtu.be/2Vrhk5OjBP8

coquelicot‭ wrote almost 3 years ago

I liked the part at 17:30, where he explains how the capacitive coupling between the wires produces current. Interesting. Did you mean this video contradict something in my answer? I don't see, on the contrary.

a concerned citizen‭ wrote almost 3 years ago · edited almost 3 years ago

coquelicot‭ The whole problem was formulated in an ideal manner, so the lamp would glow instantaneously. It no longer matters that the "bulk" of the current would only appear after 1 s due to the transmission line effect. And it will not start glowing with a brief pulse, then wait for the rest, because the induced current is constant; you can see that a few more seconds after that, at 18:05, there's a small plateau which is sustained. When the ends are opened, sure, it drops (there's an antenna, it can't hold DC), but the problem in the OP has shorted ends.

coquelicot‭ wrote almost 3 years ago · edited almost 3 years ago

@a concerned citizen. Thanks for the feedback, really interesting again. I suspect your alleged SUSTAINED plateau is an artifact of the experiment. Of course, everything is continuous in classical physics, but the meaning of what I said is that there should be a small peak, then the current should decrease, and then increases again toward the sustained regime. I need more time to analyze the video and to find the flow in the experiment. Otherwise, everything seems to me completely illogical. How could a closing switch maintain a continuous plateau by capacitive coupling in the wires when the AC part of the current last only few microseconds? Eventually, if I am unable to find the flow, I may update my answer accordingly. In the mean time, do you have some insight about how this can happen?

a concerned citizen‭ wrote almost 3 years ago

coquelicot‭ Well, if it was an artefact then it's on the owner of the video, feel free to ask him about that. I happen agree with the outcome, but that's just me.

coquelicot‭ wrote almost 3 years ago

@a concern citizen. I meant, do you have some insight about how this apparent paradox could happen, not the alleged artifact.

coquelicot‭ wrote almost 3 years ago

@a concern citizen. No need, I think I have the artifact: the line in the experiment has obviously a non negligible inductance. So, when the switch closes, the inductance of the line creates a relatively slow linearly increasing current, which can induce, indeed, a plateau by capacitive coupling in the other side of the line. In the mean time, the signal has come back and the sustained regime begins. Hence the observed phenomenon. In the ideal experiment (or with a coaxial line), the inductance is null (or canceled by the capacitance of the line), and this would not be observed. I will update my answer according to these insights, and explain everything, but a bit later this week, as I am busy. Thanks again for all!