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Background:

So, I recently watched Veritasium's video, "The Big Misconception About Electricity" and EEVblog's response to it.

Here is the circuit in question: enter image description here

I think understand why the typical distributed element line model works for:

  • Single conductor wires close together
  • Coaxial cables and twisted pair cables

My analysis of what would happen when the switch is closed:

  1. Bulb turns on for an instant because of inductive/capacitive coupling (energy flowing through field between one wire to the other wire).
  2. Bulb dims due to inductive/capacitive transients dying off.
  3. Bulb brightens depending on line impedance and reflections once the signal travels along the wire, stabilizing if the impedance is matched or when the reflection waves die off.

My questions though:

  • Is my analysis correct? In part 3 especially I am confused as to whether or not the energy is flowing along, but outside the wire in an EM field (and not between the two transmission lines). If so, are there any good visualizations of this?
  • How does distance between the two wires affect the transmission line analysis? For example, if the distance was (>>1) meter? My guess is that the inductive and capacitive coupling would be weaker with greater distance so there would be less current flow = less brighter bulb in part 1, and at a certain point the distributed element transmission line model would no longer apply?
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  • \$\begingroup\$ Could you reduce your question to a minimal working example; i.e., a circuit along with analysis (equations), that replicates your difficulty. Make it one question per page so that you can get focused answers. \$\endgroup\$
    – Syed
    Nov 28, 2021 at 5:45

1 Answer 1

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the bulb lights immediately. during the first roundtrip time, it will see a source resistance of Z0. After this time, it will only see the DC resistance of the cable and become brighter.

It doesn't die off after the "initial transient" because there are infinitely many such "transients" during the first roundtrip time, essentially they become a "plateau of transients" if you will.

When the cables are further, Z0 increases..If the bulb has a resistance of about Z0 or lower, it would thus receive less power initially. If the bulb resistance is very high in comparison to Z0, it wouldn't be much affected by the increased gap.

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