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Let's say you have a wire connected to a device that you can switch between high-voltage and ground voltage. The wire is not connected to anything on the other end. You're observing the voltage in two places: one (#1 in the pic) very close to the device and one (#2 in the pic) at the far end of the wire. The wire has some load capacitance, so between the time the voltage toggles from low to high or high to low and the time that the wire is uniformly "pressurized", #1 and #2 will have different readings since the signal does not propagate instantaneously.

My question is this: would the voltage change propagate down the wire as a "sharp line", where at some point in time #1 would be close to high and #2 would be close to low? Or would #1 and #2 rise/fall more or less in tandem with just a tiny delay between the two?

  • $\begingroup$ Take a look here: en.wikipedia.org/wiki/Telegrapher%27s_equations $\endgroup$ – Sebastian Riese Aug 7 '19 at 18:17
  • $\begingroup$ Yes, the voltage change will travel down the line as a fairly sharp rise in voltage, and then will reflect off the far end of the wire and then propagate back to the voltage source and will (in general) reflect off it and then back towards the far end of the wire where it will reflect again, etc.. This process of multiple reflections will go on many times until the voltage is fairly uniform and constant across the length of the wire. This can all happen in a very, very small fraction of a second. Google information on "transmission line theory". $\endgroup$ – user93237 Aug 7 '19 at 18:24
  • $\begingroup$ @SamuelWeir let's say you go from low to high. Once #1 sees high voltage, would it ever see low voltage again? Or would it see high voltage, and then when the reflection comes back, see momentarily super-high voltage? $\endgroup$ – PistoletPierre Aug 7 '19 at 18:27
  • $\begingroup$ @SamuelWeir I have a (relatively) long wire in a digital circuit where #1 and #2 are gate inputs that I want to act on with a picosecond-scale time gap between them; would #1's signal be stable before #2's was stable in the event of a change of voltage on the wire? Like, is this feasible or am I barking up a silly tree? (EDIT): I've confirmed that I can delay a signal by a few picoseconds by lengthening a wire from point A to point B. But I want to know if I can get from point A to point B_1 in time_x and point B_2 in time_x + time_y just by spacing them out on the same wire. $\endgroup$ – PistoletPierre Aug 7 '19 at 18:40
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    $\begingroup$ Cross-post here electronics.stackexchange.com/questions/452006/… $\endgroup$ – pipe Aug 7 '19 at 21:24

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