Electricity is magnetism

By: Adrian (a.delete@this.acm.org), December 17, 2021 5:24 am
Room: Moderated Discussions
⚛ (0xe2.0x9a.0x9b.delete@this.gmail.com) on December 16, 2021 5:22 pm wrote:
> Adrian (a.delete@this.acm.org) on December 15, 2021 4:06 am wrote:
> > Brett (ggtgp.delete@this.yahoo.com) on December 14, 2021 3:01 pm wrote:
> > > Electricity is magnetism:
> > > https://youtu.be/bHIhgxav9LY
> >
> > So the video just plays with words and it presents normal circuit behavior as unexpected.
> >
> > The only right part of it is stressing that if you have a circuit with parts close to the power
> > supply and parts very distant to it, the electromagnetic field will propagate instantaneously to
> > all the near parts, so conduction in them can start immediately, while the distant parts, regardless
> > where they are placed in the circuit, will behave as long lines so at their insertion point in the
> > circuit they might have an inductive, capacitive or resonant behavior. Depending on the details
> > of the circuit, the bulb may light immediately as in the video, or not, in other cases.
>
> Assuming the circuit is built as imagined in the video (that is: the shortest distance between the light
> bulb and the power supply is 1 meter; c = 300e6 m/s; 1/c = 3.3e-09; the total length of the cables in
> the circuit is [left+right = c+c = 2*c = 600e6 meters]): What is the minimum voltage of the power supply
> in order for the light bulb to reliably emit 1 photon in 1/c seconds after closing the switch?

>
> -atom


Whether the bulb lights immediately or not depends on other causes.


If you insert in the circuit with the bulb a cable that might be connected to something far away, maybe at the other end of the Earth, what happens immediately after switching on depends on how the cable is made.


Depending on cable construction, every cable has a parameter called its characteristic impedance, which is determined by the inductance per length of cable and by the capacitance per length of cable.

When you switch on, it does not matter how far the cable goes, the initial current, established after the near end of the cable is reached with the speed of light, equals the current that would exist if the cable would be replaced by a resistor having the resistance equal to the characteristic impedance.

For electrical cables of normal construction, the characteristic impedance is in the range 30 Ohm to 150 Ohm, which is low enough that when the cable is placed in a circuit at mains voltage with a bulb, the lamp will light immediately.

After switching on, the electromagnetic potential propagates along the cable at a speed that depends on the insulator used. The speed might be e.g. 2 thirds of the speed of light in vacuum.

At the far end, the cable will be connected to something, and after an electrical current is established at the far end, a corresponding change in the electromagnetic potential will travel back to the near end.

If the far end is left open, so that no current can pass, after the modified potential comes back to the near end, the flow in the circuit will stop and the lamp will light no more.

If at the far end the cable is terminated on a resistor equal to its characteristic impedance, then no potential change will travel back and no change will ever be seen at the near end. For different far end cable terminations, after the potential change returns to the near end, the lamp will become brighter or dimmer than in the initial moment after switching on.


In conclusion, what happens in a circuit immediately after switching on depends only on the parts that are near the power supply. The parts that are remote, they do not matter, only how the near end of the cable that connects them is made (i.e. the thickness of the wires and insulators and what type of insulator is used).

For the initial moment, any cable behaves like a small resistor, allowing current flow, and what is at the far end influences the circuit only after the electromagnetic potential travels a round trip.



























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TopicPosted ByDate
Electricity is magnetismBrett2021/12/14 04:01 PM
  *facepalm* (NT)Ray2021/12/14 05:16 PM
    in fairness what started out trollish turned into a really informative thread (NT)anonymous22021/12/15 06:55 PM
  Electricity is magnetismAnon2021/12/14 06:16 PM
    Electricity is magnetismLightning2021/12/14 06:47 PM
      Electricity is magnetismDavid Hess2021/12/14 10:47 PM
    Electricity is magnetismBrett2021/12/15 12:12 PM
      Electricity is magnetismSimon Farnsworth2021/12/15 02:30 PM
      Electricity is magnetismAnon2021/12/15 03:03 PM
      Electricity is magnetism---2021/12/16 12:19 PM
        Electricity is magnetismAdrian2021/12/16 03:51 PM
  Electricity is magnetismAdrian2021/12/15 05:06 AM
    Sorry for a few typos, I was in a hurry, but the typos do not affect the meaningAdrian2021/12/15 05:40 AM
    Electricity is magnetism2021/12/16 06:22 PM
      Electricity is magnetismEtienne Lorrain2021/12/17 02:44 AM
        Electricity is magnetismBrett2021/12/17 01:44 PM
      Electricity is magnetismAdrian2021/12/17 05:24 AM
        Electricity is magnetism2021/12/17 02:49 PM
          Electricity is magnetismAdrian2021/12/18 03:54 AM
            Electricity is magnetismAdrian2021/12/18 04:17 AM
              Electricity is magnetismanon12021/12/19 03:51 AM
                Electricity is magnetismAdrian2021/12/19 05:13 AM
                Electricity is magnetismDavid Hess2021/12/19 06:54 PM
  Electricity is magnetismzArchJon2021/12/15 11:53 AM
  This video is just really totally wrong..., sorry.Hans de Vries2021/12/15 06:26 PM
    This video is just really totally wrong..., sorry.anon2021/12/16 05:03 AM
  Electricity is magnetismBrett2021/12/19 06:02 PM
    Electricity is magnetismDavid Hess2021/12/19 07:11 PM
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