Electricity is magnetism

By: Adrian (a.delete@this.acm.org), December 18, 2021 3:54 am
Room: Moderated Discussions
⚛ (0xe2.0x9a.0x9b.delete@this.gmail.com) on December 17, 2021 1:49 pm wrote:
> Adrian (a.delete@this.acm.org) on December 17, 2021 4:24 am wrote:
> > ⚛ (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.
> The above is (from my personal viewpoint) rational, except for the "the lamp will
> light immediately" part because causality prevents the two events from happening
> at the exact same moment. 1/c seconds isn't immediate (zero seconds is).
> By asking "What is the minimum voltage of the power supply?" I didn't express very well what
> kind of question I am interested in the most. Sorry about that. Let me give it another try:
> In the video:
> - The distance between the switch and the power supply (battery) is a few centimeters
> - The shortest distance (shortest possible path) between the
> switch and the light bulb (LED) is approximately 1 meter
> +----150e6--------battery--switch-------150e6------+
> |##################################################| 1
> +----150e6--------------LED-------------150e6------+

> The video is implicitly claiming that information about the space-time event "the switch has just been closed"
> is being transmitted from the switch to the LED using the shortest possible path of approximately 1 meter (such
> as: via radio waves), instead of being transmitted in/along/via the wire of length 300e6 meters.
> Given your knowledge of the theory/theories of electromagnetism: Does the theory support the claim that information
> about the space-time event "the switch has just been closed" travels to the LED via the shortest path?
> (My original question "What is the minimum voltage of the power supply?" stems from the fact that in order
> for the LED to start emitting light there needs to be a transfer of at least 1 bit of information from
> the switch to the LED. In essence, I was asking: How much energy is needed in order to transmit this single
> bit of information, given the circuit in the "picture" above? I have no problem believing/understanding
> that the LED starts emitting light as soon as this bit of information is transmitted.)
> -atom

OK, you are right that I was imprecise with "immediately", but in the discussion it had been already mentioned many times that "immediate" means the time of propagation of the electric potential from the power supply to a point, at the speed of light.

For 1 meter distance to the bulb, that means some 3 to 5 nanoseconds (the insulation on the connecting wires slows a little the propagation speed, compared to bare copper wires in the air) until the bulb lights.

Also, you are right that the voltage of the power supply needs to be high enough so that the additional resistance added by the characteristic impedance of the cable will not dim the lamp too much.

Even at the lower mains voltage typical for USA, i.e. 120 V, the short circuit current at the initial time of switching the supply on an open cable will be between 1 A and 4 A.

A lamp consuming less than half of the short-circuit current will light well, even if slightly dimmer than when connected directly to the power supply.

While we do not know the exact type of the cable used, any lamp rated for less than 1 A is expected to light well enough, i.e. any lamp rated for less than 120 W, which should cover most consumer lamps.

So in the conditions assumed for the experiment, there was no surprise about how it behaved.

You are right that with a supply voltage much less than the mains voltage or with a very high power lamp, it would not light in the first moment, but only after a round-trip time, if a short is placed at the far end of the cable, to allow a high-enough steady-state current.

The reason why the cable behaves like a resistor until a round-trip completes is that when a voltage exists on a cable, that means that there is a charge imbalance between the 2 wires, the negative wire has more electrons and the positive wire has fewer electrons.

When the power supply is switched on, it pumps electrons between its terminals, so one terminal has more electrons and the other has fewer electrons. That creates a difference in electric potential between terminals, which starts to propagate with the speed of light in all directions.

When the difference of electric potential reaches other metallic conductors, the mobile electrons inside the metals start to move in the direction of the potential gradient, and those that move inside the lamp produce the light, while those in the wires of the cable move to create the potential difference that propagates from the near end of the cable towards the far end, and that movement corresponds to a current, because electrons enter the cable on the negative wire and are extracted from the positive wire of the cable.

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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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