Electricity is magnetism

By: Adrian (a.delete@this.acm.org), December 19, 2021 5:13 am
Room: Moderated Discussions
anon1 (anon1.delete@this.anon1.com) on December 19, 2021 2:51 am wrote:
> Adrian (a.delete@this.acm.org) on December 18, 2021 3:17 am wrote:
> > To determine the exact path of propagation from the power supply to the bulb, you need a 3-dimensional
> > geometric model of the environment, with the material parameters of everything present.
> >
> > Than you can compute the evolution in time of the charge density 4-vector and electromagnetic
> > potential 4-vector, after switching on the power supply. Then you can see the exact path.
> >
> >
> > However such an approach is seldom needed, as the actual path will not be too different from
> > the geometrically shortest path through the air, so you can approximate it like that.
>
> Here is something I do not understand (and sorry if this is very naive, my last exposure to physics of
> electromagnetism was in high school 20 years ago) — if the energy of the electromagnetic field is propagated
> to the LED on the "shortest" path rather along the wire, why is the wire even needed? That is, wouldn't
> it mean that the LED would receive energy even if were not connected to the circuit?
>
> Intuitively, what Hans de Vries wrote in his answer, makes a lot sense to me — there will be some sort of
> electromagnetic pulse received by the LED, but it will be brief and the energy transmitted will be very low.



It is not the energy that propagates through the air, but the electric potential (or in an alternative more complicated mathematical model, the electric field).

When the changed electric potential reaches the electrons inside some conductor, a force begins to act upon them, attracting or repelling the electrons.

When there are no wires connected to the power supply, the mobile electrons inside metals have nowhere to go, even if they are attracted or repelled, so they accumulate either closer to the surface oriented towards the power supply or closer to the opposite surface.

If there are wires connected to the power supply, then some electrons are extracted from the metal if it is connected to the positive terminal, or they are injected in the metal if it is connected to the negative terminal.

The wires are needed as a path for the mobile electrons. When they reach the power supply, they are pumped from the positive terminal to the negative terminal, closing the path of the electric current.

If on their path to the power supply the electrons that are attracted or repelled pass through some device, e.g. a lamp, they provide energy to the device, which is recovered when they pass through the power supply, so the energy is transferred through the flow of electrons. In stationary conditions, the electromagnetic field does not transfer energy, but only when there are accelerated electrons, which cause the radiation of electromagnetic waves.

Only in the instant when the power supply is switched on and it starts pumping electrons between its terminals, there is an acceleration of the electrons until the current is established. During this short phase, the potential difference between the power supply terminals grows from 0 to its nominal value and this potential difference begins to propagate in the environment.

Where a potential increase or decrease reaches some electrons, a force begins to act upon them and it can be said that those electrons have acquired a potential energy, i.e. if in the future they will move in the direction of the gradient of the potential, they will be able to transfer energy to whatever they encounter, like a body that falls from a height. Once the power supply is completely switched on, there is no more energy transfer through the electromagnetic field if the power supply is DC, while for an AC power supply at 50/60 Hz the energy transfer is not null but it is normally negligible.


Before the power supply is switched on, the 2 wires of a cable are electrically neutral, so the voltage between them is null.

After the power supply is turned on, if the near end of the cable is connected with wires to the power supply, electrons begin to be extracted from 1 wire of the cable and injected in the other wire of the cable.

At the near end of the cable, there is an imbalance in the electric charges and you can measure a voltage with a voltmeter. At the far end, the wires are still neutral and the voltage is null. In the near circuit, the current passes through all the circuit, because it is closed with electrons extracted and injected from the 2 wires of the cable.

On the cable, electrons continue to be extracted and injected, so the charge imbalance, together with the associated potential difference a.k.a. voltage, continues to extend from the near end towards the far end, with a speed of about 20 centimeter per nanosecond, faster or slower depending on the insulator used, until the potential difference reaches the far end.

Depending on what is connected at the far end, some current will be established through the far device, which will cause another change in the electric potential, which will propagate back to the near end. A steady-state voltage and current may become established only after several round trips.




When speaking about energy, it is good to keep in mind the difference between potential energy and other energy-like quantities, e.g. kinetic energy, work or heat.

In English, the word energy was used initially for what now is called kinetic energy. After the middle of the 19th century, the term "potential energy" began to be used, so what was initially named as energy was renamed first to "actual energy" and then to "kinetic energy". Nevertheless, nowadays "energy" is still frequently used in contexts where "potential energy" is not appropriate.

In all cases when the radiation or absorption of electromagnetic waves are not involved, the propagation of the electromagnetic potential or field may be said to transfer only potential energy.

That means that the appearance of the potential/field in some place does not cause any change yet wherever it happens, except that a force appears. The meaning of saying that potential energy has been transferred is just that conditions have been created so that, in the future, transfer of energy between bodies may happen, if the bodies will move in appropriate directions with respect to the electromagnetic potential.

The appearance of the electromagnetic field is like when you stay on some surface and you do nothing, but someone blows a huge hole immediately near you. Then you have acquired potential energy in the sense that if you will fall in that hole you will smash whatever it is on the bottom (or you will smash yourself).



















< Previous Post in ThreadNext Post in Thread >
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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