Speed of Electricity in Wiring - Why near light speed?

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Discussion Overview

The discussion revolves around the speed of electricity in wiring, specifically addressing the apparent contradiction between the slow drift velocity of electrons and the near light speed at which electrical signals propagate. Participants explore concepts related to electromagnetic fields, drift velocity, and the nature of current flow in conductors.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • Some participants note that while electrons have a slow drift velocity (2-3 mm/hr), the electromagnetic field propagates close to the speed of light in a vacuum.
  • There is a comparison made between the movement of electrons and the propagation of signals, likening it to a traffic light analogy.
  • Some participants question whether the electromagnetic field moves in the same direction as the electrons and discuss the implications of accelerating charges emitting electromagnetic waves.
  • One participant suggests that the flow of current could be viewed as a form of light, but this is contested by others who clarify that electromagnetic fields do not continuously propagate down the conductor once a steady current is established.
  • There is mention of external magnetic fields affecting current flow in conductors, with a participant expressing intent to test this with a circuit.
  • Some participants discuss the relationship between the drift velocity of electrons and the speed of signal propagation, using analogies such as water flow in a hose.
  • The impact of the dielectric constant on signal propagation velocity is also raised, with references to specific values for different materials.
  • A participant highlights the distinction between the movement of drifting electrons and those subjected to voltage, emphasizing the importance of holes in current flow.
  • Questions are posed about what exactly travels at near light speed in copper wires and the necessity of two wires for AC current.
  • There is a reference to the Catt anomaly, indicating ongoing debate in the UK regarding electric current.

Areas of Agreement / Disagreement

Participants express various viewpoints, and there is no consensus on several key aspects, including the nature of what travels at near light speed and the implications of electromagnetic fields in current flow. Disagreement exists regarding the interpretation of analogies and the relationship between electron movement and signal propagation.

Contextual Notes

Some limitations are noted, such as the dependence on definitions of current and drift velocity, as well as the complexity introduced by different materials affecting signal speed. The discussion also touches on unresolved questions about the nature of electromagnetic radiation emitted by electrons.

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  • #32
Regarding the travel of an electron in a conductor, I might suggest one examine the relationship of drift velocity to Fermi speed (velocity) and its relationship to conductance.

But still there is the category mistake of equating Fermi velocity and actual electron velocity with the propagation velocity of an electromagnetic field. And if you choose to view this from the perspective of a particle (as distinct from a wave model), then you might want to further consider the refractive index modifying the velocity of particles in a medium for an EM field..
 
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  • #33
I've read the thread but I still do not understand.

Can someone help explain how the propagation of an electromagnetic wave is near the speed of light, but the speed of electricity is slow? (the water/traffic light analogies don't make sense to me)

Does 'speed of electricity' mean the speed the electrons are traveling?

http://img59.imageshack.us/img59/9545/switchbulb.png
Could someone explain it using this picture of a bulb receiving electricity from a switch

Let's say the distance from the switch to the bulb is 100 meters. Somehow the bulb will receive electricity almost instantly. Maybe someone could show where the propagation of an electromagnetic wave comes into play?

I thought electromagnetic wave was something to do with virtual photon pairs (not sure how classical explains electromagnetic waves). Maybe this is why it can be fast. Hope someone can explain! :)

edit: I think I'm asking what is the relationship between electromagnetic waves and electrons, and how that relationship results in the bulb being quickly lit up.
 
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  • #34
It's much simpler than that. As soon as that switch is closed, a driving force (a voltage), is applied to the electrons in the wire. The bulb receives power as soon as the electrons RIGHT next to the bulb receive the signal (voltage change) to move forward; why does the bulb care if it gets the electrons coming out of the switch or not?

Also, electromagnetic waves ARE a classical concept. They follow directly from Maxwell's Laws (a collection of 4 laws that explain electromagnetic phenomena).
 
  • #35
NruJaC said:
It's much simpler than that. As soon as that switch is closed, a driving force (a voltage), is applied to the electrons in the wire. The bulb receives power as soon as the electrons RIGHT next to the bulb receive the signal (voltage change) to move forward; why does the bulb care if it gets the electrons coming out of the switch or not?

Also, electromagnetic waves ARE a classical concept. They follow directly from Maxwell's Laws (a collection of 4 laws that explain electromagnetic phenomena).
But how can it be explained in terms of speed?
If the speed of electricity is slow (millimeters per second) how can the bulb light up instantly?

Doesn't the 'electricity' have to travel the whole length of the wire? Or is it something like the single electrons pass it on to a neighboring electron?
 
  • #36
The "speed of electricity" IS the speed of the electromagnetic waves; in a way you can think of electrons as the "medium" that the wave travels in (this is not quite correct, but close).
The fact that electrons happen to move around in the medium is more or less irrelevant; there is certainly no direct relation between their speed and the speed of the wave (or equivalenty the speed at which energy can be transfered).

The bulb lights up instantly because the necessary energy is carried by the wave; not the electrons (although in order for the bulb to light up that energy has to be converted from electromagnetic energy to heat via various scattering processes).
 
  • #37
I'm beginning to have a better understanding now!

Although, Wikipedia says 'the speed of electricity' is often confused with 'the propagation speed of an electromagnetic wave'. So in that sense 'the speed of electricity' is the speed of the electrons, and not of the EM wave.
http://en.wikipedia.org/wiki/Speed_of_electricity

f95toli said:
The bulb lights up instantly because the necessary energy is carried by the wave; not the electrons
What is the wave made of? Photons?
 
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  • #38
antd said:
Although, Wikipedia says 'the speed of electricity' is often confused with 'the propagation speed of an electromagnetic wave'. So in that sense 'the speed of electricity' is the speed of the electrons, and not of the EM wave.
http://en.wikipedia.org/wiki/Speed_of_electricity
The usual terminology for the average net velocity of the electrons is "drift velocity". In all of my years of experience I have never heard the drift velocity called "the speed of electricity". Wikipedia is notoriously unreliable.

Maybe the hyperphysics page will help:
http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/ohmmic.html
 
  • #39
Thanks.

Slowly learning not to trust wikipedia ;-)
 

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