Speed of Electricity in Wiring - Why near light speed?

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Electricity (the flow of current) is said to travel at near the speed of light (75-90%), but I read that electrons have a drift velocity of only 2-3 mm/hr or something close to that. So, if the electrons are so slow, should I think it is the holes that travel fast? Suggestions.
 

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  • #2
Hootenanny
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Electricity (the flow of current) is said to travel at near the speed of light (75-90%), but I read that electrons have a drift velocity of only 2-3 mm/hr or something close to that. So, if the electrons are so slow, should I think it is the holes that travel fast? Suggestions.
Whilst the electrons travel slowly, the signal, i.e. the electromagnetic field, propagates close to the speed of light in a vacuum.
 
  • #3
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Have you ever been stopped at a traffic light ? Are the speeds of individual cars and of the information on green light the same ? Does this surprise you ?
 
  • #4
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Whilst the electrons travel slowly, the signal, i.e. the electromagnetic field, propagates close to the speed of light in a vacuum.

If it is the EM field that is moving down the wires, then would a 1 Telsa rare earth magnet cause the current in the wire to change?
 
  • #5
Hootenanny
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If it is the EM field that is moving down the wires, then would a 1 Telsa rare earth magnet cause the current in the wire to change?
Yes external magentic fields will affect the current flowing in a conductor.
 
  • #6
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Yes external magentic fields will affect the current flowing in a conductor.

Guess I'll get out a DC circuit with a light bulb and see if it changes.
 
  • #7
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Does the EM field move in the same direction that the electrons move? I read somewhere that accelerating charges emit EM waves perpindicular to the direction of movement of the charges (not parallel). Where am I going wrong?
 
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Does the EM field move in the same direction that the electrons move? I read somewhere that accelerating charges emit EM waves perpindicular to the direction of movement of the charges (not parallel). Where am I going wrong?

Based on my reading, the electrons only move very slightly and moved forward but with random motion. This suggests the electrons themselves might generate EM waves in random directions.

You are correct, that free electrons and protons that travel around synchrotrons do emit EM waves when they are accelerated by the magnetic wigglers and that the EM is perpendicular.

If Hootenanny's picture is correct that an EM field is moving down the wire, then it seems that electricity (flow of current) could be viewed as a form of light, aka electromagnetic radiation.

Does that make sense?
 
  • #9
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If Hootenanny's picture is correct that an EM field is moving down the wire, then it seems that electricity (flow of current) could be viewed as a form of light, aka electromagnetic radiation.
That is certainly not Hootenanny's picture, and is wrong. Think about the traffic light. For once, you do have a mechnical model.
 
  • #10
rcgldr
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Free electrons travel at high speed in random directions. The "drift" velocity is just the net component of this high speed travel in a specific direction.
 
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That is certainly not Hootenanny's picture, and is wrong. Think about the traffic light. For once, you do have a mechnical model.

Sorry Hootenanny for the mis-quote. I mis-read. You simply wrote that EM fields travel at the speed of light in vacuum which is an accepted concept.
 
  • #12
rcgldr
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The speed of propagation of a change in current in a wire is near light speed.

As a weak analogy, imagine billiard balls spaced 1 mm apart, and then colliding the first ball with another ball at slow speed. Each ball moves slowly, but the rate of propagtion of energy from one end of the chain of balls to the other moves much faster than the balls themselves.

There is some interaction between free electrons and the molecules of a wire. When a free electron is captured by a molecule, the molecule frees up another electron quite rapidly. I don't know how fast this "information" regarding the excess molecule is propagated. From what I understand, most of the electron movement involves free electrons, that pass between molecules as opposed to colliding with them.
 
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Hootenanny
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That is certainly not Hootenanny's picture, and is wrong. Think about the traffic light. For once, you do have a mechnical model.
Thanks for clearing this up in my absence humanino.
If it is the EM field that is moving down the wires
Let me just clarify something here. As I'm sure you know, current is a measure of the flow of charge through a particular surface. Now, the current is dependent on the applied external fields, but by no means are the Electric or Magnetic fields considered to be the current. Furthermore, the EM fields do not continuously propagate down the conductor, once a steady current has been established, the EM fields of the wire remain constant.
 
  • #15
Redbelly98
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Electricity (the flow of current) is said to travel at near the speed of light (75-90%), but I read that electrons have a drift velocity of only 2-3 mm/hr or something close to that. So, if the electrons are so slow, should I think it is the holes that travel fast? Suggestions.

I like to explain this in terms of the water-flow analogy that is popular with electrical engineers.

Imagine a garden hose full of water, where the faucet is closed but the end of the hose is open. The water is just sitting there in the hose.

Now turn on the faucet. Water starts coming out the open end almost instantly, even though water actually takes several seconds to travel the length of hose from the faucet to the opening. Actually, the time it takes for water to start flowing out the end is related to the speed of sound in water. It's the time it took for the pressure increase at the faucet end to propagate to the open end.

Like water molecules, electrons exert a force or push on each other. Electron motion at one location in a circuit propagates at the speed of light in that medium, so that electrical currents are initiated quite quickly.
 
  • #16
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Whilst the electrons travel slowly, the signal, i.e. the electromagnetic field, propagates close to the speed of light in a vacuum.

About 7" per nanosecond as opposed to 12 in real life.
 
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  • #17
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If the electrons in the wire accelerate on random directions, do the electrons emit EM radiation ? (DC current on steady state).
 
  • #18
rcgldr
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Signal progration velocity depends on the dielectric constant of the wire and the surrounding insulator. The equation is:

[tex]V = \sqrt{\frac{1}{\varepsilon _r}}[/tex]

A vacuum is defined to have a constant of 1, so it doesn't slow down the propagation rate. Air is just a bit more at 1.0059 (at 20C), while water is 80.4. So if a wire is submerted in water, the propagation rate could be slowed by a factor of almost 9. A link to some dielectric constants:

http://hyperphysics.phy-astr.gsu.edu/Hbase/tables/diel.html

This gets more complicated for AC signals. A couple of links about coax cables, signal speeds, and dielectric constants.

http://www.radio-electronics.com/info/antennas/coax/coax_velocity_factor.php

http://en.wikipedia.org/wiki/Coaxial_cable
 
  • #19
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There is little or no comparison between the movement of drifting electrons and those subjected to a Voltage (EMF). Plus in Physics, we are more more concerned with the holes left by the electrons as they move down the wire. So the direction of the current is reversed. But I digress, This is an apples vs oranges discussion and the EMF is the difference!
Edmund
 
  • #20
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Sorry guys, but I'm not sure yet.
What is it that is traveling at near light speed down our copper wires?
While your at it, why do you we need two wires for AC?
And has anyone heard of the Catt anomaly about electric current that is heavily debated in the UK?
 
  • #21
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There is little or no comparison between the movement of drifting electrons and those subjected to a Voltage (EMF).

Dozen. The term 'drift velocity' refers to the average velocity of a free electron in a conductor subject to an electric field .

Plus in Physics, we are more more concerned with the holes left by the electrons as they move down the wire. ...

Holes are of interest in P-type semiconductor materials. The usual conducting materials such as metals do not have holes is normal conditions. N-type material has no holes unless it is coerced into it. 'Holes' are valence band energy states unoccupied by electrons.

Edit: Rather than holes, you are probably thinking of 'positive current'. Positive current is a useful fiction in analyzing and designing electronic circuits. It works just as well to pretend that the current is due to positive charge carriers as negative in most any case, in these efforts.
 
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  • #22
rcgldr
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What is it that is traveling at near light speed down our copper wires?
Changes in current, or information related to voltage / current propagate at near light speed. For example the speed it takes a current to "reach" a light bulb after a switch is turned on. If the current isn't changing, then then the wire is in a steady state.

While your at it, why do you we need two wires for AC?
Two lines are needed to complete the circuit back to the AC generator, which is generating a voltage potential between the two lines, and current if there's a load in the circuit.
 
  • #24
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Changes in current, or information related to voltage / current propagate at near light speed. For example the speed it takes a current to "reach" a light bulb after a switch is turned on. If the current isn't changing, then then the wire is in a steady state.

Two lines are needed to complete the circuit back to the AC generator, which is generating a voltage potential between the two lines, and current if there's a load in the circuit.

OK.
Please rephrase "changes in current".
Please don't use the term "information" as it is a bogus term as far as I know.
 
  • #25
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Correction to my prvious post. Single line systems are possible:
http://en.wikipedia.org/wiki/Electrical_power_grid

However typical AC power distribution in the USA uses a 3 wire system, to handle the 3 phase AC output of the generators, plus a 4th ground wire (or earth ground):

http://science.howstuffworks.com/power.htm/printable


Wow, looks like someone came into my house and stole all of the 3rd wire in my extension cords and my 50 yr old house. Didn't even notice they were gone!
 
  • #26
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Delta to Y conversion. basic electrical wiring! Nothing in your house uses a Delta connection so you can have a Ground!
 
  • #27
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The speed of propagation of a change in current in a wire is near light speed.

As a weak analogy, imagine billiard balls spaced 1 mm apart, and then colliding the first ball with another ball at slow speed. Each ball moves slowly, but the rate of propagtion of energy from one end of the chain of balls to the other moves much faster than the balls themselves.

I thought current was caused by an imposed electric field driving the electrons at drift velocity and not collisions between the individual electrons? Is the speed of current propagation like a refractive index of the electric field moving through/in/around the electrons? Why is the speed of propagation less than c if its an electromagnetic phenomenon?
 
  • #28
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"For example, sound can flow inside an air-filled tube, while electrical energy always flows in the space outside of the wires, and does not travel along within the metal wires. However, electrical energy is coupled with compression waves in the electrons of the wire. Electron-waves travel inside the wires, yet the energy they carry is in the invisible fields surrounding the wires." - ELECTRIC CURRENT IS A FLOW OF ENERGY? Wrong.
from parent site http://amasci.com/miscon/elect.html"

Is the energy carried in the electromagnetic field surrounding the wire what propagates near the speed of light and not the wave caused by electrons bumping into each other? Or is the author describing them as one in the same? Do both the electron-wave and the electric field move at the same speed, and is this the speed of electricity that moves slower than c? Any help will be greatly appreciated. Thank you.
 
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  • #29
Hootenanny
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"For example, sound can flow inside an air-filled tube, while electrical energy always flows in the space outside of the wires, and does not travel along within the metal wires. However, electrical energy is coupled with compression waves in the electrons of the wire. Electron-waves travel inside the wires, yet the energy they carry is in the invisible fields surrounding the wires." - ELECTRIC CURRENT IS A FLOW OF ENERGY? Wrong.
from parent site http://amasci.com/miscon/elect.html"

Is the energy carried in the electromagnetic field surrounding the wire what propagates near the speed of light and not the wave caused by electrons bumping into each other? Or is the author describing them as one in the same? Do both the electron-wave and the electric field move at the same speed, and is this the speed of electricity that moves slower than c? Any help will be greatly appreciated. Thank you.
Since the electrons are moving they do have some kinetic energy, but this is usually very small. However, electrons are not responsible for carrying the signal, this is done by the electromagnetic fields.

Electron-waves isn't really an accurate description of the motion of electrons within a wire. When there is no potential applied to a wire the electrons move randomly in all directions with an [average] speed equal to the fermi-velocity. Now, when a potential is applied all the electrons are subjected to a force in the same direction and so begin to move in that direction, they are still moving in all directions but are all drifting in the same direction. As an analogy consider a truck with thousands of bouncy balls in the trailer all bouncing around randomly. When the truck is stationary all the balls are bouncing around randomly. When the truck starts moving forward, all the balls are still bouncing randomly inside the trailer, but they are all also drifting forward.

Does that make sense?
 
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  • #30
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Since the electrons are moving they do have some kinetic energy, but this is usually very small. However, electrons are not responsible for carrying the signal, this is done by the electromagnetic fields.

If I'm understanding this correctly, the signal of the applied potential, which causes the drift velocity of the electrons, is itself the electromagnetic field in and around the wire, or an EM wave. According to Jeff Reid:

Signal progration velocity depends on the dielectric constant of the wire and the surrounding insulator. The equation is:

[tex]V = \sqrt{\frac{1}{\varepsilon _r}}[/tex]

A vacuum is defined to have a constant of 1, so it doesn't slow down the propagation rate. Air is just a bit more at 1.0059 (at 20C), while water is 80.4. So if a wire is submerted in water, the propagation rate could be slowed by a factor of almost 9.

I don't understand the meaning of the dielectric constant, but http://en.wikipedia.org/wiki/Dielectric_constant#Practical_relevance" said that the constant was related to the refractive index of materials, a subject I somewhat understand. Assuming the two are related: since the electromagnetic signal has to travel through and around the wire, both with refractive indexes less than than of a vacuum, the speed of the signal propagation of electricity is less than c?
 
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