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Electron speed and movement through a copper wire

by Saado
Tags: copper, electron, movement, speed, wire
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Saado
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Jun5-14, 09:49 PM
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I've recently read that the electrons move relatively slowly to what I had previously thought through a metal. That the "drift velocity" is slow, but because the electrons are plentiful, they can pass down the signal very fast. Near light speeds. Could someone explain what I'm missing and this concept in general? I always thought that the velocity increased as kinetic energy did and that electrons have relatively high speeds even withing orbitals.
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nsaspook
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Jun5-14, 11:08 PM
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Quote Quote by Saado View Post
I've recently read that the electrons move relatively slowly to what I had previously thought through a metal. That the "drift velocity" is slow, but because the electrons are plentiful, they can pass down the signal very fast. Near light speeds. Could someone explain what I'm missing and this concept in general? I always thought that the velocity increased as kinetic energy did and that electrons have relatively high speeds even withing orbitals.
The Kinetic energy of a charge carrier due to it's drift velocity is incredibly tiny in a good conductor.

http://en.wikipedia.org/wiki/Speed_of_electricity
Ride the surprisingly slow electron express
Simon Bridge
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Jun5-14, 11:09 PM
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I think the bit you are missing is that the drift velocity of the electrons is an average velocity.
Electrons move in a wire like gas in a bottle - every which way. When there is no applied electric field, an electron is as likely to go one way as another, so the drift velocity is zero. With the electric field, there is a bigger chance of going one way than the others.

You can estimate the rms thermal speed of electrons in the wire at room temperature by treating them as an ideal gas and using the kinetic theory. That's much faster than the drift velocity.

This site has a widget to illustrate the motion of just one electron with different applied electric fields.

Saado
#4
Jun6-14, 06:58 AM
P: 33
Electron speed and movement through a copper wire

Okay that ideal gas description worked really well. So if I send an alternating and or direct current through a wire, I read that the speed from one end to the other would be very near c. How is this achieved? Is it due to the interactions between electrons along the way?
UltrafastPED
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Jun6-14, 04:07 PM
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Quote Quote by Saado View Post
Okay that ideal gas description worked really well. So if I send an alternating and or direct current through a wire, I read that the speed from one end to the other would be very near c. How is this achieved? Is it due to the interactions between electrons along the way?
It's not the electrons, but the electric fields which travel rapidly ... a kind of knock-on effect.

See https://en.wikipedia.org/wiki/Speed_of_electricity
and the references.
Saado
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Jun6-14, 04:16 PM
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Thanks :)
Simon Bridge
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Jun6-14, 07:02 PM
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Quote Quote by Saado View Post
Okay that ideal gas description worked really well. So if I send an alternating and or direct current through a wire, I read that the speed from one end to the other would be very near c. How is this achieved? Is it due to the interactions between electrons along the way?
So if electrons are so slooowww... how is it that electrical signals are so fast?
Well what he said - there are a lot of references about this online.

There are two commonly experienced[1] situations that shed light on the situation:
1. if you hit one end of a pipe, the sounds gets to the other end even though the individual atoms in the metal do not travel that far. How does this happen?
2. you have a hose full of water and turn on the tap - water comes out the other end almost immediately, but it takes the water much longer than that to travel the length of the pipe.
(You can do the same fort of thing for a gas - blow in one end of a pipe full of gas and some comes out the other end well before the gas from your lips could have traveled the length of the pipe.)

Considering these common experiences, is it really all that surprising that a signal can go down a wire faster than the electrons themselves travel?

----------------------------

[1] if you have not experienced these situations - then you should arrange to soon.


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