Electron drift speed in metals

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Discussion Overview

The discussion centers on the phenomenon of electron drift speed in metals, exploring the reasons behind the relatively low drift speed of electrons despite the rapid flow of current. Participants examine analogies and calculations to clarify this concept, touching on theoretical and conceptual aspects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions the physical explanation for the low drift speed of electrons in metals, seeking clarification on the apparent contradiction between slow electron movement and quick current flow.
  • Another participant uses an analogy of water flow in a pipe to illustrate that while the current appears to flow quickly, the actual movement of electrons is slow, with the signal traveling at a fraction of the speed of light.
  • A third participant notes the high concentration of electrons in metals, specifically copper, and calculates that a current of 1 Amp corresponds to a significant number of electrons moving per second, yet their drift speed remains low.
  • One participant introduces the "bucket brigade" analogy, suggesting that while electrons move slowly, they can transfer energy quickly, akin to people passing buckets of water rapidly.
  • A calculation is presented for the drift speed of electrons in a copper wire, indicating a very slow speed, while also mentioning that individual electrons can move much faster between atomic sites.

Areas of Agreement / Disagreement

Participants generally agree on the low drift speed of electrons and the use of analogies to explain the phenomenon. However, there are multiple perspectives on how best to conceptualize the relationship between electron movement and current flow, indicating some disagreement on the most effective analogy.

Contextual Notes

Some assumptions regarding the analogies used may not be fully articulated, and the discussion does not resolve the complexities of electron behavior in metals, such as the distinction between drift speed and individual electron velocities.

tomwilliam
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Is there a physical explanation for why electrons move through a metal with a relatively low drift speed? Or is it just the observed phenomenon?
I find it hard to visualise electrons moving relatively slowly through a metal despite the current flowing through it being quick...can anyone help?
Thx
 
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For a gross simplification think of a pipe full of water. If you push a little bit of water in one end then some comes out the far end almost instantaneously. It's not really instantaneous, it depends on the speed of sound in the pipe. The important thing is that it isn't the same piece of water coming out that you put in. The actual flow rate might be tiny. Free electrons in a wire can also be imagined behaving as an incompressible fluid. Apply an impulse at one end and it travels through the wire at some fraction of the speed of light, say 0.7c, but the electrons barely move.
 
Another feature is that the concentration of electrons in a metal is very large...about 10^28 per m^3 in copper.
Therefore a current of 1Amp or 1Coulomb/sec means a flow of 1/1.6x10^-19 electrons per second or about 7x10^18 electrons per second.
With a concentration of 10^28 per m^3 this means that electrons move surprisingly slowly.
ColinW's water analagy is also a very good one.
 
Last edited:
The bucket brigade is better as the electrons (charge carriers) act a slightly moving men with fast hands holding (buckets/electrons) pouring to the next mans bucket the much faster moving photons (water/energy).

Feynman’s lecture on electron interactions is great.
http://vega.org.uk/video/programme/47
 
The speed of electon drift for a copper wire with a diameter of 1 mm, and a current I=3 amperes, can be calculated to be 0.0003 m/s, a very slow 1.0 m/hour.

The big wire cross-section compared to the very small copper nucleon & electron grid size means that although the electrons are going by at slow speeds, individual electrons are jumping from one grid spot to another using much faster speeds approaching 1000.0 m/s.
 

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