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Electron drift speed in metals

  1. Dec 26, 2011 #1
    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
     
  2. jcsd
  3. Dec 28, 2011 #2
    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.
     
  4. Dec 28, 2011 #3
    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: Dec 28, 2011
  5. Dec 28, 2011 #4

    nsaspook

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    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
     
  6. Dec 28, 2011 #5

    edguy99

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    Gold Member

    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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