Roger, the speed you're referring to is called the "drift velocity" of
electrons in a metal. The drift velocity is very slow, only about 1 meter
per hour. That's not the actual speed of the electrons, however, which is
about 1 percent of the speed of light.Think of the drift velocity as
analogous to the drifting motion of a cloud, whereas the Fermi velocity is
like the speed of the air molecules in the cloud.
The original calculation of the drift velocity was derived in 1900 and is
called the "Drude Model" of conduction (pronounced Drood-eh). It says that
the conductivity of a metal is equal to ne^2t/m, where n is the density of
electrons, e is the charge on an electron, t is the scattering time (the
time the electron moves before colliding with something, like an impurity
atom) and m is the mass of an electron. The drift velocity comes into the
equation by setting the drift velocity equal to the scattering distance
divided by the scattering time. It was a very crude estimate, but embodies
the basic notion that the scattering of electrons is what determines the
conductivity. At the time, it was thought that electrons had a spread of
velocities, like that of air molecules in the atmosphere, but later it was
learned that the electrons contributing to the conduction of electricity all
travel with the same speed, known as the Fermi velocity.
73,
Jim w8zr
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Roger (K8RI)
> Sent: Wednesday, August 28, 2013 6:04 PM
> Cc: amps@contesting.com
> Subject: Re: [Amps] Electron HOLE flow
>
>
> In an introductory course we had to figure out the average speed of the
> electron drift in a #10 wire carrying 1 Amp.
> Like the pool ball experiment the one entered and the other popped out
> almost instantly, but the average speed was somewhat faster than a fast
> walk IIRC. It's quite possible that the original electron never did
> make it to the other end.
>
> Although it was a fairly easy exercise, I don't remember the steps,
> other than all atoms were considered to have a free electron. The
> number of electrons in a coulomb, and Avogadro's number for atoms in a
> mole of copper.
>
> Anyone remember how to do that?
>
> 73
>
> Roger (K8RI)
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