Re: When electricity is generated, where do the electrons originate?

Hello, Paul - sorry for the delay in replying to your question; I just 
returned from a trip offshore.   You've posed an interesting question, 
and one that is in perhaps one of the more abstract areas of science, 
so the difficulty in getting a conceptual picture of it  despite your own 
background and attempts by scientifically educated engineers on 
your team is really not too surprising.

Anyway, the short answer is that the electrons pretty much originate 
and stay within the conductor or are replenished with a source at the 
more negative terminal, and no, the matter is not changed...but let's 
start with the area of abstraction that makes this all confusing.

In a noble gas such as helium, each electron is pretty much tied to 
one specific atom. If we pass electricity through a noble gas (and we 
do with neon lights, for example), then the matter would be changed - 
the gas in a neon light is ionized because some atoms have been 
stripped of their electrons.

In simple compounds, the electrons become a little less locallized - 
they can be shared between adjacent atoms in covalent bonds.

In crystalline, semiconducting materials such as silicon, the 
electrons become even less locallized.  The s and p orbitals and 
such that we have come to know and love of the atoms in the crystal 
lattice have combined together into two energy bands. There is one 
energy band corresponding to the electrons that are tied to the 
nucleii, called the valence band, and another energy band called the 
conduction band corresponding to the relatively small number of 
electrons that have enough energy (either just due to the temperature 
or through doping methods) that they are available for conduction.




Metals also have that crystalline structure, though they might not look 
much like other crystals.  Let me quote Dr. Richard Fitzpatrick, 
Assistant Professor of Physics at University of Texas at Austin from his 
lecture on his website: The conduction electrons in a metal are
 non-localized (i.e., they are not tied to any particular atoms). 
These non-localized electrons are what give metals many of their unusual properties, 
including the reflectivity to light, electrical and thermal conductivity.

In metals, the conduction bands and valence bands basically 
overlap. The electrons are sometimes referred to as an "electron 
gas" - and some properties of this electron gas are modelled with 
variations of the ideal gas law, as if they were a microscopically thin 
layer of gas loosely adhering to the crystalline structure of the metal.

Not all of the electrons are actually conducting - 
in copper at room temperature,  less than 1% of electrons are 
available to conduct electricity. This link calculates a value of 0.4%.

Let's come up with a few numbers for a specific example. For 15 
gauge wire SWG or BWG (13 gauge AWG), with 2.63 square 
millimeters for a cross sectional area, there are 2.2e23 total 
electrons per meter.  0.4% of the electrons corresponds to 8.9e20 
electrons per meter. To put it a bit into context, an Ampere is 6.28e18 
electrons per second. 

Let's come back to the electrons in motion.  According to the 
calculations

hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic

here, the mean free path of electrons in copper  is 3.9e-08 meter -   
39 nanometers. Anyway, you asked, "But if electricity is electrons in 
motion, where do they originate? " They are mostly already in the 
metal, and the few electrons that come out of the more positive end of 
a wire are replenished at the more negative end.  At some point, 
most circuits terminate at Ground.  The earth itself, the Ground, can 
be considered an infinite source of free electrons to replenish the 
supply  of electrons. 

I hope this provides the explanation you were looking for. Please let 
me know if you need any additional clarification.

And - my best wishes for Mendell Energy for continued success in 
finding reserves of oil and gas - these efforts are increasingly 
important to our future!

Best regards,
Eric Maass