Malcolm wrote:
Hi Marshall, we certainly disagree on this one, check out positive and
negative doping of semiconductors, obviously there can be more and
looser - so to speak - electrons within lattice structures and more than
there are protons to balance them.
Nope. You are changing things. We were talking about a metal wire, now
you are talking about doped semiconductors. But despite this change the
total number of electrons is ALWAYS equal to the total number of
protons. That is a physical fact. Now depending on doping levels, more
or less of them can be unbound, or as you say looser. That is a
completely different thing. That is as you increase the doping, more of
the total electrons will be free to move about and not bound. But the
total number of electrons does not vary.
Further, due to the strains between
crystal interfaces - polish some metal and etch it and you'll see them -
various parts of any piece of metal will have different quantities of
electron propagation at some voltage across the particular crystal
interfaces.
I guess you are talking about the barrier height here and junctions.
That once again has nothing to do with the total number of electrons.
And when you have a barrier, you can have an imbalance across the
junction, which acts just like a capacitor, but the total electrons of
the whole device will equal the total protons in the device, ignoring
any surface charge. The charge on each side of a semiconductor junction
is considered surface charge, just as the charge on a particle of silver
in water is. That is the silver particle will be absent one or more
electrons, and the OH(s) attracted to it will have an extra electron
summing to 0 imbalance. But the sum of the electrons overall will be
neutral. Look it up, it is physically impossible to have an average
charge INSIDE an object (Gauss' law). Any charge can only occur on the
surface. This is basic physics. This isn't a guess, it is a
mathematical fact that is easily proven. All you have to do is
integrate the forces on each charge within an object if there are extra
or missing electrons, and ALL the electrons or (holes) will instantly
end up on the surface as surface charge.
Also the inherent resistance of various metals and alloys -
compare silver and inconel or even iron for instance - gives an
illustration of the confused and by no means linear effect of electric
pressure on so-called "free" electrons in metals.
I don't follow you here. Are you saying that ohms law is wrong?
Resistance is linear with voltage, and it does not matter if the wire is
a million volts above ground or below ground, it's resistance will be
unchanged. Ohms law has never never been proven to be in error. As far
as conductivity, there is lots of differences in conductivity between
different elements and compounds. Some have no electrons in the valence
band, and thus ALL electrons are bound, and there is essentially no free
electrons at all. These are called insulators. Some, like most metals,
have one or more electrons in the conduction (valance) band, and thus
can conduct electricity. The more free electrons they have in this band
the higher the conductivity generally. Also the structure of the
material makes it more or less difficult for electrons to go through the
material, some when cooled sufficiently make it completely free of
resistance and are super conductors. All this is known and can be found
in practically any physics book. None of this means that there are more
or less total electrons in a wire, because there simple are not. Ohms
law can be derived from the drift velocity of the electrons. In fact you
can find this done at http://en.wikipedia.org/wiki/Drift_velocity Also
on that same page you will find that current is the product of the
charge on an electron times the number of electrons that cross a
boundary per second. And of course the number of electrons that cross a
boundary per second is going to be the product of the number of
electrons moving times their speed, just like cars on an interstate.
In other words,
although the signal travels at just slightly less than the speed of
light, and though I grant that increasing the pressure on an electron
will tend to cause it to be more likely to move, the effective increase
in current in, say, a wire is overwhelmingly a matter of getting more
electrons moving than getting any one, or billion, of them to move
faster from one end of the wire to the other.
I am not sure what you mean by overwhelmingly. It is a simple vector
product of the number of free electrons times the speed they are
moving. If you double either you double the current. Nothing there is
overwhelming. The number of free electrons tends to affect the
resistance of the wire, the speed of movement is totally a function of
the voltage gradient. If you have two wires, one with a cross section
of an inch and one 1/10 inch, there will be a 100 to 1 ratio between the
areas of the wires. If you have a certain current flowing through both,
the speed of the electrons in the 1/10 inch wire will be 100 times that
of the 1 inch wire, and the number of electrons moving in the 1 inch
wire will be 100 times that of the 1/10 inch wire. It is simply
mathematics, the product of the two values. Neither the speed or the
velocity is "overwhelming", they simply are what they are, and the
product of the two defines the current.
More current causes more
heat, causes less current.
More current causes more power dissipation in a wire, which translates
into a temperature rise. Higher temperature causes the atoms to vibrate
more rapidly, making it more difficult for an electron to move without
encountering an atom and losing its energy. Thus as the wire heats up,
the resistance goes up, and if fed by a constant voltage gradient, the
current goes down. On the other hand, for a semiconductor junction,
higher heat adds to the energy of the electron, making it easier to
penetrate the junction's barrier, thus current goes UP with temperature
in most semiconductors. You have to look at the physics of what is
happening to understand why they happen.
Anyhow, that's what I was taught; perhaps the
sands have shifted from under my feet, that has happened before.
I doubt it, unless you are VERY old. I was just looking at a physics
textbook published in 1908, and it agrees with what I have said (with
the exception of anything that concerns semiconductors since they did
not exist yet). For instance:
http://books.google.com/books?id=1kMJAAAAIAAJ&pg=PA37&lpg=PA37&dq=charge+inside&source=web&ots=GlY0ND2qMR&sig=X8s3A62ADutLdgDhAPy0Z1wtfRg&hl=en&sa=X&oi=book_result&resnum=7&ct=result
"If a closed surface is drawn such that every point on it occupied by
conducting material, the total charge inside is nil. There is no charge
which is occupied by conducting material, unless this point is on the
surface of a conductor". That is Gauss' theorem.
Also page 310 of that book discusses the collision of electrons with the
atoms of the conductor causing resistance and Joule heating.
Superconductivity is another matter, and I don't know anything about
electron flow in superconductors.
Same as non superconducting flow, except that the atoms are not moving
around, so the electrons can navigate the crystal without ever bumping
into an atom. We still have the product of the number of electrons that
are mobile times the velocity defining the current.
Marshall
Take care, Malcolm
On Mon, 2008-09-15 at 11:18 -0400, Marshall Dudley wrote:
Malcolm wrote:
Ummm,
On Fri, 2008-09-12 at 15:36 -0005, M. G. Devour wrote:
Dear Neville,
You write:
[The actual linear velocity of the electrons within the wire is
proportional to the current: Zero with the switch off, and limited
by
ohm's law, ie. total circuit resistance and voltage, when on.]
As a simple example...the higher the current, the quicker the 'flow',
(forgetting ohms law for the moment)... yes?
The higher the voltage or lower the resistance, then yes, the current
will be higher, which means the electrons are moving faster in the
wire.
well not really, though more of them will be moving in the (roughly)
same direction past a given point; that is, after all, what "Current"
is.
It's not that the electrons run faster from end to end, hence increasing
the current; it's that higher voltage crowds them in more densely: for
yet another very imperfect analogy, more get stuffed into the subway
train, but the train doesn't go any faster, and so more get out at their
destination, per unit of time (hours if you live in New York!)
The number of electrons inside a wire is constant, and independent of
any voltage on the wire, it will be equal to the number of protons in
the nucleus, always. Now if you put high voltage on a wire, the number
of electrons on the surface will vary due to the capacitance effects on
the surface, but this is trivial compared to the number of electrons
inside the wire. If what you were saying were true, then applying a
positive voltage to a wire that is grounded would result in a reduction
of current as the voltage is increased, since that would result in fewer
electrons in the wire. Ohms law is correct whether the wire has a
positive voltage or negative voltage on it since the voltage on a wire
has no effect on the number of carriers inside the wire. It only
affects their average velocity. Think in terms of a pipe with water.
Adding pressure does not change the amount of water in the pipe, except
by any little amount the pipe stretches, but adding pressure drop from
one end of the pipe to the other changes the velocity of the water in
the pipe, thus the flow increases. Voltage equals pressure, current
equals flow.
It is actually pretty simply to do the math. An electron experiences a
pull when in an electric field. This pull is the vector product of the
voltage gradient and the charge on the electron. The electron
experiences an acceleration which is mathematically equal to this force
divided by the mass of the electron. However before it has gone far, it
bumps into an atom, and loses it's velocity, and the kinetic energy is
converted to heat. This is what give wire resistance. Now if you can
couple the electrons together into pairs, they can actually flow without
bumping into the atoms, and that is how a superconductor, which has no
resistance, works.
Marshall
Marshall
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