drbaker...
Without realizing it, you've actually asked two questions,
since, though it is not commonly known, the speed of
transmission of an electrical signal through a conductor
is actually quite different than the speed of an electron
through the wire.
As we shall see, you can actually walk faster than an electron
moves through a conductor! Nonetheless, the signal moves at
close to the speed of light.
To begin with, let's stop by the Naval Electricity & Electronics
Training (NEETS) module on the Tpub site:
http://www.tpub.com/neets/
Chapter 1 has a page on electric current which first notes that
electrons are always in random motion, even when there is no
voltage applied, and this motion is faster in a conductor, such
as copper, than in a non-conductor. This is called 'random drift'.
It then points out that this random motion becomes directed when
a voltage is applied to a conductor, resulting in 'directed drift'.
Figure 1-26 illustrates how, though the electron speed is slow,
the effect of this drift is transmitted essentially instantaneously.
"The directed movement of the electrons occurs at a relatively
low VELOCITY (rate of motion in a particular direction). The
effect of this directed movement, however, is felt almost
instantaneously, as explained by the use of figure 1-26.
[.../...]
Though an individual electron moves quite slowly through the
conductor, the effect of a directed drift occurs almost
instantaneously...This action takes place at approximately the
speed a light (186,000 miles Per Second)." More on the page:
http://www.tpub.com/neets/book1/chapter1/1n.htm
Now, keeping in mind that the above is a discussion of the
'theory' of electrical current, let's move on to the specifics
which can only be provided by practical field experience.
Our first hosts:
"Simula Research Laboratory (Simula) was established in January
2001, through a resolution of the Norwegian Parliament. Simula
is named after the world's first object-oriented programming
language, Simula, which was invented and developed more than
thirty years ago by the award-winning Norwegian pioneers
Kristen Nygaard and Ole Johan Dahl."
http://www.simula.no/
In a discussion of the Resilient Packet Ring, being developed
by the company, it is noted:
"The traffic stations along the RPR ring must make automatic
decisions with remarkable speed. The data packages move at
the speed of electrical current, which is about two thirds
the speed of light or about 200.000 km per second."
http://www.simula.no/project_one.php?project_id=3
As we shall see, 66% of the speed of light is a good average
figure for the transmission of data by way of electrical
current.
Our next host is the Mogami wire and cable company of Japan.
An engineer on their site has presented some educational
materials in the form of puzzles, preceded by some very
detailed information. On a page which discusses the subject
'Mystery of electric current - Speed of electricity', we
find the following data, echoing the NEETS module, but
considerably more detailed:
"...the most common material for a conductor of electric wires
is copper and the electron density in 1m^3 is 8.5e28. For
example in copper wire, of which length is 1m and the outside
diameter is 0.5mm, there are 1.7e22 free electrons..."
"...free electrons in a copper wire move to random directions
with the speed of 1.3e6m/s even in the case of no electric
current, which means it is not in electric field..."
Remember 'random drift'?
Now for the speed of electrons under charge:
"When voltage is put on both sides of a conductor, free
electrons increase the speed in proportion to the electric
field and by lattice oscillation, lattice defect, and
collision with impurities, they will be scattered to
different directions from the electric field and lose the
speed to the direction of the electric field. Therefore
it doesn't increase the speed infinitely and it will keep
certain average velocity. That means collision functions
as a kind of friction."
"As for copper, the time interval between collisions is
5.26e-45 seconds and average drift velocity is
4.62e-3 (m/s) / (v/m)."
"It means that when 1V voltage is put on both ends of 1m
long copper wire, the velocity of free electrons to length
direction is 4.62 mm/s. It seems amazingly slow but since
electric charge of electrons is -1.6e-19c, 12.6A electric
current flows in the 0.5mm copper wire with this speed."
and, finally:
"...let's say you put 50mV differential voltage, which is
almost a standard limit, on 100m of '10Base-T' cable,
which is commonly used for LAN wiring. The electric field
that is put on a conductor is 0.25mV/m. Average moving
velocity of free electrons is only 1.15m per second,
which is 4.1km per hour, and it's about the same as walking
speed of human."
http://www.mogami.com/e/puzzle/pzl-05.html
He then asks how this can be so when we know that telephone
and LAN wiring transmit information faster than an airplane
can fly. This is his 'puzzle' question. But we already know
the answer to that, right?
Our last host is Larry Spring, from the TeslaTech website.
On the following page, in entry 68, he notes that he has
independently verified the following, in 1954:
"I verified the speed of electrical current in television lead
line at from 66% to 95Z[%] the speed of light. 66% for coax
cable. 75% for ribbon lead. 90% for ladder lead line."
So, as you can see, the actual speed of transmission will vary
with the type of wiring, which includes many variables. The
conductivity of the metal wiring, the insulation, the thickness
of the wire will all play a part. Speaking as an ex-Navy
Electronics Technician, I will tell you another secret taught
in Navy training: electrons travel pedominantly on the outside
of the wire. Therefore, the thicker the wire, the better the
current flow. This is why you will see that high-end speaker
wires, e.g., are amazingly thick, even though they handle
little current!
Another factor is temperature. The concept of superconductivity
states that, when the metal conductor comprising a circuit can be
brought to its 'transition temperature' (close to absolute zero),
the electrons will flow through metal with essentially no resistance
from the collisions with other electrons which impede electron
flow in a 'normal' circuit. More here, from Superconductors.org:
http://superconductors.org/oxtheory.htm
If you're looking for a working figure for copper wire at
room temperature, I'd suggest using 66% of the speed of light,
or about 200 km per second.
Please do not rate this answer until you are satisfied that
the answer cannot be improved upon by means of a dialog
established through the "Request for Clarification" process.
sublime1-ga
Searches done, via Google:
"speed of electrical current"
://www.google.com/search?q=%22speed+of+electrical+current%22
theory of superconductivity
://www.google.com/search?q=theory+of+superconductivity |
Clarification of Answer by
sublime1-ga
on
08 Oct 2003 16:20 PDT
drbaker...
Let me address your Request for Clarification point by point.
"I would like a definitive journal-article reference for the
'drift velocity' or 'drift speed' of electrons as they are
'pushed' from one end of a wire to the other end..."
As noted in my answer, once a voltage is applied to the
conductor, 'random drift' is replaced by 'directed drift'
of the electrons, which will vary depending on the voltage
applied, the quantity of free electrons in the conducting
material, the thickness of the wire, and the resultant
'friction' imposed by collisions with other free electrons:
"When voltage is put on both sides of a conductor, free
electrons increase the speed in proportion to the electric
field and by lattice oscillation, lattice defect, and
collision with impurities, they will be scattered to
different directions from the electric field and lose the
speed to the direction of the electric field. Therefore
it doesn't increase the speed infinitely and it will keep
certain average velocity. That means collision functions
as a kind of friction."
The Mogami cable and wire engineer then clarifies by example
how the speed of this 'directed drift' will vary with the
voltage applied, and the type of wiring used:
"...when 1V voltage is put on both ends of 1m long copper
wire, the velocity of free electrons to length direction
is 4.62 mm/s."
On the other hand, when using "50mV differential voltage...
on 100m of '10Base-T' cable":
"Average moving velocity of free electrons is only 1.15m
per second, which is 4.1km per hour"
You then add:
"...that is, the speed of a 'current' pulse impressed on one
end of a wire and measured at the other end, recognizing that
the 'current' pulse is not composed of the same elecrons as it
progresses down the wire."
In my answer, I noted that the speed of the current pulse, or
signal, which is not dependent on the actual speed of the
electrons, would be much faster, approaching the speed of light,
but also varying with the type of wiring used, as noted by
the Larry Spring citation (for which I omitted the link):
"I verified the speed of electrical current in television lead
line at from 66% to 95Z[%] the speed of light. 66% for coax
cable. 75% for ribbon lead. 90% for ladder lead line."
http://www.teslatech.info/ttstore/articles/spring/discover.htm
Since he notes that he 'verified' this information, he is saying
that he found citations in the literature for the data he later
reproduced.
If I'm misunderstanding the meaning of 'current pulse', and you
are referring to the speed of the movement of the electrons, it
will be the same at both ends of the wire, and the speed will be
as discussed in my first response about 'directed drift'.
You then state:
"Also I'd like a journal reference for the 'electron migration
speed' through a semiconductor such as a transistor."
Your desire for "a definitive journal-article reference for the
'drift velocity' or 'drift speed' of electrons", and for "a
journal reference for the 'electron migration speed' through a
semiconductor such as a transistor", were not stated or implied
in your original question. If they had been, I would not have
attempted to answer the question, since I do not have access to
such journals. They require a paid subscription, and are generally
much harder to search for specifics such as you request than are
the mainstream websites available via Google.
In summary, there are no simple answers for either the speed
of the 'directed drift' of electrons through a conductor, or
for the transmission of a signal, or 'current pulse' through
a conductor (or semi-conductor), because of the numerous
variables involved in the voltage, composition and size of
the wire or circuitry, temperature and so on. Nor can I
provide you with a formula which will take all these factors
into account (if one exists). Any citations, whether from
"definitive journal-article reference" or from the internet,
as I provided, will only give you figures pertinent to the
specifics of the materials used in that particular instance.
To that end, I provided a working figure for the transmission
of a signal through an average conductor as 66% of the speed
of light.
Since I am not clear how the specific information you are
seeking is not adequately addressed in my answer, and since
I simply don't have access to the journal citations you have
now requested, I would be willing to write the editors and
request that my answer be removed, if that is your preference.
Otherwise, I would be more than willing to continue to assist
you by searching for "the 'electron migration speed' through
a semiconductor such as a transistor", by way of the resources
available to me, and to further clarify my answer to more
closely address your specific interests, if it's possible to
do so by way of those resources.
Let me know how you want to proceed.
Best regards...
sublime1-ga
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