On Sep 4, 12:41=A0pm, EskWI...@[EMAIL PROTECTED]
wrote:
> I'm wondering how one can squeeze more current through a given diameter
> wire by means of increasing the voltage.
Well sometimes you can and sometimes you can't. It depend on what the
wire is made of! Many substances follow what is termed "Ohm's Law".
This "law" states that the current through a wire is equal to the
voltage across it divided by the resistance of the wire. For such a
wire obviously (I =3D V/R) one can increase the current by either
increasing the voltage of decreasing the resistance.
Superconductivity (R =3D 0) is obviously a great way to increase
current! The only problem is that if the current gets too high it
causes superconductivity to quit.
But do not be misled by the clowns here and elsewhere who pretend that
ALL substances follow Ohm's law. They don't. Even copper, which is
more or less a standard material for electrical wires, is rather
unstable with regard to Ohm's law. When resistors are made from coils
of wire, copper is NOT the wire used for this reason. Other conductors
(like say plasmas) are FAR from following Ohm's Law.
> I'm wondering a bunch of other stuff too, but the answer to this
question
> seems basic to my eventual understanding of other questions.
>
> "More current" means that more coulombs of charge pass by a given point
> per unit of time. =A0I take this to mean that more electrons flow past
th=
e
> point.
Yes it does. Current is charge per second past a given point.
> If more electrons pass by the point per unit of time, does this mean
that
> the velocity of the electrons is greater at higher voltages, all other
> things being equal? =A0Is the means of getting more electrons through a
> given wire to increase the velocity of the electrons by means of
> increasing the voltage?
Well actually you can increase current by either having more electrons
at the same velocity (larger hose, more charge per second) or the same
number of electron density but flowing at greater velocity (Water
flowing faster in the same diameter hose) The hose analogy to
electricity is that current is like water flow and voltage is like
water pressure.
> If so, then how fast do electrons travel? =A0
Well, here's the kick in the butt! Electrons travel so slow that you
can walk faster than they move! The speed they move is called the
"drift velocity".
> I (think I) understand that the electrons do not necessarily pass
through
> the entire length of the wire, but instead, they travel between adjacent
> copper atoms. =A0When one atom near the source gets extra electrons, it
> p***** some electrons to its neighboring atom. =A0This propogates down
th=
e
> wire from one end to the other, until eventually, the current reaches
the
> load at the other end.
Yes this is it exactly. It's like a garden hose where you stuff water
in one end and water comes out the other end, but it's not the SAME
water!
> And am I correct that at higher voltages, this process happens more
> quickly? =A0If one had a really long wire, could the time difference be
> noticeable and significant?
People (College Freshmen) have ways and assumptions to calculate
"drift velocity" but the matter is very complex. It's not so easy to
find these material parameters. The "Hall effect" is one indication of
drift velocity. Also we know that a magnetic field is exactly
pro****tional to current. If we assume that somehow each electron
contributes to the field then the numbers passing a certain point is
the key factor and not how fast they are moving.
> So how fast does this effect propogate down the length of the wire?
=A0An=
d
> is velocity of travel the mechanism that allows more electrons to "pass
> through" a thin wire at higher voltage per unit of time?
Propagation of signals down a wire travels at nearly (more or less)
the speed of light! That is MUCH faster than the walking speed of
electron "drift velocity"! The signaling speed does not depend on the
drift speed. It's kind of like it may take several minutes for water
to make it's way from the faucet through a long garden hose. But once
the water is going, if you start "modulating" the faucet end by
squeezing the hose you'll find the pulses will appear immediately at
the nozzle end. Same thing in wires.
> OK, so that is my basic question, and I guess it assumes DC voltages.
=A0=
Is
> that true?
Doesn't matter.
> Taking it one step further, ISTM that with AC, the electons starting at
> the source might not get too far down the wire before polarity changes.
=
=A0
> Here in North America, we switch polarity 60 times a second.
Correct. Even FASTER and shorter distances with radio waves (which
also travel down wires)
> So are the same electrons going back and forth, back and forth, along
the
> same physical chunk of wire at AC voltages?
Yes.
> AND - what happens when we use low voltage and high frequency? =A0
Voltage just changes the current and frequency determines how far the
electrons get before reversing as you have already surmised.
> Specifically, what would happen if the switching frequency is
> significantly quicker than the time required for the voltage change to
> propogate all the way down the wire from the generator to the load?
=A0Do=
es
> the transmission system suffer weird phase problems?
Yes. But remember you are talking here about speeds faster than
LIGHT! So if you apply a step to a long wire (transmission line) That
voltage will not be seen at the far end until there has passed enough
time for it to get there at the speed of light. If the end of the wire
is shorted or open the signal may actually reflect from the open end
when it gets there and start returning to the generator. Yes that
leads to "weird" effects. If the input signal is AC and not just a DC
step, and it continues for a while, the signals reflect back and forth
on the wire creating what is known as "standing waves" Those interfere
with the lossless transmission of power in the wire. This is pretty
complex stuff and you are heading into water over your head.
> And am I starting to figure out the term "Power Factor" using this line
o=
f
> reasoning, or am I going in weird circles, and beginning my reasoning
> process with defective premises?
:Power Factor relates to the transmission of power I just talked about
above. If you are driving a resistor as a load, the power factor is
such that all power goes into the load. But if there are components
that only ****ft phase but do NOT use energy from the wire like say
capacitors or inductors, you end up with a "power factor" that
indicates that you may have a lot of current in the wire, but little
energy getting into the load. This is bad in transmission systems
because that high current heats the wire but delivers little energy
where you want it to go. Understanding this takes considerable study
and ability. It's what power transmission engineers do and deal with.
> OK. =A0Lots more than one question. =A0But I seem to have some basic
> misunderstandings or ignorance that is not letting me figure this stuff
> out in a manner I am confident of.
No not so far off, but it's so easy to ask simple questions that can
take a lifetime (or longer!) to answer!
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