Determination of direction in AC Power Flow

us:


Within the frame of the definition of the word Ampere.
One Ampere Second is a count of electrons passing a point of reference per unit time because one ampere is a rate of electron flow.
High repeat rate pulse lasers need cap banks charged by high output power supplies to keep up with the pulse rate.
That's-a-lotta coulombs.
Every element in a series circuit loop has that same flow as well. What comes out must go back in.
Yes, the coulomb is directly related to electron flow.
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Gave

Defintion of Ampere, SI - "The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 210-7 newton per metre of length."
No "electrons"
Put your definition of ampere, and the standard's name form which you took it, here.
_____________

Wrong !!!! See defintion of ampere, per SI, above.
The ampere no longer uses charge per second (even that does NOT use electrons)

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To answer the post title:
It flows BOTH ways, and it dissipates as heat in loads.
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snipped-for-privacy@thebarattheendoftheuniverse.org says...

Are we talking AC now? What relevance does this have to the discussion at hand?
--
Keith

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I SAID: "To answer the post title..."
LOOK at the post title.
Silly rabbit.
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This agrees with what I said but maybe I didn't write very clearly. I am differentiating between 'drift velocity' and 'actual speed'. Many people think of the 'two speeds' of electricity ... the effects that travel at the speed of light, and the electrons that move slowly (maybe inches per hour of drift velocity). There is a third speed. The instanataneous speed of the electrons which is very high (IIRC it calculates around 10^5 m/s using basic calcs, but there are some relativistic effects or something which led to the figure of 10^6 being used as the 'actual' typical kind of number). However, these electrons take 1000000 steps forward and 999999 steps back as they pinball around in every direction. Overall there is a tendency to move in the direction the E field pushes (trajectories are slightly curved in the direction dictated by the E field), and this results in the slow net migration of electrons, the average speed in the direction of flow over time, the drift velocity.
j
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says...

I understood your point, but the electron doesn't go banging *through* the lattice at 1E6M/S.

Ok, from one atom to the next, perhaps. How do you tag an electron to measure its individual velocity?
--
Keith


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My reply was to hob, not yourself. Looking at hob's message and rereading my post I saw that I didn't write it that well.
I believe I will continue to use phrasing like "pinballing through the lattice", it seems fine to me.

This is not required. Between observations and calculations modern science can determine an appropriate, consistent value for the typical speed of an electron between collisions.
Hang on ... just checked a basic physics text, it says speeds between collisions are about 10^6 m/s. This speed may be the thermal speed of the electron to a great extent. I don't see mention of "10^5 m/s but 10^6 m/s including relativistic (or something) effects". I wonder if I hallucinated that or read it somewheres else.
Later
j
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says...

But the individual electron doesn't go "pinballing" at 1E6M/S. Again, how would you tag the individual electron to see?

Typical == drift velocity.

It ma be 1E6M/S over 1E-10M. ;-)
--
Keith

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Does too.
Tag an individual electron. I don't know why you think science is so simple as to not be able to derive a meaningful value for a typical speed for an electron in an electron gas, or why you think each individual electron would have to be 'tagged' in order to do so. I have a dinner plate here with ten pieces of potato on it. I am lifting the dinner plate at 0.3 m/s. I conclude that a typical speed for a piece of potato on the plate is 0.3 m/s. I have not tagged any individual piece. This falsifies your tag hypothesis.

No, the drift velocity is a net change in position over a longer time interval (and might commonly be around 10^-4 m/s) and has nothing to do with typical speed (which is on the order of 10^6 m/s). 10^-4 m/s =/= 10^6 m/s. Typical instantaneous =/= average. Maybe you do not know what I mean by 'pinballing'. Which is fine.

Yes. Exactly. The mean free path between collisions may be on the order of 10^-10 m. BUT, after a collision the electron might bounce away at any crazy angle. It does NOT progress down the conductor in a series of straight, aligned, 10^-10 m steps. A typical speed between these collisions is 10^6 m/s. Based on these last four statements we see that an electron pinballs through a lattice with a typical speed of 10^6 m/s.
j
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On 11/17/06 4:33 PM, in article qzs7h.35905$ snipped-for-privacy@newsfe16.lga, "operator

According to accepted quantum theory, electrons are fundamentally indistinguishable from one another. This leads to the Fermi-Dirac statistical behavior of electron assemblages such as found in metals. This indistinguishabilty has profound consequences on semiconductor behavior, specific heat of metals, and many other aspects.
Bill -- Fermez le Bush
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wrote:

Thank you. I think we realize this, I think that is why the other poster asked how I would intend to tag an electron, implying that that the inherent indistinguishability of electrons would prevent one/me from obtaining a value for the speed of an individual electron. The quantum mechanical description you discuss is what leads to the determination of the correct value for the average speeds of electrons in a conductor, on the order of 10^6 m/s.
j
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says...

Tag one and measure its speed then.

The "average"? The average would be the drift velocity.
--
Keith

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This is like a broken record. I am comfortable that there are other ways to determine useful numbers for the average speeds of electrons in conductors. Science has done so. The speeds are on the order of 10^6 m/s.

The average speed is not the drift velocity. The average speed will be on the order of 10^6 m/s. Speed is a scalar.
Later
j
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says...

Define "average speed" then. Seems to me this would be the average of the speed of the valence electrons along the wire (a.k.a. "drift velocity").

Speaking of broken record...

So the electrons are moving at right angle to the conductor?
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The electrons are going all over the place, like molecules in a gas, with a slight slight slight tendency to move along the wire under the influence of the E field. Roughly speaking this movement along the wire gives the drift velocity, while all the whipping around - pinballing from atom to atom of the lattice - happens at a huge speed and gives rise to the average speed.

Yes. I am horrified that I have had to repeat it so many times especially considering that (1) it comes from physics texts and (2) you have already agreed with it yourself [[ "j: ...speeds between collisions are about 10^6 m/s. This speed may be the thermal speed of the electron to a great extent. krw: It ma be 1E6M/S over 1E-10M. ;-) j: Yes. Exactly. ..." ]] .

Yes. They are going in every direction wrt the conductor. They are going absolutely ape shit pinballing from atom to atom of the lattice. With no particular tendency for net movement in any direction until an E field is applied. When someone applies an E field their behavior is still close to this random crazy pinballing, though a slight tendency toward net movement along the conductor arises. The drift velocity.
j
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Gave us:

The only problem with this is the "slight, slight, slight" part.
They do not just move about chaotically throughout the lattice. There is a force and direction, as well as flow. Massive flow, so that proves the lil' bastards aren't just joy-bouncing around.
Tell us how many times each atom in a perpendicular slice of a conductor trades its valence electron(s), and then you might get a clue as to the fact that this isn't age old electrostatics, this is POWER being consumed. That's why we call it ELECTRONics. That's why it's ELECTRical engineering.
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--------
ELECTRonics and ELECTRical engineering aren't synonymous- related -yes. The name comes from the Greek "elektron" which means amber which exhibited electrostatic effects when rubbed so ELECTR... is actually based on the age old electrostatics of fossilised sap (with or without fossilised flies in it). The concept of electrons didn't exist when electrical engineering started.
In terms of circuit theory, it really doesn't matter about the actual sign of the charges involved and drift velocity is a factor which also isn't of concern. What is concerned is the macroscopic effects rather than the details of what is happening to an individual charge carrier. Even in electronics where the physics of a device is that of the individual charge carriers (and goes beyond the ideas of valence electrons jumping up and down or visiting their neighboring atoms), the final circuit model does not deal with these (call it a fictional black box equivalent which accurately describes the macroscopic behaviour of the device -but not the behaviour of a given charge carrier ). Look at the data on transistors, etc. External behaviour of the device and small signal parameters is all you get. Not a charge carrier in sight.
Since it is AC, charged particles actually do slosh around with individual charges not moving any great distance. Look at a wave- the water in that big one out there is not the same water that actually reaches the shore. How many times a given atom trades valence electrons is really unimportant.
Energy is transferred at the wave velocity - nearly the speed of light , not the drift velocity which is a crawl and averages to 0 for AC.
One can assume a given current direction (usually the conventional current) whether it is right or wrong and let the mathematical bookkeeping take care of it. This works for both AC and DC and power flow goes accordingly. If the assumed direction is wrong, then a neat little "-" sign or 180 degree shift takes care of it. There is too much emphasis laid on the direction of the actual charges involved. This interferes with an understanding of circuit theory and concepts- particularly power and energy.
--

Don Kelly snipped-for-privacy@shawcross.ca
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I would think that a thorough and rigorous understanding of the theory should lead to an increased grasp in understanding practical problems such as the direction of power flow. A similar problem as suggested by this thread might routinely arise in a complex transmission line, for example, with multiple load and generation centers.
Concepts such as current flow and how individual electrons interact on a conductor are difficult to understand. (I know I have difficulty understanding these).
When I was a young electrical engineering student, I also had difficulty imagining certain rules concerning charge distribution and concepts like Smith Charts and complex impedances.
What I learned is that if you follow the formal teachings and accept a certain amount of the theory (that you may not completely understand) but you know it has been tested (by others) to be true, then you do have a certain enhanced ability to solve practical problems.
Beachcomber
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What you say is true but the whole basis of circuit analysis and power flow, forces etc are given in terms of "conventional" current flow. The correctness of this can be questioned but the change to "electron flow" simply means that one needs more care with the polarity signs. It is quite possible to do this correctly but working on "electron" flow does present conceptual problems with AC while conventional flow doesn't.
For power system analysis, for example a load flow study, involving many generators and loads, assumes specific generator voltages and powers and load powers/ reactive needs and estimates of load voltages- then one solves for the actual voltages and line power flows and generally doesn't bother with the currents which can be found, if desired, from voltages and complex power flows. In fault studies, the currents are of concern but again the current directions are assumed and as said before, if wrong- the only choice is that the direction is reversed. Those involved in such analysis do use the "conventional current" approach. They also assume al;l power is input to the system (loads input negative power) as a standard convention so ambiguity doesn't arise. As an engineer -this is what you were exposed to. You know that typically the electrons are involved but that is actually immaterial with respect to considerations of energy flow (sign of power) or circuit analysis.
No matter how complex the circuit is, one needs to follow specific sets of rules. This is most important in complex situations where you can't "wing it".
The conventional rule is that given a current direction, a load (passive or active) will have a voltage drop in that direction. A source will have a voltage rise in that direction. Kirchoff's Laws are paramount. Sum voltage drops around any path =0 sum currents into any junction =0. If the circuit parameters are constant, then such things as loop, node, Thevenin and all those goodies follow. Could we do it differently? Yes, but there is still the need to define a set of rules for current/voltage relationships in a circuit element or the effort is worthless. That is all that we do and it has nothing to do with the actual charge carriers.
In North America traffic drives on the right side of the road. In England, it drives on the left side. Either is fine by itself but a mixed system is deadly- similarly so for any circuit analysis. Define the (mathematical) rules and stick with them. That is what your profs were trying to say.
--

Don Kelly snipped-for-privacy@shawcross.ca
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