# Determination of direction in AC Power Flow

Discussion in 'Electrical Engineering' started by Beachcomber, Nov 11, 2006.

1. ### hobGuest

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 2×10-7 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)

3. ### hobGuest

As long as you know how how do do eclectic reasoning. When you don't, a
reversal becomes the conclusion instead of merely an observation.

You do not seem capable of grasping the abstract concept of charge, and the
result is your eclectic fallacy that the elctron ois soemhow realted to the
defintion or concept of charge and coulomb.
It is a definition, used by every scientist and engineer.

5. ### daestromGuest

Agreed. In another life, we studied neutron diffusion. Neutrons born from
fission have very high velocities and slow by colliding with other
materials. Once they reach equilibrium thermal energies, they continue
colliding with materials until absorbed. The total distance a neutron
travels is important in reactor design. The distance is determined in two
parts, the 'slowing down length' and the 'diffusion length'.

Both electron movement and neutron movement are akin to Brownian motion.
Many collisions, resulting in many steps in random directions, ending with a
net movement.

The thermal energy of an electron is almost (if not all) in the form of
simple kinetic energy, so solving KE=1/2 m V^2 can give the velocity of an
electron that has 'average' thermal energies.

The rate of 'drift' due to Brownian motion is influenced by the application
of an electric field.
You two may just be argueing about symantics. The electron at any instant
may have a speed of 10^6 m/s, but it doesn't make a net progress through the
lattice at that speed. Yet, with an electric field applied, it does make
progress through the lattice.

daestrom

6. ### JoeBloeGuest

I SAID: "To answer the post title..."

LOOK at the post title.

Silly rabbit.

7. ### JoeBloeGuest

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.

8. ### Don KellyGuest

--------

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.

9. ### BeachcomberGuest

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

10. ### Don KellyGuest

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.

11. ### JoeBloeGuest

If the net effect OUTSIDE the device as you call it, is electron
flow at a specific Electromotive pressure or force, then that also has
to be what is happening INSIDE the device.

12. ### JoeBloeGuest

That, sir, is exactly what it does. Thank you.

What does one refer to the quantified measure of that progress as?

Could it be The COULOMB.

This is the reason why the same current passed through smaller and
smaller gauge wires yields higher and higher heat in the wire. It
isn't that the wire has a higher resistance, it is because one is
passing the same numbers of electron past a smaller and smaller cross
sectional bottleneck of lattice.

A light bulb should go off in one's head. et sic de similibus

13. ### Don KellyGuest

----------
Definitely not. Trying to use the coulomb as a measure of progress is like
trying to use cubic feet as a measure of flow of water. Something important
is left out. Think about it.
----------
------
Which appears as an increase in resistance to anyone who is trying to
observe it.

Note that resistance is expressed in terms of resistivity, length and cross
section. You are trying to explain on the basis of one of the terms whereas
the resistivity is the term related to the properties of the material -part
of which is the lattice structure.

The wire does have a higher resistance and what you say is one way of
explaining part of the reason for it ( which is not the whole truth but
maybe a reasonable "lie to children" -other such "lies" work as well and can
apply to conductors without a lattice structure). However what is observed
is that there is a "voltage =function of current" relationship which is
given the name "resistance".

In circuit analysis, one of the basic "circuit elements" is one where there
is this relationship between current and voltage. It is a model - a
mathematical model if you will- which correctly, within limits, represents
observed reality. Circuit analysis involves modelling of the external
behaviour of systems and components of these systems and is independent of
models of the internal physics. It really is not concerned with what the
actual charge carriers happen to be, the lattice structure, etc. What is
involved is trying to get a model which correctly predicts the current,
voltage and power relationships which are all externally observed.

The internal physics is very important but there is a tendency to use
incomplete and sometimes inaccurate models- based on a search for a simple
answer based on simple models such as the Bohr atom and a lattice structure
that looks like a Tinkertoy construction -useful but still models and
actually inadequate models on the basis of what we know now. These models do
change as we learn more of the physics involved and trying to use these to
explain electrical phenomena is a useful but possibly incorrect approach to
the physics BUT leads to confusion when one tries to apply them to circuit
theory and analysis where they aren't necessary.
In other words, don't substitute an explanation of "why" there is
resistance, for the observed relationship expressed by v =Ri (which is NOT
Ohm's Law ).

Circuit theory does not depend on the physics of particular charge
carriers, and lattice structure- treat them as separate matters. Evolution
in the understanding of physics can change your lattice and electron ideas
but has no effect on circuit theory.  