path of least resistence

Are you dumb? "The earth, newly turned, looked dark and rich, like crumbs of=20 chocolate cake. A few worms and wood lice had been disturbed and were still looking for their old homes." -- Ian Rankin.

The word is "resistAnce", and yes, electrons do not have foresight.

Force. I refuse to nitpick semantics here. Accept it or leave.

Hey Conrad, the problem is you just don't understand the advanced principles of modern quantum physics! The answer is that electrons exist and don't exist at the same time. And even though the current is split in two directions each electron can only go down one path at a time. But the probability waves they create go down both paths at once. And even though electrons can travel nearly as fast as light, the possibility waves look way ahead and decide which is the "best" (most probable) way to go. The problem is that you are trying to think of electrons as particles when they are really waves and trying to think of them as waves when they are really particles. Fact is they are both and neither at the same time, as every "modern" physicist knows full well. There. Are things clear now?

We'd like to say more, but until you learn to spell "resistence" and master the secret handshake, the above explanation will have to do!

Benj

On 20 Apr 2007 18:02:53 -0700, conrad Gave us:

Because. Ohm's law, dufus. Kirchoff's law as well.

On 20 Apr 2007 18:02:53 -0700, conrad Gave us:

No... REALLY?

The "path of least resistance" is an inaccurate but very popular statement. Current flows through ALL available paths, in an amount inversely proportional to the resistance in each path.

By Ohm's law, the current through any part of the circuit is determined by the potential difference across it divided by the resistance. Both parallel branches have the same potential across them. Therefore, most of the current would flow through the short, which has a very low but measurable resistance. Less current would flow through the resistor.

Ben Miller

I think that the conceptual problems that many have is a focus on the individual electron which, for a variety of reasons, doesn't matter a damn when one steps up to the macroscopic world of circuit theory. Somewhere common sense and observation of what actually goes on is ignored.

I really wonder how, when I turn on a light, that I get light right now instead of when the electrons from the generator get to my home, several hundred miles away, particularly for AC where they simply dither around and don't get anywhere :) --

Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer

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The electrons are just incidental; the wires are a waveguide. Stratton's book covers it well.

You asked this question is both a science and an engineering forum. Why is not a science question.

From an engineering/technician viewpoint you can use the water pipe model to understand that more electrons will fit into the larger "pipe."

Imagine a queue of people coming up to a branching of the queue, with one branch being larger than the other. Those in the queue who have no intent to select one branch or the other are merely swept along. It is obvious that more of them end up in the physically wider branch with no forecasting required.

This could make a very nice trolling question.

Think of the old hydraulic analogy: Imagine a pump putting a given pressure into a manifold with an 1/8" outlet, and and a 4" outlet. If the pressure remains static, which outlet will account for more flow? Same idea with electrons. (Electrons, incidently, don't travel at "nearly the speed of light". The electron pulse from one end of the circuit to the other, does.)

In alt.engineering.electrical conrad wrote:

| What happens at the atomic level | where electrons have an affinity for | a path of least resistance? Imagine this, | you have a parallel circuit. And at the node | where the branching takes place, current can | flow down one branch(where a resistor exists) | and it can flow down the second branch(where | we have a short). Why does the majority of the | current go down the path where we have a short? | It isn't like electrons can forecast which path has | least resistence, right? So how does the principle | 'follows path of least resistence' hold? Even when | measurements are taken, it can be observed that | the majority of current goes down the least resistive | path. It just doesn't make much sense when you | know electrons cannot forecast whether a resistor | or short lies ahead down one of the branches.

The resistance in the wiring (even if it's not much, as copper would be), is kind of like a self generated bucking force that is proportional to the current that is flowing. There is a voltage drop across any segment of wire, and you'll see a voltage that obeys ohms law relative to the current and the resistance.

The path with less reistance will have less of this bucking force. The eletrical force is really trying to go in all paths at the same time. Some paths just don't push back as much.

At the atomic level, I presume the electrons are encountering various atomic level actions that prevent their most efficient flow, such as the need to be changing electron orbital levels. The counter reactions could be thought of as that bucking force.

The electrons cannot forecast which path has least resistance. When the force (voltage) changes, that's when it can become apparent what is going on. Suppose you have two very long paths of low resistance, interrupted by one point of high resistance (e.g. a resistor inserted in the wire). In one case the resistor is near the voltage source. In the other case the resistor is nearer the other end (which might be a microsecond away at near light speed). So what is the electricity doing before it has any opportunity to "know" the circuit resistance (e.g. in the microsecond before the leading edge of turned on voltage reaches the far resistor)? The answer to that is characteristic impedance as seen in transmission lines. You could, in theory, make 300 ohm twin lead TV wire out of superconductors that offer no resistance at all (for our convenience in calculation). There's a resistor at the far end (but it's not 300 ohms). Or there may be nothing connected (open circuit). Or it may be shorted. Or the resistor might actually be 300 ohms. But the electricity that starts to flow can't "know" this for at least a microsecond of time. What that electricity will do is behave on its leading edge based on the characteristic impedance of the transmission line, which comes from the inductance and capacitance of the line itself (real wire will also add some resistance to that). When the leading edge reaches the far end, what happens next depends on what is there. If shorted, the leading edge will return on the other wire (there are 2 wires in the transmission line). If open, it comes back on the same wire. If 300 ohms is there, it is all dissipated and nothing comes back as a leading egde (a steady current will flow). Other resistances will have a lesser effect than shorted or open. That returning edge can end up reflecting back and forth between both ends until it reaches a steady state (assuming you switched on DC power). If you supply AC (changing voltage) these effects just keep on happening. Radio engineers and ham operators have to deal with this on even very short transmission lines (from transmitter to antenna) because at radio frequencies, the voltage is changing up and down in less time that it takes for the leading edge to reach the antenna. That can get very convoluted (simplified forms of it get plotted on "Smith Charts"). But it is fun to study.

In summary, electrons cannot "see" ahead. In fact they don't really even flow literally very fast or far ("electron drift"). But the "bump effect" itself moves at near light speed and comes back at near light speed and that has counter effects that "report the distant resistance" via how much has come back and when.

| I think that the conceptual problems that many have is a focus on the | individual electron which, for a variety of reasons, doesn't matter a damn | when one steps up to the macroscopic world of circuit theory. Somewhere | common sense and observation of what actually goes on is ignored.

Sometimes I think the electrons are just along for the ride. The real work is the EMF itself. If there is a cause-effect relationship (as opposed to just a mutual coexistance), then it must be EMF as the cause and electron flow as the effect.

| I really wonder how, when I turn on a light, that I get light right now | instead of when the electrons from the generator get to my home, several | hundred miles away, particularly for AC where they simply dither around and | don't get anywhere :) --

And even if they could get very far, you don't get the same electrons (if there could even be a concept of sameness between electrons) because they would not cross the boundary of transformers. If you put a battery and an appropriate resistance in series with the AC outlet, you could eventually have some of the electrons from the transformer actually in your home. But it wouldn't be the ones from the generating plants on the grid.

And there was me thinking that the path of least resistance was between a romantic candlelit meal for two and the bedroom ;)

| From an engineering/technician viewpoint you can use the | water pipe model to understand that more electrons will | fit into the larger "pipe."

Water pipe models are bad for electrical analogies. What would have to change to make them more correct is that we would use water pipes as a means to transport energy in the form of forward and reverse pressure changes (or forward only if Mr. Edison had his way) ... instead of using them as a means to acquire water.

The interesting thing in such a water based energy system is that the very same water molecules might well never go beyond a few centimeters of some point along the pipe in years, if the "AC" method is used. Of course there is a general spreading around of molecules always taking place (frozen is just a lot slower), so eventually some get into all kinds of places. But electrons do that, too.

With such systems, it's actually possible to have analogies to components like resistors, capacitors, inductors, and even transformers.

| Imagine a queue of people coming up to a branching of | the queue, with one branch being larger than the other. | Those in the queue who have no intent to select one branch | or the other are merely swept along. It is obvious that | more of them end up in the physically wider branch with | no forecasting required.

Normally they would choose the better queue. But given no knowledge, such as being blindfolded and following the crowd, this makes for a reasonable analogy.

You are barking up the wrong tree. At the atomic level, there is no resistance in the classical sense. Electrons have something like wave behavior. The waves can travel over ALL possible paths. The net wave result comes from paths near the classical path. The net result is that the wave intensity provides the relative PROBABILITY where an electron will be found.

A resistor is large compared to atomic spacings. Electrons accelerate in the conductor until they collide with something and on the average lose their velocity.

If you are truly interested, Google Feynman and "path integral."

Bill

-- Fermez le Bush--about two years to go.

In alt.engineering.electrical MassiveProng wrote: | On 20 Apr 2007 18:02:53 -0700, conrad Gave us: | |>Even when |>measurements are taken, it can be observed that |>the majority of current goes down the least resistive |>path. | | No... REALLY?

Not!

If you have a path with 26 ohms, and a 2nd path with 25 ohms, and a 3rd path with 24 ohms, you won't find the majority of current in any one of them.

Agreed: even though I prefer to stick to low frequency stuff -say 5000km wavelength - where simple circuit theory and, at most, distributed parameter T-line concepts, are more than adequate approximations to waveguide concepts.

Note the "Smiley">

Circuit theory is a perfectly good low frequency method. Doing stuff in the gap between circuit theory and geometric optics is enough to make one appreciate the value of low/high frequency approximations.

Given your usual posts, a smiley is hardly necessary :). But I thought the casual reader might be interested.

A hydraulic analogy is pretty useful in explaining electric phenomena to lay people. It is analogous to DC when compared to a hydraulic pump running a hydraulic motor, and even to AC when you consider the current at it's RMS value in many applications. Obviously, transformer theory and rotating fields don't apply here, but even then, the total of energy etc is still analogous to pumping.

I disagree. You may be right at an engineering level, but try to explain electrical basics to a group of non-electrical industrial workers. They understand the relationship between pressure, water flow, and valve restriction. They work with it every day at their sinks and showers. The electrical analogy allows them to correctly understand the relationship of voltage, current, and resistance.

I can assure you from experience that it works!

Ben Miller