path of least resistence

And you probably thought my answer early in this thread was just me being a wise-ass! Well, yeah, I was, but the question really is a serious and a mysterious one at the microscopic level. Answers at the circuit level are really just approximations to truth. The point is just HOW do electrons "look ahead" and see what's going on? In the case of a load and a short, they actually don't! Consider a wire that splits into a Y where one path is a short and the other is a resistor. You can ask how does the electron "know" that one path is shorted so more of them go that way? Fact is it doesn't work that way. If you put a sharp step into your Y wire, you'll have a wave-pulse that travels down the outside of the wire. That impulse travels usually somewhere near the speed of light and as it goes down the "stem" of the Y it really has no idea that there is a "short" ahead. It can't know anything in the "cone of silence" which is to say in regions that require speeds faster than light for communication. So therefore this energy (which actually is in the field around the wire and not in the electrons) doesn't in fact "know" what lies ahead. It is only after this pulse explores both branches and bounces around a bit that finally circuit equations begin to apply. Circuit theory is handy for quick answers but field theory or even quantum theory is needed to get at microscopic truth.

But what if we take "free" electrons hurtling through empty space. We find that if we set up two slits, each electron actually only goes through one slit at a time. Yet, SOMEHOW it also "knows" that the other slit is there because on average with lots of flying electrons the landing patterns formed will be a two slit diffraction pattern. OK, so just HOW does a single electron going through ONE hole "know" that the other hole is just over there? Good Question! Physicists do a lot of hand waving and jibber-jabbering about probability waves and the like, but it's all very imaginary and quite unsatisfying! [Go see Feynman] Does God play dice with the Universe? The world is waiting for an explanation for this that doesn't require a priesthood in Wicca to understand.

Benj (Who notes that it's a poor student who can't ask a simple question that stumps the world)

On Sat, 21 Apr 2007 21:36:06 GMT, Salmon Egg Gave us:

Which is also why the resistor heats up when current is driven through it. This is what defines a semi-conductor. Such collisions are exactly how a resistor works. Driving electrons from valence shells with EMF to produce flow.

On 22 Apr 2007 22:44:34 -0700, Benj Gave us:

That is a dopey question and proves that you don't have a grasp of the circumstances under which a circuit operates.

It is a complete loop. You need to start over with your basic electronics training.

I suggest the NEETs program.

On 22 Apr 2007 22:44:34 -0700, Benj Gave us:

You're nuts.

Hey there, SuperM, you really need to keep your mouth shut until you complete your basic electronics training... or at least wait until you get to the chapter labeled "transmission lines".

Benj (who notes it better to keep your mouth shut and be thought a fool than to open it and remove all doubt)

--------------- I hope that no one is saying that circuit equations apply to a single electron or even to the travelling wave. Circuit theory is a first order approximation to field theory and is valid only at the macroscopic level and when non-relativistic effects occur. Salmon Egg is correct in pointing out the probabilistic nature of what goes on at the microscopic level. Even at the macroscopic level, the model for a particular element-say a piece of wire can consist of a) a short circuit b)a lumped resistance c)lumped resistance and inductance d) lumped R,L,C e)transmission line d) EM models etc. depending on frequency. a-e are approximations to d. One could use d for everything but the approximations are more than adequate within bounds. A big factor is knowing what the bounds are because one doesn't use EM to deal with a DC battery-wire-lamp problem. So -is the cat alive or dead?

On 23 Apr 2007 07:49:12 -0700, Benj Gave us:

Your "dead short scenario, AND you assessment of what happens is what proves you are lost.

Too late for you then.

It is a bit more complicated than that. Electrons (or holes or even ions in electrolytes) will collide with impurities and lattice imperfections (dislocations). That is why copper for wire is pure and annealed. That is why nichrome is good for resistors. Electrons will even scatter off of lattice vibrations (phonons). That is why resistance of good metallic conductors increases with temperature.

Bill

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

It doesn't really. It divides among all paths such that the current through each path and that path's impedance satisfy the requirement that nodes common to multiple paths have one defined voltage.

Because an amount of current must flow down each branch such that the voltage across each branch (between two common nodes) is the same.

It can't. From an electron's point of view as it approaches a branch, its probability of taking one or the other depends on the potential gradient it 'sees' ahead of it (like charges repel). Along a path of lower resistivity, it takes a higher current density to produce a given gradient. So that electron doesn't have to 'see' all the way down one path. The electrons ahead of it have taken care of that already.

Its sort of like how a traffic jam works. You slow down if there's a wreck miles ahead.

Electrons follow the path of best attraction, which just happens to be the path of least resistance, or most persistence. Resistance resists attraction.

You are very persistant ;-)

According to a presentation I saw yesterday you start with Schrödinger's equation, fill in the unknowns, which for each electron is about 20 and it all essentially reduces to Newton's second law (f=ma), then you just let a computer simulate it all. Several hundred hours of computing later and your electrons will have convected, through the varying electric fields, past a few of the few hundred atoms in your finite simulation. Hopefully you will have done enough to spot the trend in the directions that the electrons are going.

I suggest the electrons don't know anything about any paths of resistance, they are only influenced by the local (to them) electric field. The models of materials usually have the electrons moving about pretty fast, but not consistently in the same direction because the electric field gradients they are seeing are huge, but over very small (interatomic) distances. Current is an average of all the movements and circuit theory applies to the averages of the quantities in time and space.

Perhaps an interest discussion would be "what is it, on an interatomic scale, the features that gives different materials different electrical resistance?"

The presentation I saw was actually about crack propagation, but essentially the same sort of stuff.

Well done Conrad, for stimulating the brain cells in the branches the thoughts only go through rarely.

Robert

c> What happens at the atomic level

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

Believe me ... some never make it to the bedroom!