Do Electrons Think? A Question About Electric Current and Ohm's Law

[webtry]

Hi, I was talking about this on another forum's general chat section and think you may be interested in answering this simple question of mine. I will copy/paste my question below. I'm sorry if this looks nonsense to you in any way since I am not an electrical engineer :)

I am confused with this question since high school. Hope anybody can help :)

Ok, the question is about electric current. Electric current is the flow of electric charge by moving electrons. Most of you know about http://en.wikipedia.org/wiki/Ohm%27s_law" .

I will define the question in a short circuit diagram but it is also valid in all circuits.

According to Ohm's law (V=IR), in the diagram below, electrons flow through the wire and when they are at point A they all turn right following the green arrow. Because there is (ideally) zero resistance there. No electrons pass through the blue arrow so there is no current/electron flow on the red wire.

http://img215.imageshack.us/img215/8076/figure031js.gif

Now, as you can see there is little a distance between the resistance (R1) and the point A. When an electron comes at point A, it has to check if there is a resistance in his way before he turns (They should also calculate the ratios of resistances if there are resistances on both wires). So electrons have to see the resistance before they turn to the right direction. In order to see and check something, you must have some kind of intelligence, don't you?

This is my question. I think this confuses me because, the level of high school physics is not enough to explain this issue. (A few years ago, I discussed this with a friend studying at electrical engineering and we couldn't find a solution. He is now graduated and probably knows the answer :) )

I certainly can presume that there is a clear explanation and love to see it if anybody has a clue.

 

[berkeman]

Think of the analogy of water flowing in pipes. When there is a branch in the pipe circuit, the water will divide according to the flow resistance in each branch. No thinking required.

 

[webtry]

I see. What confuses me is the part marked red in the above diagram. It seems electrons divide (0% to 100% for a short circuit) before they reach the resistance. If this was not the case we should be able to measure current on the red wire. They don't just go nearby the resistance, and turn back, do they?

I think, I need an improved understanding (with an advanced description) of electric current. It leads to such questions with the definition "electrons go there turn right etc."

And with the analogy of water, how do you define a short circuit? Two branches one is fully closed or more accurately, infinitively thin?

 

[webtry]

I think I have something: these two diagrams below are electrically identical. If the resistance starts just after the point A than there is no question at all. And I know it is assumed to be so in first diagram, although we have some distance between A and R1 (the red wire).

I still wonder is there current on the wire between A and R1 though :)

http://img215.imageshack.us/img215/8076/figure031js.gif

http://img395.imageshack.us/img395/853/figure040gq.gif

 

[berkeman]

Okay, the part that is confusing you is the too-ideal situation of a zero Ohm short. Even wires have resistance, so there is no such thing as a zero Ohm short. (Let's leave superconductivity for another day, okay?)

Right now, just learn how current divides among parallel resistors. In the drawing, make the "short" 0.1 Ohm of resistance, and leave the 10kOhm resistor in parallel. The current (of electrons) will divide between the two paths in inverse proportion to the resistance of that path. So you get 100,000 times more current in the 0.1 Ohm wire as you do in the 10kOhm resistor. Think of the fundamental equation, V=IR. Given some voltage across two parallel resistors, the currents in the two branches are inversely proportional to the resistance in that branch. Make sense now?

 

[webtry]

I don't have much knowledge of electricity, but I must say I definitely know everything you mentioned in your last paragraph.

I also know wires have resistance, read my first post again:
"...Because there is (ideally) zero resistance there."

I am sorry for not making myself and my question clear enough :(

 

[antonantal]

In the correct water flow analogy of the electric current the pipes are always filled with water from the beginning. Because even before you apply the voltage to the circuit the wires are filled with electrons (but they don't have a directional movement yet). You can think of the battery as of a water pump which will make the water (that already exists in the pipes) flow in the closed pipes circuit. You can think of the resistance in terms of a frozen pipe which blocks part of the (or almost the whole in this case) current.
So focusing on the red region of your circuit it's clear now that the water (which already exists) in that region won't move when you start the pump because the portion of the pipe folowing is completely blocked.

 

[russ_watters]

And with the analogy of water, how do you define a short circuit? Two branches one is fully closed or more accurately, infinitively thin?

A short in a pipe would be a wide open pipe and a resistance would be a thin pipe or valve. The water molecule (or the electron) doesn't have to "know" not to go where the resistance is - there are other water molecules (or electrons) in the way, so it can't go in that direction. It's a back-up.

 

[berkeman]

It's a back-up.

Just a quick clarification so the OP doesn't get confused. "Back-up" has two meanings. Russ means it in the sense of the plumbing getting backed up or clogged. He is not implying that electrons find that they cannot go one way, so they back up and go the other. As Russ said, there are other electrons in the way already that aren't moving down the branch with the open circuit (or clogged pipe), so the electron can only go the way the other moving electrons are going.

 

[FrogPad]

webtry:
Check out,
http://ic.arc.nasa.gov/projects/remote-agent/activities/pofo/docs/Power/2-whats-electron-flow.html

and
http://www.allaboutcircuits.com/vol_1/chpt_1/2.html

 

[webtry]

Thank you all, these answers made me think about the question again, and I remembered something that I did not clearly know while constructing the question when I was at high school.

Please correct me if I am wrong as I am not perfectly sure about this; actually electrons do not travel along the wire they transfer energy next to each other (like the well known Newtons pendulum). And this transfer (of energy not the electron flow) occurs at speed of light(?). Is this information true?

I will rethink about my question to clarify if I have anything left to ask.

@Frogpad: I will check those pages now, thank you.

[edit]Wow, this is exactly what I am talking about above (and I swear I did not read it before :) )

...A lot of people think of electron flow as electrons moving along a wire freely like cars go down a highway...

[/edit]

 

[berkeman]

Electrons do travel down the wire. The current past a point on the wire is defined as the amount of charge that is moving past per second. An Amp is a Coulomb per second.

 

[FrogPad]

Webtry somewhat misquoted the website,

... A lot of people think of electron flow as electrons moving along a wire freely like cars go down a highway. Actually, it works a little differently. ...

 

[webtry]

I disagree. It is obvious that the idea in the first sentence is incorrect. That's why I did not add the rest and put "..."

I will try to reconstruct my initial question, to suit the definition of current at NASA site.

And by the way I perfectly know that I am wrong and missing something that causes me to confuse, but I want to understand the issue without doubts.

 

[FrogPad]

I disagree. It is obvious that the idea in the first sentence is incorrect. That's why I did not add the rest and put "..."

I will try to reconstruct my initial question, to suit the definition of current at NASA site.

And by the way I perfectly know that I am wrong and missing something that causes me to confuse, but I want to understand the issue without doubts.

My bad. I was just making it clear that there was more to the website then what you said. I guess rereading what you posted, it is quite obvious :)

Anyways, just remember that electrons are not zipping along at the speed of light. Electrons themselves are moving rather slow but they push the electron in front of it, which in turn pushes the next and so on, and these "pushes" happen very fast.

 

[webtry]

Allright I attached another diagram below. What I see is, there is no reason an electron don't follow the red arrow, because the first resisting atom is at the very beginning of R1. So there must be current in |A-R1| (but there is not! (assuming the wire has zero resistance of course) ).

http://img88.imageshack.us/img88/5616/figure053oj.gif

 

[antonantal]

As I said before the wires are already filled with electrons.

You have a wrong conception about the exact moment when you turn on the power supply on your circuit. I bet that you think that electrons start to flow from the "-" terminal of the power supply and fill the previously empty wires with electrons. And indeed in this case the electron flow would divide in point A, a number of electrons would have to follow the red arrow and fill the red portion of the wire and not until then all the electrons would pass by the red wire and go through the short circuit path. So with this missconception in mind you would have to detect for a short period of time a current through the red wire. This is a missconception.

The correct reality is that the wires are filled with electrons even before you connect the power supply. They just don't move in a precise direction yet. All that the power supply does is "pump" them in a direction. So with the red wire ALREADY FILLED with electrons, no electron will be able to go through it anymore, no electron will follow the red arrow when you turn on the power supply.

 

[webtry]

I bet that you think that electrons start to flow from the "-" teminal of the power supply and fill the previously empty wires with electrons.

No I don't, since I know more or less what is an atom and every matter is made up of atoms in the universe.

So with the red wire ALREADY FILLED with electrons

Okay, tell me the diffence (physically or electrically) between the state of atoms on the red wire and the other branch of the wire; before and after power is on. When I hear a satisfying answer, this case is closed :)

 

[russ_watters]

Okay, tell me the diffence (physically or electrically) between the state of atoms on the red wire and the other branch of the wire; before and after power is on. When I hear a satisfying answer, this case is closed :)

Huh? What part are you not getting? As has been said many many times now, if other electrons are in the way, an electron at point "A" can only go to the right from point "A".

Before the power is turned on, the wires and resistors are filled with electrons and in static equilibrium. After, the electrons are moving, and the resistance influences where they go.

 

[antonantal]

No I don't, since I know more or less what is an atom and every matter is made up of atoms in the universe.

Then, in your last picture, why did you represent all those atoms but only one of them having movable electrons? That's the mistake. All the atoms have movable electrons (including those on the red wire). So in order for an electron in point A to move he would have to push other electrons which are ahead of him. Electrons ahead of him are those on the red wire and those on the short circuit path. Since those on the red wire are blocked by the resistor (supposing an infinite resistance), he can only push the electrons on the short-circuit path.

Okay, tell me the diffence (physically or electrically) between the state of atoms on the red wire and the other branch of the wire; before and after power is on. When I hear a satisfying answer, this case is closed :)

There's no difference. In the red wire as well as in the other branch the atoms have movable electrons (before and after the power is on). But the electrons in the red wire won't have where to go when we turn on the power so they will remain there and block the electrons behind them which try to go by the red wire.

 

[axawire]

I'm pretty sure the following book has your answers.

I would like to recommend the following book: Matter and Interaction vol 2 by Ruth Chabay and Bruce Sherwood, Chapter 18 A Microscopic View of Electric Circuits.

http://www.wiley.com/WileyCDA/WileyTitle/productCd-0471663271.html

If you can't get the book or borrow it the next best thing is a paper written by the authors explaining there approach in writting some of these books. This covers a lot of the physics of your question I beleive.

"A unified treatment of electrostatics and circuits"
http://www4.ncsu.edu/~rwchabay/mi/circuit.pdf

 

[webtry]

All the explanations of yours makes sense, thank you. I will also check that book axawire. Just will try my last chance to explain the point I am confused.

1.First of all please let me know, if when the power is on, all the system is effected absolutely at the same time or not. I mean is there a time delay between things happen at the beginning of the wire and the end point?

2.Then, if the actions take place step by step (not simultaneously) for the whole system:

-We have a set of atoms on the wire that has different physical properties than the rest: they resist to electron flow. And these atoms forms the resistance R1. Before the current starts flowing, are there any difference between these two:
a.The interaction between the first resisting atom at R1 and the last non-resisting atom at the end of red wire (or |A-R1| for the last diagram).
b.The interaction between two non resisting atoms at anywhere on the wire other than R1.

If you think, this does not makes sense and nothing left to answer please do not reply. Thanks again for your efforts. I am also sorry for not drawing a great diagram with all atomic structures are present; like the protons, electrons, their orbits etc.

Finally this is how I benefit from this thread:

Before: I know all current flows at the short wire, but I have questions about it.
After: I know all current flows at the short wire, but I have questions about it. I learned other things about current and electricity, and have some links/articles to read.

 

[berkeman]

I'll just chime in briefly on -1-. The actions propagate close to the speed of light, c. It's similar to a transmission line, where you have two conductors close together (like the twisted pair in Cat-5 cable), and you are forcing a potential difference between the two wires at one end. Say you put a step function voltage in at one end. That step function in voltage propagates down the twisted pair wire at c/n, where n is related to the capacitance and inductance per unit length of the transmission line. Typical cables will have n around 1.3 or so, if I remember correctly without looking it up. Lousy cables with cheap dielectric materials can get n as high as 2, I belive.

The circuit you describe is not a controlled transmission line, but the onset of current flow will behave similarly -- with the effects propagating around at some large fraction of c.

 

[webtry]

Ok I see things going fast on the system and we can disregard what's going on step by step. But things don't happen simultaneously right? I mean this happens first and then the other thing...

 

[FrogPad]

2) Imagine if I am standing behind you with my arms out facing your back and touching your shoulders (like I'm going to push you). Now imagine that you are doing the same to a person in front of you (arms out with your hands on someone’s back). The person in front of you is doing the same, and so on, for the next 50 people or so. We effectively form a chain. Now I push on your back, and "almost" instantaneously you will push on the person in front of you, and so on. My push will be felt down the chain of people at close to the speed of c. So I guess you can think of it step by step if you want, but really you shouldn't. Think about it more like this:

You have a straw full of water. You put the straw in your mouth and blow on one end. Well what happens? Obviously water shoots out the other end. You put pressure on one end and pushed all the water molecules forward. Think of the straw as a wire. The wire is already full of electrons that are MOVEABLE, so current can occur.

Read this:
http://www.physlink.com/Education/AskExperts/ae241.cfm

Now imagine this for a second. You have a battery connected to a wire that is wrapped in a loop with a switch for the battery. You flip the switch and turn the battery on. A potential difference occurs and electrons begin to move, and thusly we have current in the wire.

Now you say everything is made of atoms right? So it would be safe to assume that the wire itself is made of atoms, and the air around the wire is made of atoms right? Why do the electrons only move within the wire? Why don't the electrons start moving down the wire, then pass on into the air molecules? Well the air does not conduct, so it has effectively infinite resistance. So an electron doesn't think like "hey I shouldn't go there, I should go this way", it just CANNOT go there. Just like you can't walk through walls.

If you really want to explore this topic more in-depth. Go to the library and get an introductory book on physics (something that covers electromagnetism), an introductory book on chemistry, and an intro to circuit design.

 

[russ_watters]

Ok I see things going fast on the system and we can disregard what's going on step by step. But things don't happen simultaneously right? I mean this happens first and then the other thing...

It is very similar to when you quickly turn on a faucet (which is why the water analogy is used). Water begins flowing out of the faucet immediately, but water further away does not start moving until it "feels" the drop in pressure ahead of it, as water molecules start to move. The drop in pressure is itself a pressure wave, traveling at the speed of sound. Have you heard of the "water hammer" effect?

Electricity functions on exactly the same principle, but the "pressure" is electromagnetic repulsion and therefore the speed of the pressure wave is the speed of light.

This really isn't all that pertinent to your question, though. What is important is how the electrons are moving after the circuit has settled-down into its equilibrium. And the answer is, as already said, they move the way they do because other electrons are in the way and they can't move any other way.

 

[webtry]

Well I was checking the recent messages here but I saw I was told why electrons don't prefer to travel to air stuff. I am discouraged and lost my curiosity to read more here (this thread only not the forum:) and done with this thread.

Thanks again to all participants of this topic.

 

[Firefox123]

Well I was checking the recent messages here but I saw I was told why electrons don't prefer to travel to air stuff. I am discouraged and lost my curiosity to read more here (this thread only not the forum:) and done with this thread.

Thanks again to all participants of this topic.

Hey webtry...

First of all don't get discouraged by the answers you are getting...you have to understand that the questions you are asking are much more complicated than normal questions about current.

Most people ask "basic" questions and are satisfied with getting "textbook" answers from undergrad books. Most common explanations are either incorrect or just incomplete and often the equations simply describe the "what", not the "why". Occasionally the equations do describe the "why" to an extent, but this usually requires a much deeper understanding of the equations than one gets in a few years of school.

You want an "electron" level view of current and to understand all the forces and interactions that cause the current to "flow" (current doesn't really flow) more so through one path than another...thats fine, but you must realize the depth of such a question.

Common explanations like the water analogy, electrons "pushing" against each other, etc are somewhat incomplete and in some cases are just plain wrong.

I would recommed getting the book that was recommended by axawire and also reading the article he linked to. It seems those two resources are more what you are looking for...I may buy a copy myself after reading the short article that axawire linked to.




Russ

 

[Cane_Toad]

...

1.First of all please let me know, if when the power is on, all the system is effected absolutely at the same time or not. I mean is there a time delay between things happen at the beginning of the wire and the end point?

No, the whole system isn't affected *absolutely* at the same time. However, the events happen so quickly that you can think of it that way. The propagation of electron flow is fast, though below the speed of light by a certain amount (hence the interest in optical computing).

2.Then, if the actions take place step by step (not simultaneously) for the whole system:

-We have a set of atoms on the wire that has different physical properties

No, they have no different properties. They simply don't have anywhere to go, they are blocked off.

Think more about the people conga[?] line: if the person at the head of the line is pushing on a closed door, then the person at the tail of the line can push all he wants without getting any farther. However, if you open the door, then the person at the tail can push and advance because the person at the head is now moving past the door.

Now, think of it less as a conga line, and more of a crowd ready to leave a theater. All the people/electrons have a comfortable personal space or elbow-room (the electrons have filled all their normal shells). The exit door is closed (the circuit is closed). The outside area has only a few people roaming around by comparison (consider this the "ground").

Next, suppose another crowd of people are waiting in the hall (wire) to get into the theater for the next show. Open the entry doors (circuit to the "battery" of waiting people/electrons). The new crowd won't get in very far because the leaving crowd will resist having their person space violated (shells all become filled if they weren't already). The voltage is the difference between how overcrowded the inside is compared to the outside. The resistances are how many doors lead in and out of the theater (and/or how big the doors are).

Open the exit doors, and *all* the people start moving (current starts flowing) because people begin leaving the theater (going to "ground" in this case) making way for the new crowd. (They all don't start moving at the same time, of course, until a steady flow equilibrium is reached.)

"Heat" is generated at the resistance points as people's tempers rising by being jostled around.

OK, that was fun for me, anyway