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Do electrons think?

  1. Jun 24, 2006 #1
    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 Ohm's 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.

    [​IMG]

    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.
     
    Last edited: Jun 24, 2006
  2. jcsd
  3. Jun 24, 2006 #2

    berkeman

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    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.
     
  4. Jun 24, 2006 #3
    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?
     
    Last edited: Jun 24, 2006
  5. Jun 24, 2006 #4
    Last edited: Jun 24, 2006
  6. Jun 24, 2006 #5

    berkeman

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    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?
     
  7. Jun 24, 2006 #6
    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 :(
     
  8. Jun 24, 2006 #7
    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 dont 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.
     
    Last edited: Jun 24, 2006
  9. Jun 24, 2006 #8

    russ_watters

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    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.
     
  10. Jun 24, 2006 #9

    berkeman

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    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.
     
  11. Jun 24, 2006 #10
  12. Jun 24, 2006 #11
    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 :) )

    [/edit]
     
    Last edited: Jun 24, 2006
  13. Jun 25, 2006 #12

    berkeman

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    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.
     
  14. Jun 25, 2006 #13
    Webtry somewhat misquoted the website,

     
    Last edited: Jun 25, 2006
  15. Jun 25, 2006 #14
    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.
     
  16. Jun 25, 2006 #15
    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.
     
  17. Jun 26, 2006 #16
    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
     
    Last edited: Jun 26, 2006
  18. Jun 26, 2006 #17
    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.
     
    Last edited: Jun 26, 2006
  19. Jun 27, 2006 #18
    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.

    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 :)
     
  20. Jun 27, 2006 #19

    russ_watters

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    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.
     
  21. Jun 27, 2006 #20
    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.

    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.
     
    Last edited: Jun 27, 2006
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