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One of the most fundamental questions in electric currents (basic but very important)

  1. Feb 11, 2010 #1
    NOTE: This is NOT a homework question. I was reading a physics textbook for fun (doesn't everyone nowadays?) and could barely got through the first couple of page the chapter on electric currents because this question was not addressed in the book.

    Question 1) If you connect the negative and positive ends (of say a battery) how do the electrons know to move through the wire to the positive end (wires are pretty long)? I mean, I know that in a battery, there is a positive and negative end that is separated from each other, but wouldn't there still be an electric attractive force between them that pulls them closer to each other (even though they are resticted by the sticky material that separates the charges)? Then why would the electrons move away from each other temporarily and travel a long distance to get to the positive side? But note that whenever you cut the wire or something and you lose the connection, the electrons stop flowing to the positive side (like in a lightbulb). I researched in a couple of books and all they said was that there is an electric field in the wire and that causes the electrons to move. BUT a wire is usually pretty long (especially compared to the miniscule electron, relatively speaking) and thus the electric field would be virtually zero, right?

    Thanks!
     
  2. jcsd
  3. Feb 11, 2010 #2
    Re: One of the most fundamental questions in electric currents (basic but very import

    Not to be pushy, but NO ONE can solve this question?

    Wow! This must be tougher than I thought!
     
  4. Feb 11, 2010 #3

    berkeman

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    Staff: Mentor

    Re: One of the most fundamental questions in electric currents (basic but very import

    It's not that we can't figure it out. It just takes a lot of patience to answer basic questions like this, and we tend to tag-team on them.

    Here is a wikipedia article that should go a long way toward answering your questions. The force that pushes the little electrons along is called EMF:

    http://en.wikipedia.org/wiki/Electromotive_force

    Oh, and when you say "how do the electrons know to move through the wire to the positive end", you should try to avoid anthropomorphising electrons. They hate it when you do that. :wink:
     
  5. Feb 12, 2010 #4

    uart

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    Science Advisor

    Re: One of the most fundamental questions in electric currents (basic but very import

    Think of an analogy. Say you have a fountain with a water pump that takes water from a low pressure source (sump) and brings it up to high pressure before spraying it out of a nozzle, where it flows through the fountain and eventually back to the sump. How do the water molecules “know” to spray out of the nozzle and through the fountain? I mean the pump has high pressure at one end (the outlet) and low pressure at the other end (the inlet), so why doesn't the water simply flow back through the pump, why does it “know” to flow back through the external "circuit" instead.

    Simple answer: Because that's how pump works. It has an internal mechanism which both pushes the water toward the outlet and increases it’s pressure on the way.

    So think of a battery like an electron pump. You don't really need to know the detailed mechanism in order to solve basic circuit problems, you can simply accept that a battery does indeed work "as advertised". This is in the same way that I don't necessarily need to know the detailed workings of a pump in order to use a high pressure pump/nozzle to clean my driveway, it is possible for me to just accept that a water pump works and use it.

    If you're just starting on circuit analysis then this "logic" may apply to some other circuit elements too. While it’s always good to understand the detailed physics behind each circuit element you encounter, and I strongly encourage it, you shouldn't let it hold you back from learning circuit analysis. If you haven't covered the detailed physical reasons of why a circuit element behaves the way it does, then for the purpose of circuit analysis you should just accept that it does actually work the way the textbook describes.
     
    Last edited: Feb 12, 2010
  6. Feb 12, 2010 #5
    Re: One of the most fundamental questions in electric currents (basic but very import

    thanks for the speedy replies!

    @uart: interesting analogy! Thanks for such a detailed response!

    (I know that I don't have to know all the complicated internal workings of mechanisms in order to solve basic circuit problems, but as I stated before, this is purely for conceptual understanding. :) )

    @ berkeman: thanks! it turns out that I studied the electromotive force before I encountered this question :) I looked through the wiki article as you generously suggested but can't find anything that helps answer any of my questions, though. Because if you have the emf, then shouldn't the emf between the positive and negative charges in say, a battery, pull the charges closer to each other (even though they are resticted by the sticky material that separates the charges) rather than the charges moving through a wire that temporarily separates the charges a little more before reducing the distance?
     
  7. Feb 12, 2010 #6
    Re: One of the most fundamental questions in electric currents (basic but very import

    There is a uniform E-field within the wires if its a DC source, so the electrons are constantly being accelerated(pushed).
     
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