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(newbie question) Useful mental models for the flow of electricity?

  1. Jun 27, 2010 #1
    Hi all

    I'm a complete newbie to the subject of electricity and trying to wrap my head round it. To be more precise, I'm trying to build a mental model for myself that accurately predicts what goes on in circuits.

    I've read several accounts that suggest a model based on the flow of electrons. In my (quite possibly inaccurate understanding) of these accounts, the model is something like this: there are more electrons at the negative end of a circuit than at the positive end, and therefore there is a push, or a pressure causing electrons to flow from the negative end to the positive end. (The electrons themselves are not necessarily moving that quickly - I've seen a few cm an hour mentioned, but the flow of electrons at the positive end starts more or less instantaneously because of the pressure in the system, a bit like in a hydraulic system)

    The question I have about this model, or I should say conceptual problem, is this:

    If it really is true that electrons are being 'pushed' by the repelling forces at the negative end, then it's presumably equally true that they are getting 'pulled' by the relative lack of electrons at the positive end.

    If there is this 'pull' from the positive end, then you would expect to see the following:

    Take a circuit with a wire connecting the negative terminal of a cell to a positive terminal. Now put a break in that wire. If there is a pull from the positive terminal because of lacking electrons, then wouldn't you still get a flow of electrons to the positive terminal from the (broken) wire connected to it even though the wire was no longer connected to the negative terminal? Wouldn't this continue until the free electrons in the conductor gradually 'emptied out' until there were none left after a few hours?

    Grateful if someone can tell me
    1) If the 'electrons pushing' model is correct, then where have I gone wrong in my scenario?
    2) If the 'electrons pushing' model is not correct, or not useful, could someone point me to a source that describes accurately what is going on in such a way that the predictions of the model coincide with what actually happens in circuits?

    Thanks for your help and apologies for my newbieness!

    Jake
     
  2. jcsd
  3. Jun 28, 2010 #2
    You said, there are more electrons at the negative end that at the positive end. That is not true.

    The electron density in the wire is the same everywhere. When you apply a potential difference accross the ends of the wire, a field sets up in the wire. The field points from the higher potential end to the lower potential end. The electrons being negatively charged experience a force in the direction opposite to the field. So, they move away from the negative (lower potential) end and towards the positive end(higher potential). There are electrons everywhere in the wire. As soon as the field is set up, all the electrons start moving, and so there is a current everywhere accross the wire.
     
  4. Jun 28, 2010 #3
    Electrons can diffuse in response to concentration gradients, but that's not relevant here. There is no difference in concentration of electrons at the two ends of the wire. The electricity flows due to the electric field. You should look at the thread I just made about a physical understanding of AC signals. No one has completely answered the question there either, but there's a bit of discussion about it, and hopefully someone else will chime in as well.
     
  5. Jun 28, 2010 #4
    Thanks for your helpful replies. Very much appreciated.

    I've also just started watching the MIT open courseware lectures entitled 'Electricity and Magnetism'. I'm not sure if this forum will let me post links, but it's:

    http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring-2002/

    So far, the lectures seem great and introduce electric fields, which I was completely clueless about, and still am, except maybe a little less so..
     
  6. Jun 28, 2010 #5
    Those MIT open courseware lectures are excellent!
    Best of luck with your future...
     
  7. Jul 2, 2010 #6
    Pretty good reasoning up to this point, where you go slightly off-track.

    At the break, what happens is that the electrons on the 'far side' of the break carry on towards the end, leaving a space behind - that space creates a positive charge (there are no electrons there - they left - but the positive atomic nuclei they used to orbit are still there) That positive charge pulls back on the electrons that just left, stopping them. The new positive charge just balances the positive charge at the battery.
    At the near side of the break, something similar happens, electrons pile up, forming a negative charge that stops more electrons being pushed down the wire.

    The interesting thing is that at the break, you get one broken end of the wire becomes charged positive, the other is charged negative and if you make the break into a pair of really wide flat plates, you can put quite a lot of charge into it. Then you can take it away and use the stored electricity elsewhere just as if it were a battery.

    It's called a capacitor.
     
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