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Electricity in wire (again) - concept question

  1. Oct 5, 2007 #1
    Hi, I'm trying to understand this concept in order to explain it to a child.

    Let's say I have a battery with a copper wire attached to each end of the battery terminals. No bulbs, no resistors, etc in circuit - just a wire. The battery creates an EMF that moves the electrons in the copper wire, starting from -ve terminal of the battery towards the +ve terminal of the battery. This flow of electrons is what constitute an electric "current".

    Soon, more and more electrons are at the tip of the copper wire near the +ve terminal of the battery. My question: will the electrons then move from the copper wire *into* the battery via the +ve terminal? Will new electrons from the battery go *into* the copper wire via the -ve terminal?

    I'm asking this because I'm trying to use the bicycle chain as an analogy for such an electric circuit - in that electrons are already in the copper wire, and when the paddle moves, all electrons move simultaneously in one direction, and relating to my question above, the conservation of charges - where electons come into the wire at one end, and leaves the wire at the other end. Hust want to check if this understanding is correct.

    Thanks for your help!
     
  2. jcsd
  3. Oct 5, 2007 #2

    mgb_phys

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    Thats a reasonable explantion. Water in a pipe is a similair analogy with water-pressure equaling voltage and flow = current.
    The only problem is when you want to explain how the field in a wire moves much faster than the electron drift velocity. You might be better off saying that 'electricity; flows out of the battery rather than electrons - this also avoids the confusion about electrons flowing - -> + and current flowing + -> -
     
  4. Oct 5, 2007 #3
    The reason current is opposite to electron flow is just because of the classical definition of current being the movement of positive charge though?
     
  5. Oct 5, 2007 #4

    mgb_phys

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    Yes it's a historical mistake - but if you think of electricity in a wire as electrons moving ( which isn't true) it makes conventional current flow confusing.
    Then in solid state electronics you usually talk in terms of holes - which makes it even worse!
     
  6. Oct 6, 2007 #5
    If not "electrons moving", what is the proper explanation?

    Hi mgb_phys,

    You said "electrons moving" is a wrong concept for understanding electric current. Could you help suggest a better way to explain this concept? I'm fully aware that "water pipe", "Bicycle chain", "electrons flow" have been used for high-school physics, and some arguements against such analogies. But I'm struggling to find another analogy to explain electric current. Pls enlighten - thks!

    Xifus
     
  7. Oct 6, 2007 #6

    mgb_phys

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    The electricity is carried by a field, thats why electricity flows at nearly the speed of light but electrons only drift at cm/s.
    I can't think of a good analogy for understanding fields- but the flow one works for most practical purposes as long as you realise its not true.
     
  8. Oct 7, 2007 #7
    Hi mgb_phys,

    Thks. Yes, I understand the effect of electric field working at (near?) the speed of light.

    I relooked at some of the usual textbook explanation, which defines current as the rate at which charges flow, or I = Q/t.

    So if I put these thoughts together:
    1. It is still true that current constitute electrons (or charges) moving. But since electron drfit velocity is only at cm/s, it doesn't answer why electricity is instantaneous throughout the closed circuit.
    2. Your explanation of electric field explains that. When a potential difference is created, an electric field instanteously affect *all* electrons in the circuit to move at drift velocity.
    3. Since all these electrons are already present in the components of the circuit, their respective movement (drift) causes all electrical component to work instantaneously in that field.

    Now I will try to look for an analogy for this "electric field" effect....

    Thanks!
     
  9. Oct 7, 2007 #8

    mgb_phys

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    Yes that's about correct.
    I suppose the analogy would be a line of pupils - if you shouted, everyone take a step to the right, the effect travels at the speed of sound, even though each pupil only moves at 1m/s.
     
  10. Oct 7, 2007 #9
    I dont see how electricity works now that It's been explained like that? o_O doesnt a battery just work by going from a reductant species to an oxidant species in a circuit i.e. electrons donated by a Zn to a Cu, and electron flow is how that works, how are the fields produced?
     
  11. Oct 7, 2007 #10
    Imagine your bike chain, one link is an electron. When you start to pull one end, it will immediately move the other end while the links move quite slowly. It is similar to the electron current.
     
  12. Oct 8, 2007 #11
    Oh, quite a good analogy actually :), so as the first electon moves it creates an electric field, which perpetuates through the cirtuit at near c, but the electron doesnt move especially quickly?
     
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