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Homework Help: How do electrons flow in a circuit?

  1. Aug 17, 2007 #1
    I know how they move in vacuum when some positive/negative charge is near them.
    But I have found it really difficult to visualize their movement in a wire.
    First I used to think that due to the presence of electric field, they move from the negative terminal to the positive. But as the wire can bend, and so it doesn't make sense since electric field are straight lines.

    So, I tried this model:
    Near the positive terminal, valence electrons are being attracted, and thus the atoms that are near the positive terminal become positive, and they attract electrons from the remaining part of the wire.

    This sounds more correct, but according to this, all part of the wire from the switch to the positive terminal should be charged positive when the switch is open, and that doesn't make sense.

    Can anyone provide me the correct model?

    This have been bugging me for a lonnng time. :frown:
    Thanks a lot!!
  2. jcsd
  3. Aug 17, 2007 #2


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    It is the electric field that causes the flow of electrons... the electric field in a bent wire is changing direction along the direction of the wire...
  4. Aug 17, 2007 #3


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    Who said the electric field is always composed of straight lines? As soon as you bring a conductor into an electric field, it's sure as heck not going to be straight anymore!

    - Warren
  5. Aug 17, 2007 #4


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    Another way to think about this is to remember that real wire has some finite resistivity. If you measure the potential along a wire that connects a + and - terminal, there will be a gradual voltage drop all along the wire. Like, if you connect a 1.5V battery to long wire (that say has 10 Ohms of resistance because it is long and skinny wire), then you will get 150mA of current flowing through the wire. The "current" flows from the + terminal to the - terminal, and the actual physical electrons are flowing in the opposite direction.

    Now, if you go along the wire and measure the potential with respect to the - terminal, you will see a linear drop in voltage along the wire, from 1.5V at the + terminal, all the way down to 0V at the - terminal. Now think of a cross-section of the wire at one spot. If you think of a plane cutting through the wire, that is an "equipotential surface", at the potential that you measure with your voltmeter. So there are an infinite number of little disk-shaped equipotential surfaces stacked all along the length of the wire.

    Now, you know that the direction of the electric field vector E is normal to an equipotential surface, right? So that tells you how E bends along with the conducting wire, always pointing down the wire. Does that help you to visualize it?
    Last edited: Aug 17, 2007
  6. Aug 17, 2007 #5
    Thanks everyone for helping me out.

    Yea, I visualized them using those equipotential surfaces(if that's correct though?)

    But I am really confused about how it works(see pic).

    Last edited: Aug 17, 2007
  7. Aug 17, 2007 #6


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  8. Aug 17, 2007 #7
    u should look into some of my threads ...many of them have to do with the flow of electrons in a circuit very beautifully explained and also the links provided.:wink:
  9. Aug 18, 2007 #8
    thnx.a lot
    I will have look at them(they seem to have really good explanation!).

    just found that Irodov's "General Problems in Physics" thread. I was looking for this kind of book for a really really long time. So thanks again/
  10. Aug 18, 2007 #9
    welcome have great time solving it :wink:
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