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Electron flow in a wire?

  1. Dec 9, 2011 #1
    I heard that electrons don't travel straight down a wire but that they spiral around the outer part of the wire as they travel down the wire.

    I know about the skin effect.

    Is there a name for this spiraling of electrons as they move through conductors?

    Anyone know where I can find more information about it??
     
  2. jcsd
  3. Dec 9, 2011 #2
    I thought electron move in random direction. A potential across the wire will produce an E field and the electrons collectively move towards the +ve end. But given in any instance, the electrons are still quite random. They also move very slow. It is the EM wave that travel down the wire at very high speed. So you have to be careful talking about the electron motion. Most of what we talk about in electronics is really propagation of EM wave, not electrons. The physical electrons coming in or out of the wire are just the consequence of the boundary condition of EM wave. If you talk skin effect, you have to understand the EM model.

    Information should be in EM books, but they just never really explain it that well. In Field and Wave Electromagnetics by David K Cheng, he had a model of the parallel plate tx line diagram showing how the wave travel down, and the surface charge due to E field and surface current due to H field. Those are really the electrons that people observed, not really the electron movement from one end to the other. If you want to wait for the ONE PARTICULAR electron to move from one end of the wire to the other end, you are going to have to wait for a long time as the better the conductor, the slower they move.
     
  4. Dec 9, 2011 #3
    I agree - random motion with an average drift rate down the wire.

    Also, the skin effect only occurs with AC and there would be no net movement of electrons. They essentially slosh back and forth as the current changes direction.
     
  5. Dec 9, 2011 #4

    dlgoff

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    micohm.gif

    Microscopic View of Ohm's Law
     
  6. Dec 9, 2011 #5
    Skin effect also happen in step function, not just in AC. Like when you flip the switch on, the very first instance when the switch is on, it's the EM wave that cause the current to turn the light on very fast. But like in Fourier transform that an impulse or step function composed of infinite frequency, eventually it settle down to DC and the electrons move slowly. These are DC step function that compose of infinite frequencies.

    I actually asked this question here a while back. What if the light is connected by a single wire in US, the wire is so long that the switch is in China half a world away. The switch is then connected to the +ve of the battery in China, but the -ve side keep going in the same direction around the earth( not tracing back to the light as return path) and eventually reach back to the light after circling the whole earth. The circuit is totally isolated from the earth ground so there is no return path from the earth ground so there is no guided structure to develop an EM wave to propagate down.

    Question is how long does it takes from the switch turn on to the light bulb light up. I have not get an answer yet!!!! Because the way it set up, you cannot develop an EM as there is no return path to support an EM wave. So theoretically, the electrons has to "craw" through the wire from China to US!!!! Born2bewire said that you can look at it as a big inductor that it delay the turn on for a long long time. I feel this explanation is "thin" at best!!!!
     
    Last edited: Dec 9, 2011
  7. Dec 10, 2011 #6
    How long does it take for the light in the US to light ?

    There has to be a return path or current will not flow.
    All current flows in a loop.

    An inductor is a lumped circuit concept.
    But this is only valid when the signal wavelengths are much larger than the circuit size.
    The rise time of the step function determines its highest frequency component.
    [see the classic text High Speed Digital Design by Johnson & Graham]
    Since the circuit is the size of the Earth, and steps have short wavelengths, another approach is required.

    Light and EM energy travels at the speed of light in the medium of transmission.
    Find the characteristic impedance of the wires from the permittivity and permeability of the transmission medium [the wires] and that will give the speed of the energy which is very nearly the speed of light.

    In general the time to get to the US is how long it takes light to travel the 12000 miles modified by the refraction/impedance characteristic of the transfer medium.

    I will have to think about how large a factor this can be different from the speed of light.
    Impedance of free space is 377 ohms as I recall.
    And coax is 75 ohms ?
    So the factor could be as large as 5 [ ?? ]
    That is it could take 5 times longer than the speed of light.
     
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