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Stupid question of the day....

Discussion in 'Electronic Basics' started by AllTel - Jim Hubbard, Jul 30, 2005.

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  1. I am curious about what would happen to an electrical current in 2

    Assume that you have 2 wires that, when joined, complete a closed electrical
    DC circuit with electrons flowing thusly.....

    ------------ ============
    eeeeeeeeee eeeeeeeeeeeeeee
    ------------ ============

    If you flattened out the end of each wire where they connect , would the
    resulting electron paths be more like figure A or Figure B?

    Figure A

    --- ===
    --- ===
    --- ===
    --- ===
    eeeeeeeee eeeeeeeeeeeeeeee
    --- ===
    --- ===
    --- ===
    --- ===

    Figure B

    --- e e ===
    --- eee eeeeee ===
    --- eeeee eeeeeeeeee ===
    --- eeeeeee eeeeeeeeeeee ===
    eeeeeeeeeee eeeeeeeeeeeeeeeee
    --- eeeeeee eeeeeeeeeeeee ===
    --- eeeee eeeeeeeee ===
    --- eee eeeee ===
    --- e e ===

    (Please note that the vast # of "e"lectrons shown in Figure B is simply to
    show the path's of electrons. )

    The second portion of my question is....If the flattened portions were
    increases in mass (if each wire were connected to a metal cube and the cubes
    were brought together to complete the circuit) how would it effect electron
    flow where the cubes touch?

    Thanks for your help.
  2. Every atom in the conductor contributes an electron to the moving
    herd. If you alter the cross section or shape of the conductor, the
    total number of electrons taking part in the flow across any cross
    section changes in proportion to the cross sectional area (with cross
    section being defined as perpendicular to the local E field that
    motivates the flow).

    Since the current (number of electrons passing through a cross
    section) has to be uniform, all around a current carrying loop, the
    average velocity of the electrons must vary inversely to the cross
    sectional area. If more of them are carrying a given current, they go
    slower. If fewer have to carry that current, they mist move faster.

    I think these rules cover all your cases.
  3. <snip>

    It is not a stupid question--it is just irrelevant. Current flows in various
    ways, and in almost all cases, the details of the flow is unimportant. The
    "wires" can be made from metals, semimetals, hot glass, semiconductors,
    ionic solutions, etc. Each has a different kind of conduction mechanism.

    I have taken the probably impossible task upon myself to discourage thinking
    of conduction as a flow of electrons.

  4. TimPerry

    TimPerry Guest

    neither ... research "skin effect"

    electron flow (or hole flow is you prefer to think that way) is determined
    by total circuit resistance. (and applied EMF as per ohms law) decreasing
    total resistance by increasing contact point surface area will result in
    increased current flow if all other factors remain the same.
  5. You missed that I guess?
  6. Alexander

    Alexander Guest

    Most of the times this just aplies to AC (high frequency) circuits
  7. JoeSixPack

    JoeSixPack Guest

    Before you attack this post, saying electrons can only travel at the speed
    of light, that's incorrect. The electrons themselves can travel any speed,
    but the voltage wave produced does travel at 300,000 kms per second.
  8. John Larkin

    John Larkin Guest

    For DC or low-frequency AC, charge flow will be uniform across the
    cross-section of a round wire conductor (or, actually, any shaped
    conductor with unchanging cross-section.) If you butt two clean-cut
    wires against each other, they're now effectively a single wire, so
    current distribution is still uniform.

    The cube situation is more complex. A wire pokes a nearly uniform
    circle of current into the cubes, and the other wire (by symmetry)
    sucks it up uniformly across its cross-section, but the current
    spreads out as it passes through the large cube, most diffuse halfway
    through and necking down near the entry/exit circles at the wires. The
    exact current distribution within the cube is complex, usually
    computed using finite-element simulation. It might be possible to use
    calculus to compute this distribution, but I wouldn't want to try.

    At higher frequency AC, current in a wire tends to avoid the center
    and crowd near the surface, "skin effect."

  9. Before you attack this post for saying that electrons can travel at
    any speed, keep in mind that Joe probably understands that this
    includes any speed up to, but not including, the speed of light. ;-)

    Thanks for helping out, Joe.
  10. John Fields

    John Fields Guest

  11. John Larkin

    John Larkin Guest


    Copper does have a weak Hall effect. And the current through a round
    wire does make a circular/transverse magnetic field. So, at very high
    DC currents, is the current density a bit non-uniform?

  12. TimPerry

    TimPerry Guest

    researching skin effect will give the poster a better understanding of
    electron distribution in a conductor than short answers on the internet.

    ignore all sites that relate to car or home hi-fi audio.
  13. Alexander

    Alexander Guest

    Op [GMT+1=CET], hakte Jamie op ons in met:
    Damn perhaps Maxwell can help us out ;)
  14. John Fields

    John Fields Guest

    That's not true. The electrons diffusing through the flattened
    portion of the wire would result in a charge flow profile more like
    Figure B, given the understanding that none of the electrons would
    follow a straight-line path through any portion of the wire.
    Further, the assumption is made that the cross-sectional area of the
    wire remains constant at the connection.

  15. Jamie

    Jamie Guest

    i am glad some one is on the ball here! :))
  16. John Fields

    John Fields Guest

  17. Autymn D. C.

    Autymn D. C. Guest

  18. Thanks to everyone for the great input!
  19. Jasen Betts

    Jasen Betts Guest

    electrons cannot exceed the speed of light in a vacuum. no physical object can.

    That said the drift velocity of electrons in electric wires is rarely
    more than walking speed, the signals are transmitted by the interaction
    of the electrons electric fields - ie each electron pushes on its neighbours...

    signals usually seem to propogate through coaxial conductors at 2/3 the
    speed of light. iirc they travel no faster in any other type of conductor.

    Even in fibreoptic cables the signals (photons) go slower than 300000 km/s
    the ratio difference is the definition of the refractive index of the optic
  20. John Fields

    John Fields Guest

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