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Speed of electricity

  1. Jun 27, 2009 #1
    How fast does electricity move? It seems to move at different speeds when in lightning form. Like sometimes a bolt of lightning will be instant and then other times you can see it streaking kinda slowly across the sky.
     
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  3. Jun 27, 2009 #2

    mgb_phys

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    Like a lot pf physics - it depends what you mean!

    The speed of electrons moving a long a wire is very slow - a few mm/s, the speed of the electric field moving along a wire is a large fraction of the speed of light.

    Lightning is a little more complicated, there are lots of stages in a lightning strike, the leader current, the return current, the ionization etc - but several 100,000 mph would be a reasonable estimate.
     
  4. Jun 27, 2009 #3
    True. What I'm talking about is the visible streak that the lightning creates. Sometimes it's so fast, it looks like the entire bolt of lightning appears instantly. Then other times I can see the visible streak go across the sky. I was just wondering why there was a discrepancy.
    It's weird, so the electrons actually don't move that fast, but they act kind of like a Newton's Cradle and thus the signal gets sent quickly?
     
  5. Jun 27, 2009 #4
    Check out this wikipedia article on lightning:
    http://en.wikipedia.org/wiki/Lightning

    Although it does not address your question directly, this article suggests a lot: namely, we still have a LOT to learn about lightning. The apparent different types of lightning, positive, negative,ball, cloud to ground,cloud to cloud, etc, suggests a lot we do not understand. It would seem possible that leaders in different types of atmosphere, say varying temperatures, humidities, varying degrees of particles in the air,etc might also affect lighting propagation As is evident in the article, many lightning activities are so quick they are largely imperceptible to our senses.

    Also note the description of ball lightning which is believed to last longer than other "types"..so what you may see is not a difference is the speed of propagation but in the duration of the flash.
     
  6. Jun 27, 2009 #5

    mgb_phys

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    Yes - excellent analogy.
     
  7. Jun 27, 2009 #6
    Signals move in coaxial cables as a TEM (transverse electric magnetic) waves at about 0.66 to over 0.9 times the speed of light. The velocity of the signal depends primarily on the dielectric constant of the dielectric between the inner and outer copper conductors.
    Stanford Linear Accelerator Center (SLAC) produces beams of electrons in vacuum at about 0.999999+ times the speed of light.
     
  8. Jun 27, 2009 #7
    Electron's move at mm/s in a wire? To a rough approximation of free electrons in an infinite, vanishing, periodic lattice do we not get that electrons go about 10% of c?
     
  9. Jun 27, 2009 #8
    Not to mention the observed propogation delays in digital circuits would not be consistent with such a slow speed
     
  10. Jun 27, 2009 #9

    negitron

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    No. Look up electron drift velocity.

    As previously noted above, the speed of electrical signals you observe is not due to the motion of the electrons in a conductor but the propagation of the electric field in the space around the conductor. The velocity of this propagation is dependent upon the speed of light through the medium around the conductor; for free air this is about 98% c while for typical plastic insulations it varies between 50 and 75% c.
     
  11. Jun 27, 2009 #10

    Integral

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    Signal propagation speed is only very loosely related to electron propagation speed. The electrons move more like a train then individual cars. When a signal is applied it is felt by all electrons in the conductor. Signals propagate much faster then the individual electrons.
     
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