FAO nsaspook - speed of electricity

Discussion in 'General Electronics Chat' started by tobyw, Jun 19, 2013.

  1. tobyw

    Thread Starter Member

    Mar 21, 2013
    37
    4
    So as not to hijack the thread on current direction, this is a chance to follow up on nsaspook's comment about the speed of electricity.

    I've always thought that electricity is a wave motion, caused by the rapid movement of electrons from one atom to another, and therefore the 'speed' of that wave is pretty much the speed of light. Whereas the speed of an individual electron passing from one atom to the next to the next will be much slower - roughly a metre per hour and is referred to as the drift..

    So what's the real story? What is the speed of electricity and of individual electrons moving around a circuit? I guess it's not that straightforward?...what is really going on when you connect up a battery?
     
  2. Shagas

    Active Member

    May 13, 2013
    802
    74
    That was answered in the previous thread I think , that the electrons themselves move very slow and that 1 amp is a few inches per hour or something like that .
    The transfer itself is at the speed of light yes , but the electrons move slow
     
  3. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,221
    What you said is essentially correct.

    The speed of the wave for a transient change in the voltage/current is near the speed of light

    The actual electron speed moving along the wire due to a DC current depends upon the magnitude of the current and the size of the wire but is always quite slow for current values below the fusing current of the wire.
     
    Last edited: Jun 19, 2013
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,969
    744
    Its about 97% of the speed of light in solid Copper wire 291,000 Kilometers per second.
     
    Shagas likes this.
  5. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    It is also the case that in a "long" transmission line with distributed inductance and capacitance, that the velocity of propagation will be substantially less than 0.97c. It might be between 0.60c and 0.75c. I know that some custom cables use a foam dielectric to enhance the velocity of propagation, but I forget why it does this.

    http://www.allaboutcircuits.com/vol_2/chpt_14/3.html
     
  6. tobyw

    Thread Starter Member

    Mar 21, 2013
    37
    4
    I thought so but nsaspooks was saying its not the case. I'm interested to know what he means because I guess it's more complicated than that.
     
  7. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    I'd like to try to answer your question, but I can't find the post you're referring to.
     
  8. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,906
    2,159
    I'm going to post this link again here. http://science.uniserve.edu.au/school/curric/stage6/phys/stw2002/sefton.pdf

    The speed of electricity is governed by the propagation speed of EM fields and is 'c' (the universal speed limit) in a vacuum. In a (simple case) coaxial conductor those fields surround the outside of the (center) wire traveling from the surface of that wire into whatever is around the conductor to the other (shield) wire in the circuit. Because the energy 'flow' is in space between the conductors the electrical characteristics (dielectric constant) of that material controls the speed of any changes to the flow.

    http://www.gpssource.com/files/Cable-Delay-FAQ.pdf

    With open conductors the exact same is happening but now the fields are not totally contained between the conductors and spread out into space near the circuit. The fields that carry electrical energy in a simple static condition battery circuit are not RF EM fields that can move off into free space but are near-field reactive storage fields bound to the charge carriers in the conductor so a direct relationship between the voltage/current and EM fields is possible (circuit theory) for DC and/or slowly varying voltages/currents in small (less than a small fractions of the wavelength of the changes) circuits.
     
  9. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Why do we nod when teacher says

    Charge resides on the outside of a conductor.

    And then nod again when she says electricity moves by passing along a chain of charges in the conductor

    ?
     
  10. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,906
    2,159
    Because we want to pass the test?
     
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Because both are true statements.
     
  12. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    On the outside of a conductor, there is no surface on which the charge to reside. Unless we're talking about the dielectric.
     
  13. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Perhaps you would like to expand on these?
    Can you not see the contradiction?

    I'd go along with this. Are you happy with this state of affairs?
     
  14. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    One might therefore ask (for instance) on what principle the gold leaf electroscope functions.

    http://en.wikipedia.org/wiki/Electroscope
     
  15. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,906
    2,159
    Not really, but almost everyone one in the field of electronics eventually reaches the point where they see the limitations of not knowing the 'why' in some greater detail instead of just knowing 'what' to do to get the correct answer.

    My inspiration for knowing more was
    Richard Feynman

    http://www.youtube.com/watch?v=kS25vitrZ6g

    I don't think it's necessary to understand detailed EM field theory for learning how most circuits operate but there is a gap in learning that IMO was created when vacuum based electronics dealing with the true physical movement of electrons or ions across large spaces and how fields from plates in the tubes controlled that movement was mainly obsoleted by solid-state devices. That created an electron fixation as the 'energy carrier' in solids like metal conductors because of the need to explain PN junctions and electron/hole movement in semiconductors. What is missed sometimes is that these solid-state electron movements at junctions are extremely small in size and work by modifying the atomic scale structures into controlled 'charge carriers' for the EM energy fields to couple with and move from one point to the next. Yes, current flows but the kinetic energy contained in the electron flow is usually tiny compared to the energy in the fields moving inside the device.
     
    Last edited: Jun 20, 2013
  16. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    It's a simple answer, but this is a static charge, and I thougt this thread was about current.
     
  17. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    I think the problem lies with trying to mix engineering and physics.

    It is no more sensible to design an electric torch by studying what the eelctrons are doing than it is for a mech engineer to try to track all the gas molecules when designing an internal combustion engine. The latter uses aggregate quantities like pressure without regard to their origins.
     
  18. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,906
    2,159
    I would change that to 'trying to mix engineering and BAD physics'. :)
     
  19. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Even 'good' physics is only a model and doesn't get all the answers right.

    The trick is to select the appropriate model for the job in hand and to work within the conditions of applicability of that model. Most failures are due to one of these two causes, arithmetic errors apart.
     
  20. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,906
    2,159
    Agreed but not all engineering problems are initial design problems with a set of known requirements, some of the hardest problems to crack are usually those that need "root failure analysis" of a existing design or process. This is a recent and simple example on the job.

    We had repeated failures of an special (and costly) isolation transformer on a machine after maintenance. The secondary is at 90kV DC above ground potential connected to a ion beam generator. The machine had run for 10 years with the original transformer and ran only a month with the replacement before an internal short caused it to fail again. The tech that looked at it believed it had defective insulation and simply shorted to ground but this seemed very unlikely to me. I reviewed the SPC reports the machine generates while running and saw something strange, we were having phase-lock and tuning problems on another section of the machine isolated by an RF shield that uses 10 3kW RF amps to boost the beam. We closely examined the transformer and found the short was not directly to ground or the frame but was from the secondary winding to a internal electrostatic shield that was connected to ground.
    [​IMG]

    In our case we had high levels of unwanted high frequency AC energy generated on the secondary side that melted the insulation and wrapping separating the shield from the secondary. So the question became what is generating this energy?

    The 'good' physics, no real need for complex models or math, just a little deeper into the rabbit hole.
    The ions from the beam source are accelerated from 0 to 90keV in the space of about 1 foot, the ions are charged particles so they generate EM waves (even X-rays under the right conditions) during that acceleration (it's much worse during beam instability or arcing) that the shielding normally blocks and shunts to ground.
    http://www.tapir.caltech.edu/~teviet/Waves/empulse.html
    http://phet.colorado.edu/sims/radiating-charge/radiating-charge_en.html

    So now we have the likely AC energy source for the transformer short but what has changed recently causing it to short. A close examination of the enclosure for the transformer, ion source and shield found a set of hidden copper ground straps that had cracked over time with one completely broken and disconnected under the lug nut. So now instead of being shunted to ground by the shield this EM energy was being radiated across the enclosure into a large electrically isolated metal electronics box that was powered by the isolation transformer with an electrostatic shield not designed to handle RF energy being shunted to ground.

    Root cause: Broken ground strap during maintenance. Add procedure to check during PM.

    Two transformers: Expensive
    Down time to diagnose and repair: Even more expensive
    Knowing why it caused the problem and kicking "Murphy's law's" behind: Priceless.
     
    screen1988 likes this.
Loading...