Do electrons "move"?

Discussion in 'Physics' started by Cretin, Jun 13, 2013.

  1. Cretin

    Thread Starter Member

    Dec 13, 2012
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    Hey guys I am studying electrical engineering and am doing some summer-time reading. I found this interesting tidbit, could anyone shed some light on this paragraph from a book I picked up?



    See I was taught that electrons do "flow", that they physically move through a conductor, but this paragraph says otherwise. If that is the case, and they do collide and create a vibration, then theoretically is the last electron in a line the one with the "greatest" vibration, or are they all equivalent, sharing their vibrations cumulatively and thus creating a final output?
     
  2. Wendy

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    Mar 24, 2008
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    Yes, they move, but at a very slow rate. You have to understand the numbers involved are vast, 1A = 6.241 × 10e18 / second passing a point on a conductor.

    https://en.wikipedia.org/wiki/Ampere

    The actual speed of an electron is several inches per hour.

    The electric field, which forces this drift, travels close to the speed of light in a wire, this relates very closely to antenna theory.
     
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  3. mac566

    New Member

    Dec 14, 2012
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    What about in a CRT tube? Is the beam the electrons moving or not? Or are they moving more so because of the magnetic fields being created?
     
  4. Fieldfx

    New Member

    Jun 13, 2013
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    I joined up today as a result of this issue....and have been in the wilderness looking in for a while.
    Thanks for all organisation of the 555 circuits Bill...you made it easy for me to remember what I needed to do.

    However...without getting too philosophical...the question posed is a concept I thought we all had buttoned down historically.

    More and more I am believeing that we do not fully understand the process of current flow. It is a given that we are taught to percieve a flow of charge or even the passage of energy across the medium.

    However, as a result of what I have been doing lately, I'm beginning to believe its more like a twisting and releasing of our 3d space.
    The electron flow is more like a gaseous drift which does not leave behind any holes as as that would be ionisation right? (hence the conducting band analogy) and if so...can you imagine our copper wires giving off green light as a result? :)

    Even at DC, the jiggle of electrons is a bit hard to swallow and that would mean that there would have to be a very small quanta of defined energy at which all electrons could not jiggle below at a given temperature....I guess kT would really play a bigger part then....ok...thats noise floor then...
    Imagine the energy transfer issues with all the bumping. The losses from off angle bumping radiation....(I do Radiation Physics as part of my work)

    Consider a charged capacitor. The charge remains in place though we remove the wires....There is nothing to stop the electrons from falling back into a neutral position and if the charge has really left the plate, then the plates should glow when we return the charge.
    Has this ever been tested?

    So if you like, some good reading.
    A concise history...
    http://www.ultracad.com/mentor/what_is_current.pdf

    Richard Feynman and Hendrik Lorentz went on to build topics of science around this very notion...called QED and for which the work went on to be applied to many other areas of science. When pressed, neither of these guys knew what it was (current flow) and thought it spooky...

    However, the bit that always eludes us in class, is that the guy who distilled Maxwell's equations from the 20 odd number that they were down to the classical four we have today had some pretty good ideas himself...
    His name was Oliver Heaviside and he was a nephew of Wheatstone (wheatstone bridge fame).
    He wrote the classical Telegraph Equations that were used to balance the power along a transmission line.
    http://en.wikipedia.org/wiki/Oliver_Heaviside

    One of Heaviside's comment was that only a very small amount of the energy being transferred around a circuit was in the wires and that the majority of it was on the outside....something which Feynman agreed was happening.
    This outside bit was called the Heaviside Component and was ironed out of the mathematics as Lorentz thought it served no useful purpose...
    Mathematicians eh?!

    Hence my take on it being a twist of space...but this is just my own conjecture and musn't be relied on as fact.....but to me it seems that our circuits and such are merely conduits of sorts that allow the 'hooking on' of the energy....much like a train on a track...

    Its good to see the graduate class asking questions...the more you ask of what is, the better a designer you will become. Enjoy!
     
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  5. davebee

    Well-Known Member

    Oct 22, 2008
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    I'd say the quote in the original post is wrong in just about every sentence.

    It is almost right, it is trying to express the right idea, but doesn't quite get there.

    The point it seems to be trying to make is that electric current involves the interaction of energy and electric fields as well as actual electrons, and that much is correct.

    But the electrons do flow in a current, as I understand it. There are currents that involve positively charged ions moving in liquid, but most electric current does involve electrons moving through some kind of material.

    I like to think of electric current as like the action of a bicycle chain. When the bicycle pedal is turned, the power is almost instantly transmitted to the rear wheel, although each individual link of the chain moves around relatively slowly.

    Think of electrons as like the chain links, and think of the electric fields as like the connections between links. Apply a force to one electron, and that force is transmitted to neighboring electrons almost immediately, although each individual electron may move rather slowly.
     
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  6. nsaspook

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    Aug 27, 2009
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    In a CRT the electrons do move at high speed, moving about about 10% of light speed with typical (2kV) oscilloscope acceleration voltages and up to almost light speed in a large CRT television with 10kV+ second anode voltages. The deflection fields can be either electrical or magnetic but usually don't alter the speed by very much.
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    That electrons actually move is pretty easy to demonstrate. Take a capacitor that can be disassembled. Take it apart and measure the force of attraction/repulsion between the two parts. Put it together. Charge it up. Disconnect it. Now take it apart and measure the force of attraction/repulsion between the two parts. If they are different than before, the only reasonable conclusions is that electrons (or some charged particle) has moved from one plate to the other.

    Or even more simply, charge an object through rubbing or through induction. If it is charged, then electrons either moved onto it or off of it. Note the key work -- they MOVED.
     
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  8. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Electrons in a conductor DO flow, but the so-called electron drift is insignificant for all practical purposes. (About an inch per hour at 1A in 14 gauge wire). One case where electron movement IS significant is in vacuum tubes and plasmas.....namely the ionosphere....where actual electron movement is all important!

    So, both answers are right....but not always of equal importance!

    Eric
     
  9. mac566

    New Member

    Dec 14, 2012
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    Interesting. So with an anode voltage of 30kv+ would the beam reach or exceed the speed of light? (When you say 2nd anode voltage, are you meaning grid 2 which is usually 1kv or less and is adjustable.)
     
  10. Wendy

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    Mar 24, 2008
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    Uhh, he didn't say that, 10% is a far cry from FTL. Fact is, electrons are matter, and they can never go at the speed of light.
     
  11. nsaspook

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    Aug 27, 2009
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    The electrons can't reach or exceed light speed, just get very close to it. Electrons are so light (~1/1837 amu vs 1 amu for a proton) very little energy is required to get close to light speed in a vacuum. We have several (large) devices at work that use RF Linac acceleration for up 3MeV ion beams. Even at these levels of energy the heavy (10+ amu) ion beam speeds are only a few fractions of light speed at the target.

    The 2nd anode voltage is the post acceleration voltage after the electron gun anode voltage that extracts the electrons from the hot filament in the cathode. The actual construction varies but the design is basically the same for a simple CRT.

    http://www.tpub.com/neets/book6/21e.htm

    [​IMG]
     
  12. Metalmann

    Active Member

    Dec 8, 2012
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    In Science class, we were taught that Electrons moved.

    Did they stop moving? When?:confused:

    My wife will be pissed.:mad:

    She loves "Games of Thrones". :D
     
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  13. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Getting back to the OP (who seems absent from this thread since post1)

    I would say that the author of your book has misunderstood some theory.

    First it is most important to realise that most of the electrons in a conductor or semiconductor do not move anywhere.
    This is because it is only the outer or valence electrons that take part in electrical conduction.
    The inner ones remain bound to their particular nucleus and thes are the majority.

    Nevertheless all electrons suffer random excitation (or vibration) and can be promoted to a higher energy band by thermal or other energy, if this excitation is great enough.

    In a conductor the so called conduction band requires only a small amount of energy for the outer electrons to be promoted to. So there is always a healthy proportion of electrons in the conduction band.

    Note I said 'conduction band and earlier that the inner electrons stay with their 'nucleus' (not with their atom). This is because we can regard a conductor crystal as one giant molecule which is composed of an array of positve nuclei and the outer electron obits combine to form a band that spans the whole conductor.

    Once an electron is in this band it is free of any individual atom or nucleus and belongs to the whole crystal. It is free to roam the entire space occupied by that crystal.

    So yes, (some) electrons flow through a conductor.

    If we apply an electric field, by applying a voltage then these free electrons will move in a preferred direction and form a net current.

    Without such a field there will be no net current any any direction.
     
  14. Cretin

    Thread Starter Member

    Dec 13, 2012
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    I'm not absent, just enlightened and quite unsure of what to say :D I am really taken aback by how thoughtful you all have been in your replies and your insight. Please do not take my "silence" as a bad thing, I'm just humbled by your responses.

    So as far as I understand electrons seem to form some kind of a cloud around an atom, when this cloud is "excited" then some of these electrons are compelled to expand or perhaps drift into other electron clouds thus creating a bigger electron cloud (excuse the conjecture), and that we then have a net cloud that we interpret as a current.

    I'm probably incorrect about that too...
     
  15. studiot

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    Nov 9, 2007
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    Well yes the cloud idea is good (enough).

    And the some is a small %. Copper has 29 electrons and only one or two 'drift' off.

    although drift is the wrong word as it implies little effort. It takes some energy to detach even these few electons.

    But the main wrong impression is that the 'drift' forms a current. It doesn't.

    A current only flows if there is an impressed electric field to drive it.

    And I'm glad that you are still interested.

    :)
     
  16. mac566

    New Member

    Dec 14, 2012
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    Actually I was basing my question on this part of his comment "up to almost light speed in a large CRT television with 10kV+ second anode voltages". And knowing that many of the CRTs that I work on have an anode voltage of 30 to 35kv I was curious if this increased voltage would allow the beam to reach light speed. But that was also answered. :)
     
  17. nsaspook

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    Aug 27, 2009
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  18. BillO

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    Nov 24, 2008
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    That document, rather than clearing things up, makes it more confusing. I know no one that has ever had the misconceptions the author is trying to dispel. His explanations do not clarify this imagined turbidity.
     
  19. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    I think that the idea the energy flow down the wire is common, I had it and this article cleared it up for me. It may be oriented more towards a fundamental understanding of electricity than most engineers need. The role of fileds and the importance of maxwell's equations seems to be under appreciated. Of course any theory that depends on time varying vector fields and differential operators will be much more difficult than one that basically depends on 2 scalars ( current and voltage in a wire ).

    So while it may be a bit difficult I think that the content is very enlightening.
     
  20. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    Do electrons move?

    Throw a ball across the room all the electrons in the ball move! Is there a current, basically yes, but there are two currents of equal and opposite magnitudes so there is no net current.

    Look at a Van de Graff generator. There are net charges sitting on the belt, and they move with the speed of the belt, that is the current that charges the sphere. Is this a bit obscure, google Van de Graff generator.
     
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