Speed of the electrons

Discussion in 'Physics & Math' started by c'est moi, Jan 29, 2002.

  1. c'est moi all is energy and entropy Registered Senior Member

    Messages:
    583
    Folks,

    there's this strange thing going on
    I wondered some time ago what the speed was of electrons (e-) in a normal wire

    now, what I found out is that many people are saying different things, including scientists
    what's going on?

    The first answer which I found, and of which I believe is the correct one, is that this is VERY slow

    Bill Beaty gives the formula to calculate this as follows:

    Bulb power: about 100 watts, about 100V at 1A
    Value for electric current: I = 1 ampere
    Wire diameter: D = 2/10 cm, radius R=.1cm
    Mobile electrons per cc (for copper, if 1 per atom): Q = 8.5 x 10^+22
    Charge per electron: e = 1.6 x 10^-19


    cm/sec = ________I_______ = .0023 cm/sec = 8.4 cm/hour
    Q x e x R^2 x pi

    a good friend of mine, studying for industrial ingenieur says the same thing, he's got it in his physics book

    so why does your light bulb (for example) almost immediately light up the moment you turn on the electricity?

    the e- movement gives of EM energy (heat) which travels at the light of speed and which provides the energy for your bulb
    there's no need for the e- to be fast

    But there's a second big group saying that the electrons move at almost the speed of light. Disturbing here is that I think this is simply not possible because of the metal (ion) grate (or how do ya call this in english?) which prevents the e- to move fast. It's a little bit like jelly packed up.

    Also disturbing is that another friend of mine whom studies physics at the university has this second answer in his book (with all the maths etc.). So this is really getting extremely confusing. They're all using different formula's etc.

    If you go to askascientist.net you'll find that two people have asked this question
    now, one scientist answers with the first group's answer, ie very slow, the other says almost speed of light

    What is it gonna be??

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    how is it possible to have so much confusing for such a basic thing......
     
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  3. Crisp Gone 4ever Registered Senior Member

    Messages:
    1,339
    Confusion is easy in this one

    Hi C'est Moi,

    Most of the times, two electron velocities are mentioned when talking about the "speed" in a wire:

    - The instantaneous velocity: in the classical theories of conduction, electrons can scatter while they move along a conducting wire. Because of this scattering, they move back and forward/up and down in an almost random fashion. The instantaneous velocity is the velocity along the scattering pattern. These velocities are mostly dependent of temperature (I have no immediate idea on how large they are, but I guess about 1 meter/second or something).

    - The drift velocity: this is the net speed at which electrons drift in the direction of the current (you could see it as the averaged instantenous velocity). The drift velocity is a lot lower than the instantenous velocity, simply because the electron moves forward, back a bit, another bit back, forward again, etc etc... and this results in a net speed along the current. Typical values for the drift velocity lie about 20 cm / second or thereabout.

    The reason why light turns on instantaneously is because the electric field that drives the electrons is propagated at (almost) the speed of light. There are electrons in the wires between the switch and the lightbulb, and from the moment you turn on the light, all electrons begin to move (this "starting to move" propagates at more or less the speed of light, exactly because that's the speed at which the electric field propagates).

    Bye!

    Crisp
     
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  5. Imahamster Registered Senior Member

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    135
    (Whoops Crisp got a fine answer in first. Well it doesn’t hurt to have more explanations so this hamster will also respond.)

    There are two separate phenomena, the flow of electrons and the changing EM field. When a chemical battery discharges there is transport of electrons from one terminal to the other through a wire. This is DC current and the electron velocity is relatively low.

    In the presence of an EM field charged particles move. Moving charges produce EM waves. The result is a propagating EM wave. The EM wave velocity is different from the charge velocity. For AC the average charge velocity will be zero while the EM wave velocity is very high. (Image a row of billiard balls. Strike one end and almost immediately a ball moves at the far end even though the intervening balls moved very little.)

    (Referring to “light speed” in a wire is misleading as an EM field may travel faster than light through a wire but does not travel faster than “c”, the speed of light in a vacuum.)

    “electrons move at almost the speed of light”

    Likely your friends meant that an EM field moves at almost “c”. Under special conditions electrons may move at almost “c” but those conditions have little to do with house lighting.
     
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  7. esp Registered Senior Member

    Messages:
    908
    Ve

    In my professional opinion,
    (which is pure conjecture):

    we want to find the speed of an electron in a piece of conductive wire.

    We know that electricity is (for mathematical purposes) the transition of +ve ionization from static atoms. (Easier to think of as movement of electrons).

    And we know that the rate of flow of electrons is called current, (I).

    We know the amout of electrons flowing from the voltage, (v).

    All we need to know to calculate the speed of the electron in flow is the amount of material through which the electron has the oportunity to flow, which can be calculated from resistance, if you can be bothered to work the whole thing!

    (If you do, tell me the answer)
     
  8. Crisp Gone 4ever Registered Senior Member

    Messages:
    1,339
    Hi all,

    Imahamster,

    "Referring to “light speed” in a wire is misleading as an EM field may travel faster than light through a wire but does not travel faster than “c”, the speed of light in a vacuum.) "

    Interesting. Do you have some more information on this, since on the EM theory of light, you would expect the electromagnetic waves to propagate just as fast as light.

    esp,

    "We know the amout of electrons flowing from the voltage, (v)."

    I don't think the voltage can be used to determine the amount of electrons flowing (this is exactly the current I). The voltage is the driving force that makes the electrons flow (voltage = a potential difference = electric force on the electrons).

    Bye!

    Crisp
     
  9. Adam §Þ@ç€ MØnk€¥ Registered Senior Member

    Messages:
    7,415
    V= IR
    I= V/R
    R = V/I

    Where I=Current, V=Volts, and R=Resistence.

    1 Amp = 6.25 x 10^18 electrons per second.

    Power(Watts) = I(squared)R
    R=P/I^2
    et cetera...

    You gave Power as 100 Watts, or 100 Joules/second.

    Power is also equal to the product of current (I) and voltage (V). You said 100 Volts I think, multiplied by 1 Amp, gives 100 Watts. Yay, all works out.

    Voltage is how many are moving, current is how fast they are moving, resistence is how much energy is transferred from the current to the conducting (or not conducting) material.

    How fast is it all moving? Pretty damn slow, a few millimetres per second. But since the circuit is already chockas with the moving matter/charge, it all moves at roughly the same time, like a train. You know, push back of train, front carriage arrives almost immediately further up the track, even though the bit you pushed will take longer. Because of this, the charge effect moves along at C/refraction-index of the material, even though the moving eletrons are slower. I haven't got the formula here, but the speed is about 4x10^-5 metres per second in your average wire circuit.
     
  10. ImaHamster2 Registered Senior Member

    Messages:
    220
    Crisp, this hamster was repeating information from an article concerned with interconnecting chips. One point of the article was that in certain cases electrical signals travel faster than light would in the same wires. (The author was making the point that optical interconnects weren’t necessary faster than electrical interconnects.) As this hamster can’t produce that reference, this hamster will attempt a different justification. (Take it with a large grain of hamster salt.)

    Different frequencies of light travel in glass fiber at different speeds. As described in this reference.

    http://www.lightreading.com/document.asp?site=lightreading&doc_id=7303

    What this hamster would like to conclude is that GHz EM waves in certain wires could travel faster than light EM waves in those same wires as this hamster recalls from the article. However this hamster’s gut starts to churn. Comparing the light wave guide fiber model to EM field propagation in a wire might be viewed as comparing apples to oranges. Such models often contain unstated assumptions. Crossing between models to do a comparison strikes this hamster as shaky reasoning. A best this is a hint that this hamster's statement might be correct.

    Maybe you or someone else could venture an opinion or provide a better reference giving a direct comparison. Sorry.
     
  11. hexaaaaa Registered Member

    Messages:
    1
    SUPERB Crisp..

    Dude Crisp... superb explanation... awesome.. hats off to you...
     
  12. jone Registered Member

    Messages:
    1
    hellow

    I have a couple questions.
    I've read all the comments on here and I was wondering if I can maybe get a better explanation, I seem to be confused whether or not you are referring to AC or DC? and whether or not you referring to electron flow or the field that the electron flow creates.
    I hope that you could explain this a little better.
    The reason why I am trying to figure this out is because I need to know whether or not the speed of the electrons flowing through a wire are relevant to this size of the field the decree when they're going through a coil referring to a coil to create a magnetic field.
    So I hope that you could explain the subject in more detail
    Donovan
     
  13. Q-reeus Valued Senior Member

    Messages:
    2,497
    There is a persistent 'urban myth' perpetuated in some physics textbooks and various online sites, expressed too in some posts here. That drift velocity in metals and some semiconductors is typically at a snail's pace; fractions-of to a few mm/s. Wrong. I cannot yet post links, but it has been accepted (but not widely enough disseminated!) since Sommerfeld's pioneering work back in the 1920's that the true drift velocity in metals is a large fraction of the Fermi velocity, and is typically of the order ~ 10^6 m/s. Only a very tiny fraction of the 'free electrons' in a metal actually contribute to electrical conduction at any given time. I agree though with other remarks about sharply distinguishing between drift velocity and wave propagation velocity when it comes to circuit behavior.
     
  14. leopold Valued Senior Member

    Messages:
    17,455
    measuring the voltage drop across a known resistance will give the current flowing through that resistance after applying ohms law.
    you must know at least 2 of the three required variables, in this case voltage and resistance.
    knowing voltage alone will not tell you the current
     
  15. Aqueous Id flat Earth skeptic Valued Senior Member

    Messages:
    6,152
    With a good lab instrument you can detect an out of band signal propagating a near light speed straight through a coil. The in-band delay through the coil is not due to the physics of the wire, but to the inductance of the coil. Frequency response due to inductance and propagation velocity due to the properties of the material (crystal lattices in wire) are not the same. One is a systemic effect of turning the wire on a form, and the other is intrinsic to copper (or whatever material is used) itself.

    As to the second question: no, the amplitude of the current in the wire does not normally change the propagation velocity. There are some exceptions, for example if you get the wire to glow red hot from excessive current you can expect the propagation velocity to change slightly. Also as current increases, the power coupled with the air (and free space) around the coil increases. At audio frequencies and high power, for example, the coil behave like a transducer and set up acoustic waves in the air (such as hum, either because the coil is vibrating, or some turns have freedom to vibrate, or some ferrous material in the vicinity, as in a transformer, is free to vibrate and hum). At radio frequencies even a coil will behave like an antenna and transmit through the air.

    If you rethink this, the coil has two "speeds" of propagation. The out of band signals pass through it a near light speed, although attenuated, and the in-band signals are delayed by the coil's reactance with little attenuation (approaching its DC resistance) at all. Thus you can model a coil as an inductor in parallel with a resistor.
     
  16. wellwisher Banned Banned

    Messages:
    5,160
    One way to visualize this is with the propagation of a tsunami or tidal wave across the ocean. The wave is moving at high speeds, but the local water molecules, anywhere along the path, are essentially staying in the same position. They may be displaced vertically and then fall back down after the wave passes. The electron particles are moving slowly, but the electron waves are moving through the wire medium at near the speed of light.

    This is a good proof of electrons being particles and waves, with electricity demonstrating the electron particles and waves can also be induced to move separately. The particles are barely moving while the waves go near C. Over time, the swapping of waves still allows each particle to retain a wave function but these don't have to the originals.
     
  17. origin Trump is the best argument against a democracy. Valued Senior Member

    Messages:
    9,822
    No problem, just write out the link, I am sure that anyone interested could copy it into the address bar. I would think that many people here would like to know that our physics books were perpetuating an urban myth!
     
  18. Prof.Layman totally internally reflected Registered Senior Member

    Messages:
    982
    I think it would be even more interesting if someone was able to show the speed of electrons through air, and then measure the speed of a spark was that same speed. I don't think anyone would be able to do that. I think there is good reason why there is no mathmatics of electron flow and their speed in electron flow theory. I don't think the mathmatics of the speed of an electron really applies to anything. Like I mentioned the other day in another thread related to this one, the electric field is designed into circuitry so that it operates at a faster speed. To me it just sounds like something somebody just made up to support this theory, and it is not actually true and has allowed the validity of this theory to be overlooked! If you took out the capacitors on ground from a circuit out, it would still operate at the same speed! The capacitor is the component in electronics that creates an electromagnetic field that would travel at the speed of light!

    According to the math the velocity through air would be much slower than wire, when lightning strikes it is caused by an imbalance between the electromagnetic field, but during the flash this imbalance is then balanced so the number of electrons are equalized in this time. Lightning flashes do not last for hours and days or months even!!!!
     
  19. Prof.Layman totally internally reflected Registered Senior Member

    Messages:
    982
    I would think that the speed electrons travel through a material would be how long it takes for a voltage to transferr to another voltage.

    http://en.wikipedia.org/wiki/Voltage

    Voltage is the electric potential difference between two points, caused by the electromagnetic field. The potential difference between two points could then represent the number of electrons it would take to generate this potential difference. So then when one electron travels to one point to another point, it would then subtract the amout of electromagnetic force that one electron could create from the difference of potential between these two points. Then an electronic high would be a certain number of electrons that can create that much voltage or difference in potential. The rate that this difference in potential can change would then be how fast the electrons can then travel to one point to another or through the medium. The rate the volatage can change in a medium is very fast, a square wave is proof of that. The difference in potential or volatage of a circuit can change almost instantly or close to the speed of light. The vertical line in oscilliscopes is actually filled in artificially by some models. In older models a square wave will not even have vertical lines on the square waves that it displays.
     
  20. leopold Valued Senior Member

    Messages:
    17,455
    the simplest way i can explain this is:
    imagine a garden hose 50 feet long filled with marbles.
    the hose is the wire, marbles the electrons.
    pop a marble into one end of the hose and a marble immediately pops out the other end although the actual electron never left the other end.
    voltage doesn't produce an EM field, current does.
     
  21. Read-Only Valued Senior Member

    Messages:
    10,296
    There is some good info in this thread along with some pretty horrible nonsense.

    The actual speed of electrons in a conductor is VERY slow - so slow, in fact, that it's refered to "electron drift." In ordianry wire under normal conditions,the electrons are flowing at the rate of −0.00029 m/s.

    The reason it seems so fast is that the wire is already filled 'full' of electrons. Much like water in a pipe from the municipal water plant - it comes out of the faucet as soon as you turn it on rather than having to travel the entire distance before it shows up at your house.

    Also, the signal (intelligence) traveling along a wire conductor (like voice or data) travels at about 2/3 c.

    You can forget much of what else is presented in this thread.

    Also, if you happen to doubt what I've just said, you can verify it at Wikipedia and many other sources.
     
  22. Prof.Layman totally internally reflected Registered Senior Member

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    982
    "Voltage is equal to the work which would have to be done, per unit charge, against a static electric field to move the charge between two points" - http://en.wikipedia.org/wiki/Voltage

    So then you only lost one marble, so then how could you lose all of your marbles so quickly?
     
  23. leopold Valued Senior Member

    Messages:
    17,455
    wrong.
    POWER is needed to do work, voltage by itself just sits there like a dumb shit.
    i haven't tagged any electrons so i can't really tell you how fast they move.
     

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