current and electrons

Discussion in 'Off-Topic' started by samjesse, Aug 7, 2009.

  1. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    Hi

    please take what I am going to say with a grain of salt, I am a beginner and trying to crawl, I would appreciate your help though.

    if current is the flow of charge, and that charge moves inside the wire very slowly.
    then it is not possible to have a 5 amp in a DC "every-day" circuit since 5 amps means 5 coulomb per second, and one coulomb is equal to 6.5x10^15 charge, i.e.
    5 amp = 3.125x10^16 charges flowing per second.
    that does not seam very slow to me when I learned that charge is a substance-like from which the atom is built.

    I may still have not settled it, as what is a charge and what is an electron.

    I know electrons do not flow inside the wire, i.e. current is NOT the flow of electrons because if it is then
    how many electrons will flow in a 5 amp circuit, and if there are 2 wires of different materials in the circuit (say copper and silver) then the electrons from the copper wire will mix with the electrons of the silver wire and after n time we will have a one material wire equal to the mix of all the electrons and that just does not happen.

    I have been reading a lot here and there and things do not add up.

    Thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    In wires, current is the movement of charge. Electrons are the charge carriers. The number of electrons in a given cross section of wire (or any conductor) is huge, so the actual motion of any one electron is not that great for any current.

    Can you say how you know that -
    As far as electrons are concerned, there is no particular flavor put on them if they are associated with any metal or any other atom. So mixing electrons from two differeing conductors will not produce the effect you predict -
    Current involving several differing metals happens in many cases, like every solder joint on a printed circuit board. The copper traces never start to become lead-like, not does the tin ever become like copper. Those electrons do not define an atom, nor can they transmute an atom into another substance.

    You might want to review the material that said something about charge. It should not have taught you -
    Charge is a property that partially defines the electron. It is not a substance, however. The electron carries a negative charge as the proton carries a positive charge. Atoms "like" to be in charge balance, but that has nothing to do with their substance. Neutrons, which have no charge, are also important parts of the substance of an atom.
     
  3. bearing01

    Member

    Jul 29, 2009
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    If you run current through a mixed metal junction you can get corrosion or the junction can heat up or get cool based on the direction of current flow. This is how those refrigerators/coolers you plug into your cigarette lighter works. Basically, the more conductive metal has to heat up to become resistive so its conductivity matches that of the other wire. To make it heat or cool you just reverse the current direction.

    As for electron speed, they do flow at almost the speed of light. There is electrical thermal noise that slows them down, which based on the specific metal property, has a conductivity or mobility factor to account for charge velocity. But don't think of the necessity for one single electron to travel from point A to point B in order for the current to have been created. It's not like that. Think of it like a pipe filled with marbles. If you push another marble into one end that will force one out the other end at the same instant. The marble you pushed in did not have to roll down the pipe. Just like a river flows water molecules. If the water height is to stay the same, if a cup of water is poured into the river at 100 miles away, one cup will simultaneously flow out your end instantly. The balance of water or charge must remain the same inside the river or wire. If you add charge in one place, to maintain the charge balance in the wire, charge comes out somewhere else.

    Oh, and also, the wire does not provide the supply of charge or electrons to make all this current possible. The battery supplies the force to push the existing electrons around. All the electrons simultaneously flow out and then back into the battery to form a loop. The wire is just the river or pipe that enables the battery's electrons to flow through itself. The battery is what pushes those electrons around the loop. Think of a water pump and a hose connected from the pump outlet back to its inlet. Spin the pump and the water will flow around the circuit. The spinning energy of the pump is what pushes around the same number of fixed electrons, over and over, through the cycle, until the pump's energy is used up (your hand gets tired). The battery acts like the pump in doing this.

    Battery chemistry is probably a little outside this conversation, but it too is interesting. Just charge, in the form of ions, that wants to unwind the battery's chemically stored energy by flowing in the externally provided circuit loop.
     
    Last edited: Aug 7, 2009
  4. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    that I learned from William Beaty

    which then proves that electrons do not flow at almost the speed of light. they flow very very slowly just one marble distance, the thing which flow at the speed of light is energy "joules", i.e. the flow of energy "joules per second = watt".

    am I missing something?
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Beaty can be the source of some confusion. His views are highly individual and contrarian. He definitely marches to a different drum.

    Our Ebook presents the conventional and accepted explanation of electricity/electronics. If you look at that material, the subject becomes a great deal less confusing. Beaty is not a particularly good source for information - his material is mostly opinion.
     
  6. Ratch

    New Member

    Mar 20, 2007
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    beenthere,

     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    This is old ground. Beaty is not considered an authoritative source. Further discussion will be in Off Topic.

    The OP has already indicated considerable confusion due to Beaty's nonsensical explanations. Beaty has strong opinions, but they are in conflict with convention. We are aimed at a first learner/hobbyist. In the same way that promoting advanced concepts does not fit with initial electronic theory, Beaty simply confuses every issue.
     
    Last edited: Aug 8, 2009
  8. Ratch

    New Member

    Mar 20, 2007
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    beenthere,

    Why is it that I cannot get you to say Beaty is wrong about his facts? If he is not wrong, then he is a credible and authoritative source.

    The OP had a problem understanding the tremendous quantity of electrons that could be moved, even though they move slowly. He did understand that electrons move slowly, but respond quickly to a voltage field, thanks to Beaty's analogy. The OP even corrected a responder who was wrong about that fact. So Beaty did not confuse him.

    What is wrong with that? As long as they are correct. Challenge convention!

    Nothing advanced about what the OP read about charge flow. Any confusion was not from Beaty. And from what I see, there is a fair amount of confusion from e-books too. That is part of the learning process.

    Ratch
     
  9. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    Aha, so the thing that pushed the marble from one end of the pipe is voltage, and the stronger the voltage "static electricity" the stronger the push, i.e. flow of energy "watt".

    what about the electrons from different materials? are they identical, I mean you can have an electron from a gold atom in a silver atom and they are an identical replacement?

    I must admin that I never understood about electricity till I read Beaty's work, for that I am very thankful.
     
  10. bearing01

    Member

    Jul 29, 2009
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    I never heard of Beaty before. As for the pipe and hose analogy, seriously, I just made that up off the top of my head to help explain it to you. I never read that analogy before.

    Yes, all electrons in all materials are the same. All materials are made of the same things, being protons+neutrans and that's balanced out by clouds of electrons floating around in an orbit to balance out the positively charged nucleus with negtively charged electrons. What makes materials different is the size of nucleus (number of protons) and therefore number of electrons in the oribitals or shells that swarm the nucleus in order to make the whole thing electricall neutralized. Then you got like the electrons spinning and that too creates a magnetic field about the atom we're talking about. The size of these atoms all together, and their attractiveness, is what makes up the different elements in the periodic table of elements used chemistry. Then, from this table, if particular elements loose or gain one electron it becomes electrically un-neutralized and tends to be attracted to other oppositely un-neutralized atoms (called cations or anions) and that forms new - different species of atom clusters called molecules. That's where water or salt or skin comes from - different molecules. Everything is made of the same basic building block, being protons+neutrons, forming a positively charged atom nucleus, and then that surrounded by clouds of negatively charged electrons to neutralize out the net overall charge.

    Back to topic, Everything that conducts electricity, all these materials whether it be silicon or lead or gold all have their atoms orgainzed in a crystal lattice structure. All the clouds of electrons coexist and only some of them actually form a sea of electrons that can conduct like a fluid that can move around. The electrons don't just exist in a swarm but rather exist in discrete energy bands, regions of discrete energy level that exist in a position relative to the position / distance from the nucleus. There's something called a valence band and a conduction band. It is the charges (electrons) in this conduction band that are the ones free to move about, like water in the sea, to freely conduct. Electrons in this valence band don't typically participate in conduction or in current flow. Any such electron in this band has to first be promoted into the conduction band to do so. One could be promoted by heat energy added to the material, or maybe sunlight, or maybe by external electrical energy being added. The jump in energy required to promote such an electron is called the band-gap. The amount of energy for an electron to cross the "band gap", say some number in eV or electron-volts. This is getting into semi-conductor physics and I don't want to get too far into that. But in a metal, this valence band and conduction band overlap. Meaning that you don't have to add much energy in order to get bound electrons unbound/ promoted to participate in charge flow. A minute electric field is sufficient to get the electrons to flow. Whereas, say a semi-conductor diode, you have to supply as much as 0.7V to first get electrons to start moving. It really has to do with material physics and I'd have to go read up on it to remember it all again.

    As for electron velocity, it is important that it be fast. When getting into radio frequency circuit design you need the electrons to flow through the transistor channel really fast in order for the transistor to handle the high frequency of operation. The conductivity (C) of a material is related to the average electron velocity (vd) as well as the density of electrons in the material available to participate in conduction (N) and then that's scaled by the charge of an electron (e) which is 1.6e-19

    Therefore, C=N*e*(vd/E)
    where E is the electric field applied across the material.

    To look at another way,
    vd = C*E/(N*e)
    where electron velocity goes up if electric field strength E is increased or if the charge density N, for a given C, is reduced (you need faster electrons if there's fewer of them in order to maintain the same number of electrons per second arriving at the desitnation).

    This vd/E factor is assumed a constant ratio. It being equal to something called the mu-factor or mobility factor. This relationship holds up to a particular value of E. Once E exceeds a certain strength then this ratio no longer holds. Once this happens, if you apply stronger E to the material the electrons won't flow any faster. Then the velocity is considered to reach its saturated point, where vd = vd,sat. In RF semiconductor design, when you reach this point, this is what limits the operating frequency of the semi-conductor technology you're working with. The only way around it is to make the transistors smaller and smaller so the distances that electrons have to travel is shorter, to make the electrical delay shorter. But there's more to it than that. That would be a long conversation. I'm only touching the surface, the type of things you learn so you can later take for granted and forget. There are books and courses written to cover the whole topic.
     
    Last edited: Aug 8, 2009
  11. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    Thanks for this explanation. I will have to chow on it a bit.

    did you mean speed? (velocity is the rate at which an object changes its position and it is a vector quantity).

    back to the flow of electron problem I am still having.
    let current be the flow of electrons; for copper

    does electron have a mass, if so, it must have a size, what is its size (s)?
    5 amp flowing means how many electrons (n)?

    I am going to multiply s by n to find the size of this piece of copper that must move to create the 5 amp flow.

    please do not quantize it by saying it is very small or ..., I want to see the math.

    my math was as follows
    s =
    n = 5 x 6.25^15 = 3.125 x 10^16 electrons

    6.25^15 is coulomb in one amp, coulomb is the quantity of charge, so n above is the quantity of charge for the 5 amp, but that does not mean the quantity of electrons since I do not know if 1 electrons = 1 charge

    but this is as far as I can go with this.
    if you tell me electron does not have a size, then how can it neutralize the proton which should have a size, unless that also does not have a size.
     
  12. bearing01

    Member

    Jul 29, 2009
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    http://www.google.com/#hl=en&q=electron+mass&aq=f&oq=&aqi=g10&fp=1&cad=b

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

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

    As for calculating the necessary wire gauge (AWG) to handle a specific current, it typically comes down to desired resistance drop across the wire. You also have to consider the length of wire. This is why sometimes you buy a tool and the manual states if you're using a 16AWG chord you must not exceed 25' whereas if you use a 12AWG chord you can use a 50' one. Bigger the diameter wire (lower the gauge #) the lower resistance the wire will be, and therefore the long the chord can be. If you notice, for every step up in gauge (or every couple steps, or something) the cross sectional area of the wire doubles.

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

    If you want to rack your brains around doing this on paper then I'd start by taking a wire diameter and length and then find its volume. Them multiply that by the density of your metal. Then divide that by the molar mass of the metal, to give the number of moles of atoms in your volume of metal. Then multiply that by Avogadro's constant to find the number of metal atoms in the volume. Then your job will be to do the quantum physics, or whatever, to determine the probability of how many electrons are available for conduction... or whatever...

    http://en.wikipedia.org/wiki/Avogadro_constant
     
  13. Ratch

    New Member

    Mar 20, 2007
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    samjesse,

    OK, so be it.

    A electron has mass, size, and one negative charge. You can find all those constants in a good physics book. 5 amps means 5 coulombs per second. Don't forget the time factor when you refer to amperes. Charges are quantized as whole numbers, never fractions.

    Your answer of 5 times the number of electrons in a coulomb is correct, but you should specify electrons per second. Don't forget the time with amps.

    Electrical neutralization has nothing to do with size, only charge. A proton is much, much heavier than a electron. Yet a electron neutralizes the plus one charge of a proton.

    Included with this post is a scan of a couple of example problems from a good physics book. It calculates the drift velocity of a electron. I believe that is what you are trying to do.

    Remember, physics and mathematics are two of the foundation sciences of electrical science. If you don't have a good knowledge of those two, your understanding of electrical science will be vague and superficial. You should have access to books on those subjects.

    Ratch
     
  14. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    what is the short answer for "what is the size of an electron"??

    and if there is no short answer, why?
     
  15. bearing01

    Member

    Jul 29, 2009
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  16. JoeJester

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    Apr 26, 2005
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    what is the size of an electron?

    I don't know, I've never seen one with the naked eye. I couldn't tell you how many flowed through my body when I got hung up on a 900V supply either, but I doubt it was just one.

    From WikiAnswers:
     
  17. samjesse

    Thread Starter Active Member

    Sep 14, 2008
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    electron radius r = 2.8 x 10^-15 cubic meter
    electron volume s = 4/3 pi r^3 = 9.2 x 10^-32 cubic mm
    number of electrons flowing in 5 amp n =
    5 x 6.25^15 = 3.125 x 10^16 electrons
    total volume of electrons flowing in one second = n x s = 2.8 x 10^-14 cubic mm
     
  18. beenthere

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    The numerical value for electrons/sec in one amp is incorrect. From Wikipedia -
    5 amps is 3.120753 X 10^19 electrons/sec. This does not relate to size, but to the charge carried per electron.
     
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