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Confused about "What is current flow"

Discussion in 'Electronic Basics' started by Brian, Aug 23, 2003.

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  1. Brian

    Brian Guest

    A very simple approach:
    Let's say that we have a battery with a resistor connected across it. Let's also
    say that the wire that connects the resistor to the battery, has a diameter of
    one atom. At the positive end of the battery, the battery terminal pulls one
    electron away from the atom at the end of the wire. That atom then becomes
    positive charged. This positive charged atom, then pulls an atom away from the
    atom next to it (that is closer to the negative terminal of the battery).
    Through chain reaction, this positive charge works it's way to the negative
    terminal of the battery (which is called conventional current). The electron
    that is pulled from each atom, is called electron current.
    Hope this helps,
    Brian
     
  2. Many years ago I attended the Navy's electronics technician school at
    Treasure Island, CA.
    I was taught that current flow is the flow of electrons from the negative
    terminal of the battery
    (or any power supply source) to the positive terminal. I also remember that
    the instructor
    briefly mentioned something called "conventional current" which he said was
    a theory that
    said that current is really an "effect" that is opposite to the direction of
    electron flow. This was
    basically, as far as I can remember, all that was said on this subject, and
    the course proceeded
    for the remainder of the 38 weeks it lasted, to treat electron flow and
    current flow as synonomous.

    Now I am studying a book which contains the following passages:

    v = iR Eq. 1

    "Equation 1 implies a specific relation between reference directions for
    voltage and
    current. This relation is show explicitly as:

    i R
    ------>
    -------/\/\/\/\/\/\/\-------
    + v -

    the arrow defines the positive flow of current (flow of positive
    charge) is directed * in *
    at the resistor terminal assigned to be positive voltage. This
    convention is generalized
    to an arbitrary element as follows:

    i
    ----<-------
    | +
    |
    -------
    | | v
    -------
    |
    | -
    --------------

    The variables v and i are called the terminal variables for the
    element. Note that the
    values of each of these variables may be positive or negative
    depending on the actual
    direction of current flow or the actual polarity of the
    voltage."

    I am completely confused by this passage and subsequent ones which depend on
    it. My poor
    mind is fixated on what I learned in my younger days, i.e., that current
    flow IS the flow of
    electrons out of the NEGATIVE terminal of the source and into the POSITIVE
    terminal of
    the source, which, of course, results in the following:

    i R
    ------>
    --------/\/\/\/\/\/\/\---------
    - v +

    which is the reverse of what the book shows. The book's statement about
    "flow of postive
    charge" is really confusing because positive charges are protons and they
    certainly don't flow
    unless you split the atom. I guess most students today are taught this
    convention from the git-go
    and so they don't have to unlearn the previous convention. Can someone
    explain this new
    way of thinking about current flow to me, or perhaps point me to some
    website that does so.
    Thanks.

    Regards,
    Jeff S
     
  3. Once upon a time people thought that current was a flow of electrons
    from positive to negative. They were wrong, but we still pretend that's
    the way it is. That's it. There's nothing more to understand on the
    matter.

    It doesn't make any odds what way you say they flow, it all cancels out.
    If what is black is really white, but everyone calls everything that is
    black is white, there is no net difference.

    There is no current "flow". "Current" already contains the notion of
    flow. Current means "flow of charge".

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  4. Mark Haase

    Mark Haase Guest


    For the OP's sake:

    http://www.ibiblio.org/obp/electricCircuits/DC/DC_1.html

    Some experimenters speculated that invisible "fluids" were being
    transferred from one object to another during the process of rubbing,
    and that these "fluids" were able to effect a physical force over a
    distance. Charles Dufay was one the early experimenters who
    demonstrated that there were definitely two different types of changes
    wrought by rubbing certain pairs of objects together. The fact that
    there was more than one type of change manifested in these materials was
    evident by the fact that there were two types of forces produced:
    attraction and repulsion . The hypothetical fluid transfer became known
    as a charge .

    One pioneering researcher, Benjamin Franklin, came to the conclusion
    that there was only one fluid exchanged between rubbed objects, and that
    the two different "charges" were nothing more than either an excess or a
    deficiency of that one fluid. After experimenting with wax and wool,
    Franklin suggested that the coarse wool removed some of this invisible
    fluid from the smooth wax, causing an excess of fluid on the wool and a
    deficiency of fluid on the wax. The resulting disparity in fluid
    content between the wool and wax would then cause an attractive force,
    as the fluid tried to regain its former balance between the two
    materials.

    Postulating the existence of a single "fluid" that was either gained or
    lost through rubbing accounted best for the observed behavior: that all
    these materials fell neatly into one of two categories when rubbed, and
    most importantly, that the two active materials rubbed against each
    other always fell into opposing categories as evidenced by their
    invariable attraction to one another. In other words, there was never a
    time where two materials rubbed against each other both became either
    positive or negative.

    Following Franklin's speculation of the wool rubbing something off of
    the wax, the type of charge that was associated with rubbed wax became
    known as "negative" (because it was supposed to have a deficiency of
    fluid) while the type of charge associated with the rubbing wool became
    known as "positive" (because it was supposed to have an excess of
    fluid). Little did he know that his innocent conjecture would cause
    much confusion for students of electricity in the future!
     
  5. Mantra

    Mantra Guest

    It *is* just a convention, but there is a reason: the algebra of
    doing circuit analysis tends to be less confusing - you'll have a
    higher likelihood of a spurious "-"s that wreak your calculation when
    you analyze with negative electron flow. Positive or negative, the
    calculations all turn out the same, but only *if* don't accidently
    flip any voltage drops in a loop.


    MM
     

  6. Electronics teachers and authors of textbooks are often chided for
    passing on an error to their students: the false idea that electric
    current is a flow of positive particles in one direction, when it
    really is a flow of negative electrons in the opposite direction.
    In fact, those who do the chiding are themselves mistaken. They are
    laboring under the misconception that "electricity" is invariably made
    of negatively-charged particles called electrons. This is wrong, and
    it leads people to wrongly conclude that electric current is really a
    flow of negative particles. Actually, in some situations, electric
    current can really be a flow of positive particles. In other
    situations, the flow is negative particles. And sometimes it's both
    positive and negative flowing at once.
    "Electricity" is not made of electrons (or to be more specific,
    Electric Charge, which is sometimes called "Quantity of Electricity,"
    is not made of electrons.) It actually comes in two varieties,
    positive and negative particles. In the everyday world of electronics,
    these particles are electrons and protons.
    Because the negative particles carry a name that SOUNDS like
    "electricity," people unfortunately start thinking that the electrons
    ARE the electricity, and that protons (having a much less electrical
    name?) are not. Some text and reference books even state this
    outright, saying that electricity is composed of electrons. In reality
    the electrons and protons carry electric charges of equal strength. If
    electrons are "electricity", then protons are "electricity" too.
    Now everyone will rightly tell me that the protons within wires cannot
    flow, while the electrons can. Yes, this is true of solid metals.
    Metals are composed of positively charged atoms immersed in a sea of
    movable electrons. When an electric current is created within a copper
    wire, the "electron sea" moves forward, but the protons within the
    positive atoms of copper do not.
    However, SOLID METALS ARE NOT THE ONLY CONDUCTORS, and in many other
    substances, the positive atoms *do* move, and they *do* participate in
    the electric current. These various non-electron conductors are
    nothing exotic. They are all around us, as close to us as they
    possibly can be.
    For example, if you were to poke your fingers into the anode/flyback
    section inside a television set, you would suffer a dangerous or
    lethal electric shock. During your painful experience there obviously
    was a considerable current directed through your body. However, NO
    ELECTRONS FLOWED THROUGH YOUR BODY AT ALL. The electric charges in a
    human body are entirely composed of charged atoms. During your
    electrocution, it was these atoms which flowed along as an electric
    current. The electric current was a flow of positive sodium and
    potassium atoms, negative chlorine, and numerous other more complex
    positive and negative molecules. During the electric current, the
    positive atoms flowed in one direction, while the negative atoms
    simultaneously flowed in the other. Imagine the flows as being like
    crowds of of tiny moving dots, with half the dots going in one
    direction and half in the other. The crowds of little dots move
    through each other without any dots colliding.
    So, in this situation, which direction did the electric current REALLY
    have? Do we follow the negative particles and ignore the positive
    ones? Or vice versa?
    Batteries are another example of a non-electron or "ionic" conductor.
    When you connect a light-bulb to a battery, you form a complete
    circuit, and the path of the flowing charge is *through* the inside of
    the battery, as well as through the light bulb filament. Down inside
    the battery, within the wet chemicals between the plates, the electric
    current is a flow of both positive and negative atoms. There is a
    powerful flow of electric charge going through the battery, yet no
    electrons flow through the battery at all. What is the real direction
    of the electric current while it's between the two plates of the
    battery? Not right to left, not left to right, but in both directions
    at once. Out in the metal wires, the flow is from negative to
    positive. But inside the battery's wet electrolyte, the charge-flow is
    in two opposite directions at the same time.
    There are many other places where this kind of positive/negative
    charge flow can be found. Electric charges in the following list of
    devices and materials are a combination of movable positive and
    negative particles. During an electric current, both varieties of
    particles are flowing past each other in opposite directions.
    TWO-WAY POS/NEG ELECTRIC CURRENTS CAN EXIST IN:
    • batteries
    • human bodies
    • all living organisms
    • the ground
    • the ocean
    • electrolytic capacitors
    • aluminum smelters
    • liquid mercury
    • ion-based smoke detectors
    • Geiger counter tubes
    • electroplating tanks
    • electrophoresis gels in medicine (esp. DNA testing)
    • air cleaners, smoke precipitators
    • particle beams
    • the vertical "sky current" in the atmosphere
    • gas discharges, which include:
    • electric sparks
    • flourescent tubes
    • neon signs
    • the Aurora
    • lightning and corona discharges
    • arc welders
    • thyratron tubes
    • mercury vapor rectifiers
    • sodium and mercury arc streetlights
    The above list of conductors which contain both positive and negative
    flows is not short. Again I ask you, what is the REAL direction of
    electric current? We cannot solve the problem by belittling it, or by
    pretending that it pertains only to something exotic, to something not
    part of everyday life.
    Let's get down to the details of the problem. When trying to
    understand electric circuits and electrical measurements, how can we
    make measurements of the important entity named Electric Current?
    Won't we first have to figure out how much of the current is composed
    negative particles going one way and positive the other? Yes, but only
    if we want to know everything about the electric current. The negative
    and positive flows are usually not equal, and the speed of the
    positives in one direction is usually not the same as the speed of the
    negatives in the other. However, there is a nasty trick we can pull
    which avoids having to look at the particles at all...
    The main effects produced by electric current are magnetism, heating,
    and voltage drop across resistive conductors. These three effects
    don't care about the amounts of positive and negative particles, or
    about their speed, mass, charge, etc. If a hundred positive particles
    flow left per second, this gives just as much magnetism, heating, and
    voltage drop as a hundred negative particles flowing to the right per
    second. (Note: this is because reversing the polarity of the particles
    reverses the current, and reversing the particle flow direction
    reverses the reversed current!) Magnetism, heating, and voltage drop
    together represent nearly every feature that is important in everyday
    electrical circuitry. And so as far as most electrical devices and
    circuits are concerned, it makes no difference if the current is
    positive particles going one way, negative particles going the other,
    or half as many negatives flowing backwards through a crowd of half as
    many positives.
    So, to simplify our measurements and our mental picture of Electric
    Currents, we cut away the unused parts of the picture. We
    INTENTIONALLY DEFINE the electric current as being a flow of
    exclusively positive particles flowing in one particular direction. We
    don't care about the real polarity of the particles. We don't care
    about their speed, and we don't care about their number. We ignore the
    chemical effects and the effects of the velocity and direction moving
    particles. We ignore the collisions between positive and negative
    particles. All we care about is the total charge which moves past a
    particular point in the circuit. The real charges are too complicated
    to deal with, and the added complexity gets us very little
    information, as long as we're only interested in voltage drop,
    magnetic fields, and heating.
    Once we start ignoring the speed and direction of the charges, we can
    then build electrical instruments, "amp meters," which measure the
    electric current in terms of the magnetism it creates, or by the
    voltage drop which appears across a resistor, or by the temperature
    rise being created in a calibrated piece of resistance wire. These
    three types of meter will agree that "current" is "current," then we
    can use these meters everywhere. In nearly every situation they will
    tell us all we could ever want to know about flows of charged
    particles in any circuit. An amp-meter might not be appropriate when
    used in an exotic physics experiment. But for more than 99% of
    electricity and electronics, the direction of the particles is
    irrelevant, and an ammeter tells us the "real" current.
    We do cause some problems in choosing to simplify "Electric Current"
    in this way. For example, what if we think in terms of simplified
    electric current for many, many years? Couldn't we all eventually come
    to believe that this oversimplified concept of electric current is
    REAL? Yet it's not real, it is simply one aspect of flowing particles.
    For this reason, we might start to see "Electric Current" as a sort of
    abstract, invisible, difficult-to-visualize thing. We might lose track
    of the facts that electric current is an actual flow of matter, or
    that there are real, visible particles flowing along inside that
    circuit, or that they have a particular average speed, mass, and
    direction.
    Because it is so incredibly useful, the simplified interpretation of
    Current takes over and becomes more real than the real world. It is
    incredibly useful, and it lets us understand parts of physical science
    which otherwise might be too complicated to think about. But in
    letting it take over, some nagging questions are left behind, such as

    "WHICH WAY DOES THE ELECTRICITY REALLY FLOW?"

    Animesh Maurya
     

  7. No its not short, but is not really relevant in the context of electric
    circuits. If we are dealing with general purpose electronics, in the
    majority of cases, we are discussing electron flow.

    Again I ask you, what is the REAL direction of
    This is not really sufficient. Charge flow is not just a flow of matter
    particles. If this were so, the ammeter placed in a capacitor would read
    zero under AC conditions. Charge is a net effect of photon movement, it
    is not restricted to residing within a charged particle.


    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  8. The best article I have ever read on this issue.

    Should be saved and used in a FAQ for this newsgroup, on web sites for
    beginners and used whenever the issue comes up.

    You are a very good writer.
    The only (positive) criticism I can give you is that you could insert
    more empty lines here and there, to break up the text into smaller
    blocks, to make it even easier to read.
     
  9. Fred Abse

    Fred Abse Guest

    Would that be wheat or rye?
    All the above refer to ions, which are charged atoms, ie atoms having an
    excess or deficiency of electrons. It can be argued that the ions
    constitute a conductor, in which the current still consists of electron
    flow. Experience tends to bear this out, since current flows just as fast
    in an ionized medium as it does in a "conventional" conductor, whist ions
    move relatively slowly. remember "ion traps" in older CRTs?

    -
    Then there's duct tape ...
    (Garrison Keillor)
     
  10. Fred Abse

    Fred Abse Guest

    Why? If you just change the convention so that negative becomes positive
    and vice versa, the math's the same.

    The math doesn't know about electrons, electrons don't do math :)
     
  11. Wim Lewis

    Wim Lewis Guest

    Actually, electrons don't move very quickly in a conventional conductor,
    either. Certainly not as fast as the change in voltage propagates
    through the conductor. The electric *field* propagates quickly, but
    the individual charge-carriers move much more slowly.

    Take an analogy of electrical wiring being like plumbing. Now imagine
    you're in the bathroom and you turn the shower on, drawing hot water
    from the water heater. Water starts flowing out of the shower head
    immediately, and (nearly) simultaneously, starts flowing into the
    other end of the pipe, from the water heater. But it takes
    several seconds for the water coming out of the shower head to
    become hot. Why is this? Why should it take several seconds for
    the hot water to arrive even if the water-current starts flowing
    out of the water heater immediately? It's because the *change in
    water pressure* caused by turning the tap on moves through the
    pipe much more quickly than the actual *water* does.

    It's the same with electricity. Change in voltage moves quickly --- at
    the speed of light, or a bit slower if you include transmission-line
    effects. But the electrons (or ions, or whatever is transferring the
    charge) typically move much more slowly than that.

    (If you really want to, you can calculate the average electron
    velocity --- the drift velocity --- from the density of electrons
    in the material, the charge per electron, and the current (charge
    per second):
    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html )


    As for the original question: I usually think of current as the
    flow of positive charge, without worrying too much about what's
    carrying the charge. As it happens, the most common charge carriers
    are electrons, and since they have negative charge, they move in
    the opposite direction to the current. Kind of like if I want to
    give a company some money in return for goods: I could hand them
    some cash (positive charge), or they could send me a bill. The bill
    represents a debt, a "negative charge" of money, and it moves in
    the opposite direction to the "money flow".
     
  12. You said that potential difference is established almost at the speed
    of light across a conductor. We further know that potential difference
    is necessary for the flow of current.

    If we define electric current in a metallic conductor as a flow of
    electron then there must exist some phase difference between voltage
    and current.

    Current must be lagging as the drift velocity is very-very small. But
    we know that in a pure conductor voltage and current are in same
    phase.

    Any explanation please ?

    Animesh Maurya
     
  13. Ratch

    Ratch Guest

    Sure, drift velocity is small, but all the charge carriers from one end
    of the conductor to the other move at just about the same time. Therefore
    current exists at the speed of light difference between the ends of the
    conductor.

    By the way, current is charge flow. Therefore "current flow" is
    actually charge flow flow, which is a redundant phrase. If you talk about
    current, which already means charge flowing, say that it exists, not that it
    flows. If you want to use the word "flow", then say charge flows. Ratch
     
  14. Actually, this is not correct. Charge flow runs similar to mass flow,
    that is things continue to keep moving in a straight line unless acted
    upon by a force etc. If you start accelerating a charge, and then turn
    of the voltage it keeps moving. In fact, superconductors illustrate
    this.
    Yes.

    The current, accepted theory of EM is quantum electrodynamics, qed. This
    explains *all* EM effects by the exchange of photon momentum between
    charges. The net effect of this momentum exchange is dependant on the
    distance between charges. If an electron starts to move nearer to
    another one, the electrons will react to this effect at the speed of the
    photons, i.e. light. One can argue that charge flow, is the cumulative
    effect of photon motion. That is, it is a number that summarises the net
    backwards and forwards momentum flow of photons. That is, there is not
    such a thing as a real object actually flowing in a way mass would flow.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
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