Current Flow Direction

Discussion in 'The Projects Forum' started by Oxbo Rene, Aug 3, 2009.

  1. Oxbo Rene

    Thread Starter Active Member

    Feb 20, 2009
    200
    0
    Lets see;
    When I learned my electronics, we were taught that electrons migrate from atom to atom toward a point that was deficient of electrons (+ ions) = atoms wanting/requiring electrons to become stable (equal number of protons and electrons).
    And vice versa, wanting to depart from (- ions) atoms that have an over abundance of extra electrons, requiring to get rid of the extra electrons to achieve stability.
    Thus, electron flow is from negative (-), toward Positive (+).
    And, I can understand the opposite concept referring to "Hole" flow.
    However, I see more and more these days where folks demonstrating current flow show current flowing from (+) down to (-) (gnd). Did I miss something ?
    I was watching TV the other day and there was a demo of electro-plating a chrome vehicle wheel.
    They showed emmersing the wheel and the bar of (whatever the material is) into solution and had the wheel connected to the (-) and the bar connected to the (+), and, showed that the bar material would diffuse through the solution and attach to the wheel, thus chrome plating the wheel.
    I thought, that's not right, they'll coat the bar with the steel of the wheel, etc.
    Then in a couple of other instances I see where folks show current flow from positive to negative. That just throws me off, who is responsible for teaching this absolute opposite theory to the world these days??
    Maybe I'm mistaking "current path" with "current direction", but still, if they have an arrow on the end of the line that shows me "direction of flow, etc..
    Just thought I'd ask about this......
    Oxbo
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
  3. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    Movement of charge in conductors can be taught at numerous conceptual levels. Of course, for detailed analysis of real devices, one needs the correct physics. However, one of the most useful lessons in electronics is that either the "correct" or "incorrect" convention for current flow is adequate to solve circuit problems as long as the convention is consistently adhered to.

    I remember this useful tidbit from the only EE course I ever took -- the teacher solved the same network both ways and showed that the results for current flow were the same. It was a simple, illustrative example.
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    People use the conventional flow of charge (current) which is from positive to negative and they assume that the charge flowing is positive. This is how Benjamin Franklin determined the flow of charge in the old days without knowing the real direction of flow.

    The real direction of flow of charge in the wires is from negative to positive because the charge is negatively charged (electrons).

    You can use whatever you like if you use it correct.
     
  5. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287
  6. Oxbo Rene

    Thread Starter Active Member

    Feb 20, 2009
    200
    0
    Well,
    I sure don't know where I've been.
    This "conventional" method is to me just another "spin" to give the truth a dual and opposite meaning.
    Kinda like eggs are good for ya, then, a year later, they're bad for ya, then another year later, good for ya etc.
    How is one supposed to grasp reality when someone always comes up with a change of definition?
    I can deal with it, but, cheese ! !
    Relate to this ->
    I needed a new battery for my cell phone, went to Radio Shack, they say "all" phone batteries are $50.00 ! I look over on the shelf and see my exact phone for sale (not on sale)...... FOR $29.95 ! !
    So, I'm better off buying a new phone (with batt), rather than just buying a new battery !
    I just can't get my mind around that concept ! It's ridiculous ! ! !
    But, that's the new reality of today.
    I told the fellow, "ya know Bub, that's what's wrong with the world today, absolute insanity !
    I know somewhere, someone is making a buck on the concept, but I say it's just crazy stupid.
    However, current (electron flow/migration) is from negative to positive ! !
    PERIOD ! ! !
    Appreciate your enlightenment.
    Tx's for lettin me share .....
    Oxbo
     
  7. rspuzio

    Active Member

    Jan 19, 2009
    77
    0
    > I thought, that's not right, they'll coat the bar with the
    > steel of the wheel, etc.

    As it turns out, in the electrolyte, the direction of flow is
    really from + to -! The reason for this discrepancy
    is that, in that case, the charged objects which are moving
    are not negatively charged electrons but positively
    charged chrome ions so that is why the chrome goes on
    the + terminal and the wheel on the - terminal. To coat
    the bar with iron, you would have to reverse the connections
    so as to get positively charged iron ions to move from the
    wheel to the bar.

    The point is that electric current consists of movement of
    charged objects, which can either be positively or negatively
    charged. To compute the current, we multiply the charge
    by the velocity. This means that, if the moving object has a
    positive charge (like a chromium ion), the current is in the
    same direction as the velocity whilst if it is negatively
    charged (like an electron) the direction of current is opposite
    of the direction of motion. Thus, the direction of current flow
    may or may not be the same as the direction in which the
    charged objects are moving.

    To understand this business of current conservation better,
    let's look at your example in a bit more detail. At the surface
    of the bar, a neutral chromium atom loses three outer electrons
    to become an ion. The three electrons move away from the
    bar towards the power source whilst the ion moves in the
    opposite direction towards the wheel. At the wheel, the opposite
    reaction happens --- the ion combines with three electrons
    coming from the other side of the power supply to make a
    neutral chromium atom which sits on the wheel.

    Thus, in this example, the actual flow is in one direction in the
    wire and the opposite direction in the plating vat. Furthermore,
    there are three times as many electrons flowing as chromium
    ions. The definition of current is a convenient bookkeeping device
    which lets us keep track of what is going on without having to
    worry about the nature of the charge carriers. If, say there
    are 3 billion electrons passing through the wire each second
    in one direction and a billion ions passing through the solution
    in the opposite direction each second, we have a current of
    3 billion elementary charge units per second passing through
    the whole circuit and the direction of the current flow is the
    same in the wire and the solution, even though the flow of
    charged objects happens to be opposite.

    While on this subject of current, there is one more thing to
    mention, but it is an advanced topic so you might want to
    skip this paragraph for the time being. Even if there
    are no charges around, one can still have a current because
    a time-varying electric field also counts as a current (known
    as "displacement current" --- this was Maxwell's big discovery).
    To understand this, consider passing AC through a capacitor.
    If we understand current simply as charges in motion, then
    we would say that the current stops when the electrons pile
    up on one plate of the capacitor, is zero between the plates
    and starts again when electrons come off the other plate.
    However, there is an electric field between the plates and, as
    it turns out, the rate of change of this field is proportional to
    the current flowing through the leads of the capacitor. Thus,
    if as Maxwell suggested, we consider the rate of change of
    the electric field (multiplied by a suitable constant) as a type
    of current, then we have no break in the circuit --- there is the
    same amount of displacement current passing between the
    plates of the capacitor in the same direction as the
    conduction current passing through the leads.

    The bottom line is that current is a quantity which we define
    in such a way as to make the current continuous throughout
    a closed circuit. There is no swindle here as long as we are
    consistent --- there is leeway in exactly how we can make such
    a definition, sort of like how British and Americans drive on
    opposite sides of the road. Of course, once we make a choice,
    we have to stick with it consistently but either choice is equally valid.

    > How is one supposed to grasp reality when someone always
    > comes up with a change of definition?

    You need to check which definition is being used before
    interpreting statements. In particular, if someone tells you
    that they measured, say, a current of 2.5 A, then you need
    to know whether they are using the old Franklin convention
    or the opposite convention before you can completely
    understand how that statement describes the physical reality
    of what is going on in their circuit.
     
  8. Oxbo Rene

    Thread Starter Active Member

    Feb 20, 2009
    200
    0
    Appreciate that.........
     
  9. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    Electro chem. is one of the areas where thinking that electron flow and current flow are the same thing shows up its shortcommings. Some people will go nuts if you even say current flow because current is flow ( see rio grande river and pizza pie ). This forum mostly goes with the book and has current go from - to +. Physics, chem..... have current go from + to -. It makes no difference if you are consistent right down to the equations you use. If you study physics and use - to + you may have some trouble as the right hand rule becomse the left hand rule......
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    One of the reasons why we confine our Ebook to electrons in wire comes from the site's introductory statement -
    Please note that ionic acticity in a battery or electroplating cell or electrolysis cell do not change the curent in the wires supplying/using that activity.
     
    Last edited: Aug 6, 2009
  11. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    nice summary by the way.

    And where is Ratch?
     
  12. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,170
    395
    Enough has been said, but I'm tired & out of my head. I have a heck of a time shoving electrons from a cold plate to a hot cathode, so gave up and let them flow from cathode to plate. Does'nt bother me,if terms are just defined up front.
     
  13. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    One question - off topic. Is that your PT-26 Peashooter?
     
  14. Oxbo Rene

    Thread Starter Active Member

    Feb 20, 2009
    200
    0
    Huh ?...........
     
  15. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Sorry - the PT-26 question was for Bernard.
     
  16. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,170
    395
    Still have it hanging in my "office". 'Only know of one still flying at Planes of Fame, Chino CA. One of my alltime favorites.
     
  17. jimecole

    New Member

    Sep 15, 2009
    1
    0
    i learned electron flow theory in chemestry class. i learned conventional current flow theory in electronics class. then.... i learned that ya use whichever theory is in use in your field at the "present".... different fields of electrical / electronics / chemestry / physics have all used one or the other or both at the same time over the last 50 years that i have been in the field.
     
Loading...