Direction of current flow

Discussion in 'General Electronics Chat' started by Yakima, Jan 14, 2015.

  1. Yakima

    Thread Starter Member

    Jan 23, 2012
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    When I went to the university to get my BSEE we were taught current flows from positive to negative and we did all of our calculations with this convention. The textbooks throughout our electrical engineering courses used this convention. And yet we were taught the truth: that current actually flows from negative to positive. Nevertheless, science persists in the conventional positive to negative direction. The calculations are based on this presupposition and they work no matter which way the current actually flows. Okay, so far so good. But then I run into electrical technician sources and they insist on thinking in terms of negative to positive such that the little arrows on the diode and transistor symbols are backwards. Why is this counter intuitive system of thinking gaining ground? I, for one, will never draw my KVL loops backwards, for that would involve the complication of most of the numbers being negative. I like to work with positive numbers. Any thoughts on this?
     
  2. profbuxton

    Member

    Feb 21, 2014
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    Not this current flow thing again! I too worked on positive to negative for a long time and still do. You just need to be adaptable for the particular situation and think in the correct flow for the application as warranted!
     
  3. WBahn

    Moderator

    Mar 31, 2012
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    There's been LOTS of discussion on this.

    In either convention, current (i.e., the flow of electrical charge) flows from positive to negative. People that use "electron current" try to claim that current flows from negative to positive and ignore the fact that it is the charge carries (i.e., the electrons) that flow from negative to positive and since electrons are negatively charged (which they don't dispute or try to change), there is a negative current flowing from negative to positive. But because they want to insist that it is a positive current flowing from negative to positive they run into all kinds of sign errors if they try to describe all but the simplest circuits in a mathematically rigorous way. But many folks in this camp seldom do this, in part because this error has made it so that they can't, and so they have learned to work at a level and in a way that they can jiggle themselves around all of the tripping points, like how having a positive current into a particular terminal of a capacitor can somehow result in there being a negative charge on that side when, by definition, a positive current means that a positive charge is flowing in that direction.

    Many people in this camp also rely on sheets of formulas instead of applying basic, consistent concepts. You see this in the ton of formulas that abound for working with combinations of inductors and capacitors where the formulas require the user to distinguish between inductive reactance and capacitive reactance. For all but the simplest things, this ends up in a mish mash of special rules when if they understood the power and elegance of complex impedance their lives would be much simpler.
     
  4. bzznxlad

    New Member

    Dec 29, 2014
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    It's like a question why electrons have negative charges and protons have positive charges. Once in history scientists agreed to treat electrons as particles with negative charge and protons which have opposite charges as positive.

    Electrical current from definition is -> i=dq/dt. In brief, electrical current is a motion of electrical charges in time.

    In perfect conveyor situation is quite simple because electron current flows in with in direction from "lower" potential to "higher" potential, as definition says, negative electrical current flows from "-" to "+" and thus positive electrical current flows from "higher" potential to "lower" potential. However, it was about perfect conveyor.

    Electrical current could be classified to four options:
    - electron current
    - hole current
    - ion current
    - moving charged particle is also considered as current
     
  5. crutschow

    Expert

    Mar 14, 2008
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    Originally the charge polarity of the electricity particles were unknown and the current direction was arbitrarily assigned to be from positive to negative, implying the charges were positive. Further measurements determined that the charges were actually negative (for example, the charges being boiled off a hot filament being attracted by a positively charged plate).

    So the current flow convention is still positive to negative but the electron flow is negative to positive. I'm used to current flow and prefer that (I like the semiconductor symbol arrows pointing in the direction of the current flow), but which you use is arbitrary and either will give you the correct answer if you are consistent in your nomenclature (and remember that some circuit rules, such as the right-hand rule for magnetic field direction, become the left-had rule).
     
  6. WBahn

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    Mar 31, 2012
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    I agree that that the key is consistency. The problem is that "electron flow" users are almost always not consistent. They want to say that a current of one ampere flows from negative to positive through a resistor when a twelve ohm resistor is placed across a twelve volt battery. But since one ampere is, by definition, one coulomb of charge per second, they are claiming that one coulomb of charge is flowing from negative to positive each second. That's not the case. there is minus one coulomb of charge flowing from negative to positive per second which means that the current flowing from negative to positive is minus one ampere. If they were being consistent, this is what they would conclude. But they don't and a consequence of their inconsistency is that they have to layer on more and more inconsistencies in order to patch things up, like having the plate of the capacitor that positive current is flowing onto somehow magically end up with a negative charge.
     
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  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    People who believe in the superiority of the “electron flow convention” may have other personality disorders so you should be on guard.

    Look, *you* are the engineer here, they are the technicians. You have a greater in depth training over a technician and I would say you have a duty to teach and train them in those things they may not know or have been taught incorrectly.

    Over my long career I have only occasionally seen this pop up, though I agree lately the problem comes up more and more frequently. I would blame that on the interwebs where anyone can post anything and claim authority on the subject.

    Other conventions do exist. I have worked with customers who insisted current flowing into a device is positive, and current flowing out is negative. This directly leads to things like the output current for a positive voltage regulator being a negative current. This lead to thousands of dollars of parts stuck in test because out test program measures the short circuit current at 5 amps maximum, but they insisted it was minus 5 amps maximum. Do note that -10 amps is a smaller number than -5 amps so only devices that did not work could pass the spec. Interestingly, the engineer for that customer stated “that may be true in mathematics, but not in electronics.”

    My fellow engineers here when I currently work all use conventional flow without question. Of course, as always, the correct way to do it is how your boss does it. I can work either way, but my brain sees conventional as correct, so working with other conventions is a good way for me to make mistakes.
     
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  8. Externet

    AAC Fanatic!

    Nov 29, 2005
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    To make life more miserable,
    Yes, electrons flow from negative to positive outside a battery.
    Yes, electrons flow from positive to negative inside a battery.
     
  9. ramancini8

    Member

    Jul 18, 2012
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    The military electronic schools used to teach negative current flow and most still do; many of us learned that way and are reluctant to change our habits. But, the positive current flow convention is used in higher education, most of us converted to positive current flow. Makes no difference unless you are afraid of being mistaken for a tech!
     
  10. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Just ask the techies what direction alternating current flows in.

    roflmao.gif banghead.gif
     
    Last edited: Jan 15, 2015
  11. BR-549

    Well-Known Member

    Sep 22, 2013
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    Which one?

    Even though the physical entity that is moving in a conductor is negatively charged.........Charge flow in a conductor is bi-directional and balanced.

    Of course WBahn has nailed it.

    We usually draw and talk about our circuits with positive voltages and currents.

    But when a mechanic gets in there and has to work with the innards...... he has to be mindful of the moving entity.

    It can be confusing at first.
     
  12. alfacliff

    Well-Known Member

    Dec 13, 2013
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    I havnt worried about the direction of flow for decades, as long as the current flow indicator on diodes, tubes, transistors isnt changed, what does it matter? are you going to put an electron version oof a water wheel in the wire to get mechanical energy? when you know whether the polarity is correct, the flow direction isnt important.
     
  13. bzznxlad

    New Member

    Dec 29, 2014
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    It is not fully explained because inside battery electrical current is in from of ions and electrons.
     
  14. WBahn

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    Mar 31, 2012
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    Not miserable at all. That's like noting that water goes from high to low down the water slide but from low to high in the pipe feeding it. The difference in both cases being that another form of energy is being consumed in order to drive something in the direction opposite from the one it would otherwise take.
     
  15. #12

    Expert

    Nov 30, 2010
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    Convention: an agreement among people.
    A completely different argument: Whether electrons move toward the front of the CRT or protons jump off the front of the CRT.

    The point: A convention doesn't have to agree with physical reality.
    Work with the convention or work the other way, it's just a convention.
     
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  16. WBahn

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    Mar 31, 2012
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    The point I am trying to make isn't whether you should use one convention or the other, but rather that people using one particular convention are inconsistent in how they use it.
     
  17. studiot

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    Nov 9, 2007
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    The difficulty arises becuse there is not one sign convention but two.

    It is the interaction of the two sign conventions that causes the difficulty.

    Convention 1 is the positive direction chosen for current flow.

    Convention 2 is the sign of charge, which is equivalent to the direction of the voltage potential arrow.
     
  18. ErnieM

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    Apr 24, 2011
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    Once again the CRT beam is given as a counter example.

    I have yet to see the argument why this is in disagreement with conventional current flow.
     
  19. WBahn

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    Mar 31, 2012
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    It's not. You have a current of -|Magnitude| coulombs/second directed from the cathode to the screen for a current of |Magnitude| amperes from the screen to the cathode.

    Notice that I didn't have to say which convention I am using. The only thing that determines it is whether I choose to abide by the definition that the charge on an electron is negative.
     
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  20. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I agree with ernie's question.

    For a cathode ray, I would not say that a current of any description flows from the screen to the cathode, any more than I would say a negative aircraft flying from london to New York is the same as a positive aircraft flying from New York to London.
     
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