Conventional Versus Electron Flow
"The nice thing about standards is that there are so many of them
to choose from."
Andres S. Tannenbaum, computer science professor
When Benjamin Franklin made his conjecture regarding the direction of charge
flow (from the smooth wax to the rough wool), he set a precedent for electrical
notation that exists to this day, despite the fact that we know electrons are
the constituent units of charge, and that they are displaced from the wool to
the wax -- not from the wax to the wool -- when those two substances are rubbed
together. This is why electrons are said to have a negative charge:
because Franklin assumed electric charge moved in the opposite direction that it
actually does, and so objects he called "negative" (representing a
deficiency of charge) actually have a surplus of electrons.
By the time the true direction of electron flow was discovered, the
nomenclature of "positive" and "negative" had already been
so well established in the scientific community that no effort was made to
change it, although calling electrons "positive" would make more sense
in referring to "excess" charge. You see, the terms
"positive" and "negative" are human inventions, and as such
have no absolute meaning beyond our own conventions of language and scientific
description. Franklin could have just as easily referred to a surplus of charge
as "black" and a deficiency as "white," in which case
scientists would speak of electrons having a "white" charge (assuming
the same incorrect conjecture of charge position between wax and wool).
However, because we tend to associate the word "positive" with
"surplus" and "negative" with "deficiency," the
standard label for electron charge does seem backward. Because of this, many
engineers decided to retain the old concept of electricity with
"positive" referring to a surplus of charge, and label charge flow
(current) accordingly. This became known as conventional flow notation:
Others chose to designate charge flow according to the actual motion of
electrons in a circuit. This form of symbology became known as electron flow
In conventional flow notation, we show the motion of charge according to the
(technically incorrect) labels of + and -. This way the labels make sense, but
the direction of charge flow is incorrect. In electron flow notation, we follow
the actual motion of electrons in the circuit, but the + and - labels seem
backward. Does it matter, really, how we designate charge flow in a circuit? Not
really, so long as we're consistent in the use of our symbols. You may follow an
imagined direction of current (conventional flow) or the actual (electron flow)
with equal success insofar as circuit analysis is concerned. Concepts of
voltage, current, resistance, continuity, and even mathematical treatments such
as Ohm's Law (chapter 2) and Kirchhoff's Laws (chapter 6) remain just as valid
with either style of notation.
You will find conventional flow notation followed by most electrical
engineers, and illustrated in most engineering textbooks. Electron flow is most
often seen in introductory textbooks (this one included) and in the writings of
professional scientists, especially solid-state physicists who are concerned
with the actual motion of electrons in substances. These preferences are
cultural, in the sense that certain groups of people have found it advantageous
to envision electric current motion in certain ways. Being that most analyses of
electric circuits do not depend on a technically accurate depiction of charge
flow, the choice between conventional flow notation and electron flow notation
is arbitrary . . . almost.
Many electrical devices tolerate real currents of either direction with no
difference in operation. Incandescent lamps (the type utilizing a thin metal
filament that glows white-hot with sufficient current), for example, produce
light with equal efficiency regardless of current direction. They even function
well on alternating current (AC), where the direction changes rapidly over time.
Conductors and switches operate irrespective of current direction, as well. The
technical term for this irrelevance of charge flow is nonpolarization. We
could say then, that incandescent lamps, switches, and wires are nonpolarized
components. Conversely, any device that functions differently on currents of
different direction would be called a polarized device.
There are many such polarized devices used in electric circuits. Most of them
are made of so-called semiconductor substances, and as such aren't
examined in detail until the third volume of this book series. Like switches,
lamps, and batteries, each of these devices is represented in a schematic
diagram by a unique symbol. As one might guess, polarized device symbols
typically contain an arrow within them, somewhere, to designate a preferred or
exclusive direction of current. This is where the competing notations of
conventional and electron flow really matter. Because engineers from long ago
have settled on conventional flow as their "culture's" standard
notation, and because engineers are the same people who invent electrical
devices and the symbols representing them, the arrows used in these devices'
symbols all point in the direction of conventional flow, not electron flow.
That is to say, all of these devices' symbols have arrow marks that point against
the actual flow of electrons through them.
Perhaps the best example of a polarized device is the diode. A diode
is a one-way "valve" for electric current, analogous to a check
valve for those familiar with plumbing and hydraulic systems. Ideally, a
diode provides unimpeded flow for current in one direction (little or no
resistance), but prevents flow in the other direction (infinite resistance). Its
schematic symbol looks like this:
Placed within a battery/lamp circuit, its operation is as such:
When the diode is facing in the proper direction to permit current, the lamp
glows. Otherwise, the diode blocks all electron flow just like a break in the
circuit, and the lamp will not glow.
If we label the circuit current using conventional flow notation, the arrow
symbol of the diode makes perfect sense: the triangular arrowhead points in the
direction of charge flow, from positive to negative:
On the other hand, if we use electron flow notation to show the true
direction of electron travel around the circuit, the diode's arrow symbology
For this reason alone, many people choose to make conventional flow their
notation of choice when drawing the direction of charge motion in a circuit. If
for no other reason, the symbols associated with semiconductor components like
diodes make more sense this way. However, others choose to show the true
direction of electron travel so as to avoid having to tell themselves,
"just remember the electrons are actually moving the other way"
whenever the true direction of electron motion becomes an issue.
In this series of textbooks, I have committed to using electron flow
notation. Ironically, this was not my first choice. I found it much easier when
I was first learning electronics to use conventional flow notation, primarily
because of the directions of semiconductor device symbol arrows. Later, when I
began my first formal training in electronics, my instructor insisted on using
electron flow notation in his lectures. In fact, he asked that we take our
textbooks (which were illustrated using conventional flow notation) and use our
pens to change the directions of all the current arrows so as to point the
"correct" way! His preference was not arbitrary, though. In his
20-year career as a U.S. Navy electronics technician, he worked on a lot of
vacuum-tube equipment. Before the advent of semiconductor components like
transistors, devices known as vacuum tubes or electron tubes were
used to amplify small electrical signals. These devices work on the phenomenon
of electrons hurtling through a vacuum, their rate of flow controlled by
voltages applied between metal plates and grids placed within their path, and
are best understood when visualized using electron flow notation.
When I graduated from that training program, I went back to my old habit of
conventional flow notation, primarily for the sake of minimizing confusion with
component symbols, since vacuum tubes are all but obsolete except in special
applications. Collecting notes for the writing of this book, I had full
intention of illustrating it using conventional flow.
Years later, when I became a teacher of electronics, the curriculum for the
program I was going to teach had already been established around the notation of
electron flow. Oddly enough, this was due in part to the legacy of my first
electronics instructor (the 20-year Navy veteran), but that's another story
entirely! Not wanting to confuse students by teaching "differently"
from the other instructors, I had to overcome my habit and get used to
visualizing electron flow instead of conventional. Because I wanted my book to
be a useful resource for my students, I begrudgingly changed plans and
illustrated it with all the arrows pointing the "correct" way. Oh
well, sometimes you just can't win!
On a positive note (no pun intended), I have subsequently discovered that
some students prefer electron flow notation when first learning about the
behavior of semiconductive substances. Also, the habit of visualizing electrons
flowing against the arrows of polarized device symbols isn't that
difficult to learn, and in the end I've found that I can follow the operation of
a circuit equally well using either mode of notation. Still, I sometimes wonder
if it would all be much easier if we went back to the source of the confusion --
Ben Franklin's errant conjecture -- and fixed the problem there, calling
electrons "positive" and protons "negative."
Lessons In Electric Circuits copyright (C) 2000-2011 Tony R. Kuphaldt,
under the terms and conditions of the Design