In my class lecture, my professor mentioned that "the ac supply
frequency is 50Hz.So the time period is 20ms.The current which is
sinusoidal in nature behave as, for 0 to 10ms the current flow from A
to B (assume A and B are any two points at different potentials) so
the electron flow is from B to A. Similarly in the next half cycle
from 11ms to 20ms current flows from B to A so electron flows from A
Looking from the point of view that direction of flow of electrons is
opposite to that of current then the above explanation is right.
But does this happen really?
This question is almost as old as electrical theory itself.
'Current flow' is a term used in various ways. In early work, it was
assumed that the 'current' flowed from positive to negative. I think this
was even before the theory of the 'electron' even existed.
When the electron was discovered and found to have a negative charge, it was
obvious that since like charges repel, electrons must flow from negative to
positive. Most folks stick with 'electron current flow' for most things.
Metals have a loosely bound 'sea' of electrons surrounding the atoms, so it
is easy to visualize that these electrons are pushed along from negative to
positive in an electric circuit.
But when you start studying semi-conductors, you find that the 'hole' where
an electron has been displaced from can be thought of as 'flowing' in the
opposite direction. As electrons bump from one location to the next,
generally in the direction from negative to positive, you will notice that
the 'vacancy' or 'hole' left behind appears to be jumping from positive to
Then Maxwell came along and said it isn't the movement of individual
sub-atomic particles (electron or hole), but the electromagnetic wave
traveling along the conductor that explains things the best.
You can work with old-fashioned 'current flow', the more modern 'electron
flow', the semiconductor 'charge carrier flow', or Maxwell's e-m waves.
Each one can give you good results within their area of usefullness. But if
you start mixing them up, you can get pretty confused and make mistakes.
As to what 'really happens'.... who knows for sure. These are theories
that explain the experimental evidence of their day.
Forget all this electron business. Circuit theory ia mostly an exercise
in linear algebra. Ity just turns out that the reuyltant mathematical
model describes the behavior of electrical circuits very well indeed.
Stick to the mathematical model as well as you can, Most of the time,
microscopic understanding of current flow is unnecessary. Moreover, as
the previous post indicates, it can end up causing confusion.
Electric circuit theory developed using electrochemical (Voltaic) cells.
Conduction inside the cell was by ions, not electrons. In the external
circuit conduction in the wires took place, but the microscopic going
ons inside this wire was of little interest. The key concept was that of
charge transfer and potential differences. The unit of charge was the
Faraday associated with the plating out of an equivalent weight of
I would agree that electrons tend to migrate one direction during one
half cycle, then they will tend to migrate back the other way in the
next half cycle. Electrons would not 'drift' very far in each half
cycle of ac in normal applications. Probably micro- or nanometers.
Actually the "conventional current" flow is what is used in engineering and
physics. Call it,if you wish the direction of flow of positive charge. Many
electronic institutes of the 1930's- 50's did use electron current simply
because the electron was the active carrier of charge in vacuum tubes. Some
still do (why?).
I don't think any university level course used "electron current" at any
time (I can only personally attest to about 60 years but older texts push
this back further) because it was simply a pain in the ass to change the
sign in some expressions on the basis that the electron was found to be
"negative" . Rather than rewrite expressions , it is much easier to attach a
negative sign to the electron in cases where it matters.
In circuit theory and measurement it doesn't matter what the electrons are
doing- a negative charge going left is the same as a positive charge going
east. The real problem with electron flow concepts applied to circuit
analysis, is that chasing electrons around as they drift slowly or, for AC
merely wobble around a bit without going anywhere is useless as it simply
leads you into confusion. Don't sweat it.
Salmon Egg has it right. In circuit theory, you are looking at linear
algebra which work (and are a limited approximation to field theory). Stick
with the conventional (as Daestrom indicates- the old approach-(as given by
your prof) because, contrary to what he says, is the basic approach that is
In the situation that your prof is indicating, note that (for resistive
cases to avoid phase complications) when A is positive compared to B
instantaneous current flows from A to B and when A is negative compared to B
the instantaneous current is reversed BUT, in both parts of the cycle the
energy transfer is in the same direction. That is the key point and has
nothing to do with the sign of the charge carrier. It appears that right
now, you are just getting into the beginning of AC circuit theory- and
haven't yet met concepts such as RMS and phase.
Don Kelly firstname.lastname@example.org
remove the X to answer
ram27 email@example.com wrote previously in alt.engineering.electrical:
Yes, absolutely, electrons flow in a direction opposite to "current flow".
When electrical experiments were conducted on chemical baths (batteries),
current flowed in the wire from one terminal to the other. Some guy had to
choose which battery terminal would get a "+" and which would get a "-". No
one had no idea which way the current actually flowed thru the wire, so he
flipped a coin, and the "+" got stuck on one of the terminals. Electrical
current was defined as flowing from were it "contained more" (+) to were it
was "less" (-).
That's the convention. We are just stuck with it.
In chemical solutions "ions" carry the charge, as the "ions" could be
positive or negative, ion flow could be either/both ways, from positive
terminal to negative terminal or the other way around.
Later it was discovered that electrons were the real carriers of electrical
charge in wires. But the convention didn't change, so, electrons flow in a
opposite direction as the "electrical flow".
Blame it on Benjamin Franklin. Under the right circumstances of birth
and intellectual nurturing, Franklin could have been as great a
scientist as Newton. As it it turned out, he was merely an outstanding
scientist good at many other things as well. He came up with the single
fluid theory of electricity and arbitrarily defined positive and
negative. At the time there were no batteries marked with + and - signs.
Electrical charge was static charge. Electrons did not "exist" until the
20th century. Sure, Edison carried out some experiments that hinted at
electrons earlier. So what?
So it turns out that in an external wire made of copper,free electrons
in the metal travel slowly from negative pole to the positive pole. When
that happens, electric current travels from the positive pole to the
negative pole. Until the 20th century, no one would know that little
negative thingies were moving in the wire. If it turned out that little
positive thingies were moving from the positive pole to the negative
pole, the effect on circuit theory would be virtually nil.
It is only with the arrival of vacuum tube like devices, that the actual
sign of moving elementary charge became significant. That however,
effects how the device is described, and not anything fundamental about
I have no problem thinking that current flows into the plate (anode) of
a tube even though the plate is collecting particles rather than
emitting them. If you take a look at an electrical circuit consisting of
devices in series, the current is the same everywhere although the
carriers for individual devices may differ from one another. The sooner
you can shed the hang-up about carrier sign, the sooner you can devote
yourself to understanding circuits. The sign is a red herring.
Save your hang-up over sign for when you think about devices.
Indeed, they don't move far. A bit OT, as it's about RF... It's been
a while since I looked at the physics of electron flow but I recall
that at "normal" 50 MHz RF current densities for signals in copper
wire the phenomenon is well described by Samuel Coleridge Taylor in
the "Rhyme of the Ancient Mariner", viz: "Backwards and forwards half
a length, with a short uneasy motion". A well respected electronics
author in the 1950's first invoked this notion - it was "Cathode Ray"
in "Second Thoughts on Radio Theory" (1956), p14.
BTW, who was "Cathode Ray"? I know he was English.
No he isn't.
"Cathode Ray" was the pseudonym used by the author of a column in the the
magazine "Wireless World" (formerly known as "The Marconigraph") now a
small circulation Magazine called "Electronics World"