Electric Current


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
to B.
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?
Regards
Ram
Reply to
ram27
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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 negative.
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.
daestrom
Reply to
daestrom
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 element.
Bill
Reply to
Salmon Egg
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.
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j
Reply to
operator jay
----------- 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 still used.).
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 snipped-for-privacy@shawcross.ca remove the X to answer
Reply to
Don Kelly
ram27 snipped-for-privacy@gmail.com wrote previously in alt.engineering.electrical:
Short answer: Yes, absolutely, electrons flow in a direction opposite to "current flow".
Long answer: 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".
Reply to
Antonio Perez
But in detail, it doesn't matter; the formulae all work, regardless as long as you don't mix conventions.
Reply to
Twayne
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 circuit theory.
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.
Bill
Reply to
Salmon Egg
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. Cheers, Roger
Reply to
Engineer
In article , Eng> BTW, who was "Cathode Ray"? I know he was English.
If you are talking abouty who discovered cathode rays, it was J J Thomson.
Bill
Reply to
Salmon Egg
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"
Reply to
Stuart
M. G. Scroggie. You can look him up on Wikipedia.
Reply to
Tony
Thought it was but wasn't sure.
Reply to
Stuart

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