Electron flow direction: Convention VS what's exactly happening

  • #26
sophiecentaur
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Where is there a "contradictory flow"?
Positive charges (never mind the carriers) appear at the positive terminal, having been given Potential Energy by the chemical action in the cell, and travel around the circuit, transferring their Potential Energy and ending up at the negative terminal. Thence, they are moved to the positive terminal, acquiring more Potential Energy. This energy comes from the chemical reaction in the cell.

In the metal wires, the carriers are electrons, which are attracted to the Positive Ions at the + terminal of the cell and the neutral atoms build up there. Negative Ions are forced towards the - terminal where they release their electrons into the wires. I don't find that confusion. Have I missed something?
 
  • #27
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I believe you said everything right, if you are talking about chemical reaction that mainly produces electrons in the process(which in all batteries does happen). Electrons combine at the positive anode and make that anode thicker(that is why the symbol for source has positive bar bigger).

http://ocw.mit.edu/courses/physics/.../video-lectures/lecture-10-batteries-and-emf/

this video best describes this


Like Lewin said: Why would negative Ions go the the negative side of the battery? By doing so, they engage in a chemical reaction that yields more energy than it costs.
 
  • #28
sophiecentaur
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Soooooo
Where is the "contradictory flow"? It all ties up very nicely - as one would expect.
 
  • #29
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The only case I know of where DVM could tell the difference between positive and negative charge carriers is the Hall effect. The cross-current polarity P semiconductor is the opposite of N material.

Studiot, please send your 64K dollars to me, care of PF forums. Cash preferable.
 
  • #30
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Studiot, please send your 64K dollars to me, care of PF forums. Cash preferable.
I'm sorry what does the Hall effect have to do with the internal working of a battery?

:confused:
 
  • #31
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I'm sorry what does the Hall effect have to do with the internal working of a battery?

:confused:
I was wondering that myself XD
 
  • #32
sophiecentaur
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It doesn't but it is relevant in that it one of the few instances in which the sign and direction of flow of the charge carriers is actually relevant. Other instances would be some forms of Cathode Ray tube and a Mass Spectrometer, neither of which are part of a conventional circuit.
 
  • #33
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I'm sorry what does the Hall effect have to do with the internal working of a battery?

:confused:
Oh, I see. Nothing. So, we'll split the difference; you can have half. ;P

On the other hand, I increasingly find the notion of charge as contained in itsy-bitsies and specks as primitive;ind likewise untenable, but this is a minority opinion.
 
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  • #34
sophiecentaur
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Charge is charge. How it's carried is another matter.
If people really want to talk in terms of particles that's their problem. It just makes life harder for them and doesn't make any difference to 'understanding'. I wonder what they think when they watch TV. Do they obsess about the electrons hitting the front of the screen or do they just sit back and watch the TV programme?
 
  • #35
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Where is the "contradictory flow"?
I am amazed that SC has not encountered this before.

I have attached a small circuit with two batteries to emphasise the point.

The whole basis of our circuit analysis is that we claim the curent flows without break in one direction only - clockwise or anticlockwise - and has to complete the loop.

So starting at A with conventional current flowing clockwise from A (+) towards B (-) through R, everything is as predicted.

But what happens from B to C?
If we maintain the clockwise direction we are stating that current now flows from negative to positive.

Continuing on clockwise we have clockwise current from C (+) to D (-) which is as predicted.

But again from D to A to complete the loop we have to posit our clockwise current as flowing from negative to positive.
 

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  • #36
sophiecentaur
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If there is an escalator and a slide between the two floors of a building, people can go up the escalator (gaining Gravitational Potential Energy) then walk over to the slide and slide down (losing that PE and eventually losing the KE they gained). There is a continual, non-contradictory, flow of people. No problem, so far?
Now do the same with charges. The battery gives them Electrical Potential Energy and they transfer the energy they gained in the battery as they pass through the resistive load. They are going round in the same direction all the time. Now, OF COURSE, they are going from - up to + inside the battery. But that is because they are being given Potential Energy. Where is there anything paradoxical about that - unless one decides (with no good reason) that 'Charges always go from positive to negative? The Charges are Not Energy. There is nothing in any theory that says that charges 'always' go from positive to negative so there is nothing to disagree with. Theory says that charges acquire potential as they move towards the positive and lose it as they move away.
I can't think of any other ways of saying it. It's so bleedin' obvious that there is no paradox / contradiction / confusion as long as you follow what the theory actually says and not what some primary School teacher told you. That, btw, is a good principle to work by in all of Science.
 
  • #37
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I'm sorry but it is replies like this that cause so much confusion in students learning circuit theory and the reason why the OP was

The convention is that the electrons flow from + to -
But in reality they flow from - to +


I don't get it. Why don't we just mark it like it happens in reality? Why decided to do it otherwise? Isn't it just confusing, esp. to new students
I’m quite sure FF (who has been really quick to catch on in other threads) is simply quoting what she was told.

Now you are telling her that

Now, OF COURSE, they are going from - up to + inside the battery.
No wonder she is confused.

I observe huge numbers of students confused over this.
 
  • #38
sophiecentaur
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The problem is when you say "there is a convention. . . .". You are missing out the second part of the correct version. The complete statement should be: "charges flow from positive to negative where energy is being transfered from the charges into a load". When energy is being transferred to the charges they are moving from negative to positive. (Which is what the definition of Voltage is all about!)
 
  • #39
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The problem is when you say
I didn't say that. I understood circuit analysis long ago.

I reproduced (and said so) the Original Post (OP).

I didn't originally offer advice to FF because I knew (sadly ) she would be offered a hotchpotch of conflicting advice - which she was.

I think FF posted the message that is being taught to students (wrongly IHMO) around the world, because it is confusing.

I agree that it is confusing.

Charge, current or current density are the fundamental quantities in electricity, not potential. It should be possible to set this out without recourse to potential, which you had to do to 'explain' the contradiction.

Indeed there is another thread currently raging where your method would fail. Is it therefore fair on students to introduce it?
 
  • #40
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I think you can't take same circumstances for the inside the battery. Battery is giving out electricity. Current is going against its path because of chemical reactions.
 
  • #41
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The problem is when you say "there is a convention. . . .". You are missing out the second part of the correct version. The complete statement should be: "charges flow from positive to negative where energy is being transfered from the charges into a load". When energy is being transferred to the charges they are moving from negative to positive.
I think you can't take same circumstances for the inside the battery

So you want to tell rookies that current sometimes flows from positive to negative and sometimes from negative to positive?
Presumably there are also a set of rules to tell which way and when?

I cannot see the gain in this approach, making what is probably the simplest subject in Physics so complicated, especially when it is so unnecessary.

Listen to Maxwell himself who clearly understood what was going on when he discussed the laws then proposed by Kirchoff.

'avoids consideration of potential'
Kirchoff originally proposed (here is an English translation);

The conditions of a linear system

1) At any point of the system the sum of all currents which flow towards that point is zero.

2) In any complete circuit formed by the conductors the sum of the electromotive forces taken around the circuit is equal to the sum of the products of the currents in each conductor multiplied by the resistance of that conductor.


I agree with Antiphon that we should clearly distinguish between engineering circuit analysis which has been developed for the most efficient way of calculating important quantities in electric circuits and the Physics of what is really happening. Circuit analysis from first principles really is the most difficult and confusing way to go about it .
Remember also that this is an engineering thread.

That is why I prefer the original version of KVL, in that it allows us to completely sidestep the issue of what goes on inside a transformer, battery or indeed any source of EMF.

Incidentally I agree that there are many good reasons for retaining the 'conventional' direction of current. However I don't think it fair to load all that onto students who are just starting Ohm's law.
 
  • #42
sophiecentaur
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I want to tell "rookies" that current flows ROUND a circuit. (How could you have mis-read what I wrote and how could you have ignored the fact that I actually stated a simple rule for the direction?) This cannot be reconciled with the notion of 'from A to B'. It is the fact that "rookies" have been told that current always flows from + to - (à la primary School teacher) has never done them any favours.
I find it hard to agree that Potential has nothing to do with the question. If it weren't for the fact that 'Electricity' involves the transfer of Energy then we would hardly have been interested in the stuff. Charges, just sitting there are of little interest - even at the atomic and sub-atomic level. It's only when some change is involved that it gets interesting. Energy flow is an important concept in circuits and also in many other mechanical systems. Even in a simple system of a handle operating a machine, the whole thing is reversible unless you take into account the energy flow and friction. Energy considerations in a circuit will tell you which way things are happening.

I have to assume that anyone involved with Electricity must just come to terms with the conventional current direction and the electron flow direction. Without appreciation of that, there is no hope for them - and there wouldn't be, even if we'd happened to have chosen the opposite convention in the first place. Negative vectors come up in one form or another all through Science; they are a fact of life. It's just something students need to come to accept and there's no way round it.

So why should charges circulate and not always flow 'from A to B' in circuits? If they didn't, then they would accumulate at B and go on accumulating at B, increasing the Potential there (can you avoid the concept of Potential here?) without limit. How can charge conservation possibly confuse anyone? A source of emf (let's say a chemical 'pump' of some sort, as in a battery) will cause a displacement of charges towards the + terminal which, when open circuited, will balance when the PD across the terminals has reached the emf value. If a external path for the charges is provided, via a resistance, then charges will flow 'downhill' through the resistance (losing energy) because of the potential difference (the original energy they were supplied with). The action of the battery will be to maintain the flow by the Chemical energy causing a buildup of ions on each plate.

These questions are always difficult to discuss because we all have our own 'history' of our learning which colours our own picture of an archetypal student who needs an explanation. If you tell students that they will be confused by something and that the explanations are wrong then you can guarantee that outcome. If you give them a consistent explanation then at least they have a chance. There is nothing inconsistent with what I have written. Can you point out something?
 
  • #43
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You keep refering to primary school teachers

In which primary schools is electric circuit theory on the curriculum?

I return to my point - there is a difference between engineering circuit theory and the physics of electricity.

Whilst agreeing 110% that the physics is more 'fundamental' I ask do you differentiate or integrate every expression you need to from first principles?

I think the first couple of pages of chapter 2 of Hammond says what's needed very well about the physics of the situation for starters. Yet these days we throw Griffiths at them instead.

I repeat my question "If many students express confusion, as many have, is this the fault of the students, the teacher or the syllabus?"

The most important current a rookie should never encounter (but will do) does not flow around . I mean of course fault disconnection currents to earth.
 
  • #44
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Charge is charge. How it's carried is another matter.
If people really want to talk in terms of particles that's their problem. It just makes life harder for them and doesn't make any difference to 'understanding'. I wonder what they think when they watch TV. Do they obsess about the electrons hitting the front of the screen or do they just sit back and watch the TV programme?
No, that's not what I mean. It's a physics thing. The more I learn about continuous fields the more I dislike the notion of electrons, photons, and even atoms to be reducible to particles. I prefer to speak of an electron field, where such a field is only the divergence of the electric field, and no more, whose time evolution is not through the nonrelativistic Lorentz force but dictated by energy-momentum continuity, rather than popular model of a collections of tiny charged ball bearings living in disparate electromagnetic fields.
 
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  • #45
sophiecentaur
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The "primary School teachers" bit was poetic licence but it is taught to 12 yr olds and often taught by total non-specialists. They, of course, like the idea of particles because that's the way they were taught (not being ancient, mostly) and the way the educationists have told them to teach it. To me, this seems to be a potential disaster. No one can 'see' current, charge or electrons so I can't see why electrons are such preferred favourites at the moment.

The OP is actually Wrong in what it says about how conventional current is described when taught. She has reported it wrongly and probably because the teacher said something in a confusing way. A text book wouldn't have said that.
Instead of just pointing this out and saying that it was a bit unfortunate that a less suitable (not wrong) choice was made before electrons were discovered, the topic was picked up and shaken like a terrier with a rat by me and several others. Any post that was as wrong as that original one, really doesn't need all that hassle but just needs to be set straight, pending a further post. But we're on our third page of waffle now and poor old Femme_Physics will have given up long ago - sorry FP.

Of course there are different areas of Science but Engineering says nothing that Physics would disagree with. Both groups mean the same thing by the words Charge and Potential. This rather confirms my idea that electrons are best left alone, initially. It really would be just as valid to draw little Positive charges bombing around a circuit. Actually, as the electrons are going sooo damned slowly, it's hardly relevant in most cases.

There is some sort of snobbery about particles vs fields and it's often totally misplaced (in either direction, actually). Neither approach is nearer the 'truth' because there isn't a real 'truth' - just models that work to some degree. Electrons as 'little bullets' is a notion that is just as full of holes as Photons as other little bullets.

D'you know, I think there is some danger here that we may be finding a rather unhealthy amount of agreement on this forum, as it slowly peters out.

btw, if the Earth current doesn't somehow, get 'around' a complete resistive circuit, there would only be a few uF of Capacity to lead to the demise of the unfortunate student.
 
  • #46
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D'you know, I think there is some danger here that we may be finding a rather unhealthy amount of agreement on this forum, as it slowly peters out.
:cool:

This is an engineering question (FF is studying mechatronics) in an engineering forum.
As such electric circuit theory should not be about little (or large) balls.
It should be about black boxes with one port/2ports/3terminals etc, connected by wires.
The properties of interest are presented wholly at the terminals, we do not need to know what goes on inside the box, and we do not need to trace currents through the boxes.
 
  • #47
sophiecentaur
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You are right, of course. But the gravitational analogy - and the situation where you mechanically move two charged objects together and then apart both make it quite reasonable to think of charges being moved to a higher potential and then falling to a lower potential, whilst energy is transferred. There seems to be absolutely nothing contradictory about it, whether you call them 'charges' or electrons going the other way. It's all perfectly consistent.
 
  • #48
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But the gravitational analogy - and the situation where you mechanically move two charged objects together and then apart both make it quite reasonable to think of charges being moved to a higher potential and then falling to a lower potential, whilst energy is transferred
Only sometimes.

But if your black box is an ideal voltmeter, what charge is transferred?

or moving to something in mechatronics

If your black box is a 473 terminal microprocessor, of what use are Kirchoff's laws?

Or perhaps we should consider explaining why zero volts (ground) = logic 1 in TTLspeak and +5volts = logic 0, by Thevenin?
 
  • #49
sophiecentaur
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Only which times does the potential not change with separation? You push similar charges together, doing work (like the battery) and you let them separate and do work against a load.
There's no point in talking about an ideal voltmeter because they don't exist. There is always some energy involved in making a measurement.

You can choose whatever convention you like for Logic circuits. You would also be loony to try to apply Mr K's exceedingly good laws to anything so complex as a microprocessor but you could look at any part of it and they could be useful. We can't apply them Willy Nilly at frequencies at which electromagnetic radiation becomes significant but that's nothing exceptional for a scientific 'Law'. They all run out eventually.
 
  • #50
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I think the points are made we are done here.
 

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