Talk:Electromotive force

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February 2003 – February 2006

The discussion revived!

I wish I had put this article on my watch list as participating in this conversation would have been fun. Why is the accuracy tag still present? Did you guys give up before this was settled or did you all agree to disagree? Alfred Centauri 00:53, 4 April 2006 (UTC)

I deferred (or is it demurred) to the superior knowledge of the Asst Prof.--Light current 01:00, 4 April 2006 (UTC)

So, what then is still in dispute? From a quick read of the discussions on this page, It appears that there was some type of agreement although I'm not really sure. Alfred Centauri 01:17, 4 April 2006 (UTC)

The rest, as they say, is in the edit history (and here). This is all there is! Sad!--Light current 01:19, 4 April 2006 (UTC)

The actual definition of emf is disputed. Perhaps you could help?--Light current 01:28, 4 April 2006 (UTC)

Surely emf is simply a measure of the ability of the reaction to occur? I know it can be related to many other things but the main thing i think about when faced with problems including emf is the Gibbs free energy of the system and its ability to actually do the electrical work.

Consider an isolated chemical battery. We know that there is an electric field outside this battery due to the different charge densities on the terminals of the battery. Further, we know that this external field is conservative. That is, the work associated with a unit test charge moving from the positive terminal to the negative terminal of the battery is independent of the path taken as long as the whole of that path is external to the battery. Equivalently, the work associated with a unit test charge moving in a closed path is zero as long as the whole of the path is external to the battery. The value of work per unit charge in moving form the positive to the negative terminal is called the potential difference between the positive and negative terminals.

Now, consider the region inside the battery where, within the electrolyte, there are free charge carriers. Free charge carriers accelerate in the presence of an electric field yet, inside the battery, there is no current. Apparently, in the region internal to the battery, there is no electric field. Yet, we know that between the terminals of the battery, there is an electric field so we can only conclude that, inside the battery, there is also an equal but opposite electric field between the terminals.

Integrate the net electric field along a closed path where part of that path is from the negative to the positive terminal through the inside of the battery. This part of the path contributes nothing to the integral since the electric field is zero inside the battery. Thus, the only part of the path that contributes to the integral is the external path but we already know what the value is for that part of the path - it is precisely the potential difference between the terminals of the battery.

Now, for a different perspective, let us decompose this electric field into a conservative part and a non-conservative part. The conservative part is due soley to the charge densities on the terminals of the battery and the non-conservative part exists only in the region inside the battery. The integral of the conservative componenent of the field along any path from the positive to the negative terminal is defined as the potential difference between the terminals. If we integrate the non-conservative part of the field along a closed path, there is no contribution from any part of the path outside the battery. The only contribution to the integral comes from that part of the path inside the battery. Thus, we can equate the line integral inside the battery (from the negative to the positive terminal of the battery) of the non-conservative part of the field to the line integral inside the battery (from the positive terminal to the negative terminal of the battery) of the conservative part of the field. Or put another way, the line integral inside the battery of the conservative part of the field from positive to negative is equal and opposite to the line integral inside the battery of the non-conservative part of the field from positive to negative.

I'll have some more thoughts later... Alfred Centauri 16:31, 4 April 2006 (UTC)

Charge densities on cell terminals

Youre not on vacation again are you? Im going to go thro your text para by para. In your first para, you state that the charge densities on each terminal of the battery are different. Did you mean to say they were the same but of opposite sign? After all, is this not a case of charge separation as in a caopacitor? ;-). I Dont see how they could be different-- unless you know better! I see no reason to disagree with the rest of your para 1. But one question I have is: Why are you allowed to exclude the interior of the battery when doing the line integral? --Light current 16:44, 4 April 2006 (UTC)

Equal but opposite means equal in magnitude but opposite in sign which, quite clearly, is different. Charge density can be positive or negative.

I exclude the region inside the battery in paragraph one for the reason given in the following paragraphs. Within the battery, there is a non-conservative field that is superimposed on the conservative field originating from the charge on the terminals of the battery. Outside the battery, the electric field is conservative. To be more precise, in this idealized example, the curl of the electric field is non-zero (in fact, infinite) at the boundaries of the battery. Alfred Centauri 17:37, 4 April 2006 (UTC)

a)Is the magnitude of the charge density equal on both terminals?

b) You must not exclude the interior of the battery. Add an opposing field if you like inside, but I cant see how exclusion is legitimate. A conservative and non conservative field can sum in a region of space, can they not? --Light current 22:37, 4 April 2006 (UTC)

a) There is no need for the charge density to be equal on both sides just as long as there is more electrostatic repulsion on one side then the other. Physics require an unbalanced force for movement not an absence of oposing forces.69.213.70.93 19:43, 8 September 2006 (UTC)

Current inside batteries (cells)

With regard to your paragraph two, I have to take issue with your statement that there is no current inside the battery You have stated that there are free charge carriers - correct. You have also stated that in the presence of an electric field, charge carriers will accelerate (correct). You have stated there is an electric field within the battery (correct). Flow of charge carriers is defined as electric current. Ergo, there must be a current inside the battery!! Im still considering the interesting ramifications of the rest of your para 2. Watch this space!--Light current 17:09, 4 April 2006 (UTC)

Uh, I stated that "Apparently, in the region internal to the battery, there is no electric field." I justify this conclusion precisely because there is no current in the battery. Alfred Centauri 17:37, 4 April 2006 (UTC)

You have not shown that there is no electric field inside the cell. You must justify your statement that there is no current in the the battery explicitly by demolishing my above argument.--Light current 22:39, 4 April 2006 (UTC)

There is no need to demolish any argument. That there is no current circulating inside a battery is an observed fact. If you believe otherwise, please point me to a reference that supports your belief.

But, if you insist, I will demolish your argument. I never stated that there is an electric field within the battery. Alfred Centauri 00:20, 5 April 2006 (UTC)

Ah now you seem to be changing your statement. What do you mean by 'circulate'. Do you mean a flow wholly contained within the confines of the cell, like coffee circulating in a cup after stirring it?--Light current 00:52, 5 April 2006 (UTC)

The things that move inside the cell are the electrons, which the negative plate shoves off through its electronic circuit to the negative cell terminal from which it goes to the load circuit, and does its work. Then the electron returns to the cell positive terminal, where it moves through the positive plate electronic circuit to the positive plate, where it causes the positive plate material to shove the electrons into the electrolyte which transports them back to the negative plate.WFPM (talk) 16:09, 1 September 2009 (UTC)The driving power in all of this is the emf potential of the chemical reaction at the negative plate/electrolyte interface, which forces the electron through the circuitry and overpowers the chemical reduction action in the positive plate.WFPM (talk) 04:01, 2 September 2009 (UTC)

The electric field inside a cell

When you say apparently there is no electric field inside a cell, on what evidence or thinking do you base this statement? You seem to have generated a circular argument with no justification.--Light current 22:52, 4 April 2006 (UTC)

Para two, second comment. You say:we know that between the terminals of the battery, there is an electric field Correct. It is generated by the internal drift of charge carriers in the cell due to the chemical action . Wheres the problem? I think you may have mentioned this problem before--Light current 22:57, 4 April 2006 (UTC)

Para 2. Another question. Are you saying this:

The internals of a cell constitute a generator of emf. As such, you cant actually measure it becuase its generated by or involves a non-conservative field,- just like an electromagnetically induced emf. --Light current 23:14, 4 April 2006 (UTC)

There is no circular argument, LC. The argument goes like this:

(1) There is a conductive medium inside the battery.

(2) An electric field within a conductive medium causes a current.

(3) There is no current inside an isolated battery (excepting some minute leakage current that depends on the physics of the battery). If you disagree with this, then please point me to a supporting reference.

(4) Thus, there is no (net) electric field inside the battery.

It would seem to me that (1) and (2) are statements of fact. Also, it would seem to me that (3) is as statement of fact but if you can show me otherwise, I'll recant. The conclusion (4) is a logical consequence of (1) (2) and (3). There is no begging the question fallacy in this argument.

The static electric field is not generated by the internal drift of charge carriers (that is typically called a current). The static electric field is due to the accumulation of electrons on one terminal and by the removal of electrons from the other. This accumulation and removal of electrons occurs because of the chemical reaction between the electrolyte and the plates.

Regarding you final question. How do you propose to measure emf? Alfred Centauri 00:29, 5 April 2006 (UTC)

What's this about non-conservative electric fields? All static electric fields are conservative! There must be an electric field inside the battery. Pfalstad 00:40, 5 April 2006 (UTC)

Hmmm... where exactly did I claim that a static field is not conservative? In fact, I said that the electric field due to the charge on the battery terminals is conservative. However, inside the (isolated) battery there is no electric field. I'm not making this up - allow me to cite a source:

"These charges [on the battery terminals] give rise to an electrostatic field intensity E both outside and inside the battery. Inside the battery, E must be equal in magnitude and opposite in direction to the nonconservative Ei produced by chemical action, since no current flows in the open-circuited battery and the net force acting on the charge carriers must vanish.". This is from pg 206 of Field and Wave Electromagnetics, 2nd Edition, David K.Cheng [1]

I will cite additional sources if required. Alfred Centauri 03:21, 5 April 2006 (UTC)

Unfortunately I don't know much about batteries and I don't have that source. But if there's no electric field inside the battery, then the field is not conservative, because in a conservative field, all closed-loop line integrals are zero. If you integrate through the battery and then outside to the other end, you will get a nonzero answer if there is no field inside the battery. Pfalstad 20:17, 5 April 2006 (UTC)

(1) is wrong, or misleading. A battery does not act like a conductor. Otherwise, there couldn't ever be a voltage difference across the two ends of it. Pfalstad 00:43, 5 April 2006 (UTC)

Ah yes, but two equal and opposing voltages may exist across a conductor, resulting in zero current. I suspect this is what Alfred is driving at.--Light current 00:56, 5 April 2006 (UTC)

Your not thinking right, Paul. Recall that a length of wire disconnected from any external circuit and immersed in an appropriate changing magnetic field will have a measurable potential difference across the ends of this conductor. Nonetheless, there will be no (net) electric field inside the wire. How? Within the wire, the induced electric field exactly cancels the static electric field due to the charge at the ends of the wire. Similarly, a battery disconnected from any external circuit will have a measurable potential difference across the terminals but there will be no (net) electric field inside the battery. Alfred Centauri 03:21, 5 April 2006 (UTC)

If there's a changing magnetic field, it's not electrostatics. But an isolated battery, with no current flowing, is an electrostatic case. So the electric field must be conservative.. Inside and outside the battery. Pfalstad 20:17, 5 April 2006 (UTC)

Yes charge separation occurs due to the chemical reaction. Drift was the wrong word to use. I withdraw it. Alfred, it is up to you to provide a reference against the widely held belief and evidence that current flows within a cell.(KCL). Im talking about when its on load - are you? Ive just seen your term 'isolated' Do you mean not connected to anything at all?? I do not propose to measure the emf. If you look at my previous posts, you will see that I have argued the case that it is not possible to do so directly. All one can do is measure a pd across the battery terminals. Maybe one could measure the pd inside the cell as well (Wet cell). Any way,you have not answered my question about whether you are stating that the internal field is non conservative and that is why it cannot be measured. Please do so.--Light current 00:48, 5 April 2006 (UTC)

Correction

If Alfred is talking about an isolated battery sitting alone on a shelf with nothing connected to its terminals, then, in this case:

1. there is no external current
2. therefore there is no internal current from one terminal to the other (not sure about circulating currents inside)
3. there is an external field.
4. this must be balanced by an internal field of opposite sense

Is THAT what you are saying? If so I agree.--Light current 01:06, 5 April 2006 (UTC)

Ah! Now I understand where our disconnect was. Yes, I was referring to a battery that is not connected to an external circuit. Alfred Centauri 02:16, 5 April 2006 (UTC)

Your last para

Ive just read this carefully, and I think I agree with what I think it says (I think!). In fact, Im sure I said something similar myself (but not as well put) further up the page. i.e

e.m.f= -pd

We must now move on!--Light current 01:22, 5 April 2006 (UTC)

We agree. The sign of the emf is opposite of the potential difference. That is what I had hoped to show with my battery example. So, does professor Steve disagree with us on this? Alfred Centauri 02:21, 5 April 2006 (UTC)

Dunno! He says the emf is the sum of the potential drops in the (closed) cct and then the signs are all correct. But he doesnt give an explanation for his conclusion. So he seems effectively to be saying the same as we. --Light current 03:47, 5 April 2006 (UTC)

It does now seem likely that the same defn can be used for electrochemically generated emf and induced emfs. Would you agree?--Light current 04:25, 5 April 2006 (UTC)

Early scientists and emf

Did the early scientists appreciate this subtlety of distinction between emf and pd when they were bandying around this 'emf' term ? especially as they could only measure pds with potentiometers. Or did they just ignore the sign difference? I think these experiments would have been before the days of Maxwell, and so these scientists may not have had the advantage(?) of vector calculus to help them. If they did appreciate the opposite signs, they appear to have been much more perceptive than todays electrical engineers (and certainly wiser than I!). Yet the voltages they measured they always referred to as emfs! So maybe they were using a shortcut or maybe they just referred to the magnitude and didnt give a hoot about the sense? Does anyone have any comments on this as it may be good to put in the history para of the article.

This subtle difference betweeen emf and pd is in danger of being forgotten unless we get this article right and as Alfred Centauri has pointed out in the past, the concept of emf is essential to understanding the fundaments of electrical engineering--Light current 16:05, 5 April 2006 (UTC)

Where is the magnetic field?

I believe that there is a serious problem with my analysis of the isolated battery.

First, I do believe that there cannot be a net static electric field within the electrolyte as there are mobile ions that, like the electrons in a conductor, will redistribute themselves in such away as to cancel any applied field. However this alone cannot create a non-conservative electric field!

Second, I argued that this lack of an internal electric field leads to a non-zero closed path integral of the electric field if part of the path was through the battery. But, if this is true, then by Stokes' theorem, the flux of the curl of the electric field through the surface bounded by the path must be non-zero. But then, according to Maxwell, there must be changing magnetic flux through this surface. This cannot be physical. I am not aware of anyone measuring a constantly changing magnetic field in the vicinity of a charged battery sitting on a shelf!

Thus, despite the citation I gave, I do not see how there can be a non-conservative electric field inside the battery. Reading Professor Steves' comments again, the word contact potential jumped out at me. I feel certain that when this contact potential is properly taken into account, the closed line integral of E will always be zero even when the path is through the battery. More thoughts later. Alfred Centauri 23:58, 5 April 2006 (UTC)

<This is in response to LCs' not so stupid question>

OK, restore stupid question: :-)

So, you are thinking that any source of emf has to be produced by a non-conservative field. Am I correct in this assumption?--Light current 00:04, 6 April 2006 (UTC)

Well, if we define emf by

${\mathcal {E}}=\oint _{C}\mathbf {E} \cdot d\mathbf {l}$

then yes, emf is due to a non-conservative field. We know that a non-conservative field can drive (deliver energy to) an electric charge moving along a closed path whereas a conservative field cannot.

However, if by emf, we mean a measure of the strength of 'something' that can 'pump' electric charge along a closed path, then maybe a non-conservative field is not absolutely required. For example, can we not think of a (rechargable) battery as a analogous to a capacitor where electrical energy is stored in chemical bonds (which are, after all, electrical) rather than the electric field between the plates? Alfred Centauri 00:49, 6 April 2006 (UTC)

Contact Potential - I see that there is not an article here on Wikipedia but there shall soon be one. The contact potential is a "discontinuous jump of the electrostatic potential φ at the junction" [2]. A discontinuous jump in the potential implies an impulse in the electric field. Thus, as we integrate E along a path through the battery, the value of the integral suddenly jumps at the first terminal/electrolyte interface and then again at the other electroyte/terminal interface. In between, while we're in the electrolyte, the integral doesn't change. These jumps are in the same direction and must add up to the opposite of the potential difference between the terminals in order for the line integral to be zero. There's the 'emf' and it is indeed equal to -p.d. Alfred Centauri 01:05, 6 April 2006 (UTC)

As we cannot have infinitely thin interfaces, then the integral function will not be discontinuous. This is a minor point I think you will agree with that does not affect the argument.--Light current 14:21, 6 April 2006 (UTC)

Yes that article will be a welcome one as I have already refered to contact potential under 'other sources of emf'--Light current 01:08, 6 April 2006 (UTC)

Here's why we can have a conductive medium (electrolyte) inside the battery yet not discharge the charge stored on the plates of the battery: Contact_electrification#Electrolytic-metallic_contact. Alfred Centauri 01:10, 6 April 2006 (UTC)

Because theres no pd or emf across the electrolyte and all the emf is generated in infinitely (or very) thin layers at each electrode? Neat!--Light current 01:21, 6 April 2006 (UTC)

I agree. At the interface, the integral (potential) changes very rapidly over a very small distance compared to the overall distance along the path. Alfred Centauri 14:38, 6 April 2006 (UTC) OK. good!--Light current 16:35, 6 April 2006 (UTC)

Status of disputed tag

LC, we still have a disputed tag on this article but the comments by the person who put it here are now archived. I think we should have a summary of the point(s) that are disputed, address them, remove the disputed tag, and then archive all of the comments regarding the dispute in a separate archive. What think? Alfred Centauri 14:01, 6 April 2006 (UTC)

I believe the correct procedure would be to summarise Steves comments from the archive page and post the summary here, addess the points, and then, if no further comments, remove the tag. Yes.--Light current 14:08, 6 April 2006 (UTC)

Oh boy... yes, this article still needs a lot of work. I just found it after splitting Voltage ("pd" in this context) off from Volt. I think our first task ought to be to collect definitions with cited sources; without them, all this talk is just talk. Melchoir 00:28, 12 April 2006 (UTC)

I think you'll find most of the article is correct. But please go ahead and find some refs!--Light current 00:39, 12 April 2006 (UTC)

Copied from archive page by --Light current 08:20, 2 May 2006 (UTC)

disputed

I don't think that the sign convention has anything to do with the difference between electomotive force and potential difference. Potential difference is, strictly, the line integral of electric field, $\int \mathbf {E} \cdot d\mathbf {s}$. This is not well-defined in a circuit with a time-varying magnetic field, because it is path-dependent. Emf, as I understand the term as it is used nowadays, is simply a generalization of potential difference to handle such cases, by including a "fictitious" potential drop over things like ideal inductors, in exactly the right amount to make Kirchhoff's voltage law valid. I'll double-check a reference book when I get a chance, but meanwhile I'm adding the disputed tag because I'm dubious about the current text. Even if it is commonly used simply to mean the driving voltage(s) in a circuit, which is what the current text seems to suggest, I don't recall seeing a consistent difference in the sign convention. —Steven G. Johnson 18:22, 26 February 2006 (UTC) PS. Sorry about adding the disputed tag yesterday without explanation. WP gave me an error message and I thought that the edit hadn't occurred.)

---

emf

I looked in a couple of standard reference books. In Jackson, Classical Electrodynamics, he only considers the induced emf in circuits without chemical batteries. In this case,

${\mathcal {E}}=\oint _{C}\mathbf {E} \cdot d\mathbf {l}$

(as l.c. quoted above). i.e. it is the sum of the potential drops around the circuit, which is non-zero when you have time-varying magnetic fields and a nonzero inductance. In circuit theory, this emf is usually included as a fictitious "potential drop" in the circuit to make Kirchhoff's voltage law valid.

In Landau and Lifshitz, Electrodynamics of Continuous Media, they also consider the emf in electrostatic situations where you have a chemical battery. In this case he defines the emf as the sum of the contact potentials around the circuit. The contact potential is the discontinuity in the potential as you go from one conductor to the next, due to the difference in work functions for the materials (the work required for a thermodynamically reversible removal of a charged particle through the surface of a conductor). This is not the same as total potential difference around the circuit, which is zero for electrostatics, because it does not include the ohmic potential drop IR within the conductors. When you have a battery, he writes: "the circuit includes conductors in which the current is carried by different means (e.g. metals and solutions of electrolytes). Because the work function is different for different charged particles (electrons and ions), the total contact potential in the circuit is not zero even when the conductors at each end are similar."

In both cases, the emf is essentially the driving voltage in the circuit. It is related to a sum of potential differences $\int \mathbf {E} \cdot d\mathbf {s}$ around the circuit but is not the same. (There is no sign difference as long as we are talking about potential drops.)

—Steven G. Johnson 19:43, 27 February 2006 (UTC)

I dont really see any conflict here between what Steven is saying and what the article says. Can anyone? --Light current 08:26, 2 May 2006 (UTC)

The article seems to claim that the only difference between emf and potential difference is the sign. This is false as far I can tell; the sign convention is a minor thing compared to the underlying physical distinctions. —Steven G. Johnson 00:19, 3 May 2006 (UTC)

Im not an expert on this by any means, but it seems that Alfred Cetauri is saying that emf can only be generated by a non conservative field whereas pd is not so restricted. If this is correct, then maybe the article needs to say it.8-)--Light current 00:34, 3 May 2006 (UTC)

emf is defined on a loop; pd is defined between two points. They are generally incomparable. Melchoir 00:38, 3 May 2006 (UTC)

Wher is the loop in a chemical cell? 8-|--Light current 00:46, 3 May 2006 (UTC)

It passes through the cell and around the vacuum outside. Since the force on charge carriers outside the cell is uniformly zero, it doesn't matter which outside path you use, which is why one speaks of the emf of a cell. Melchoir 00:53, 3 May 2006 (UTC)

Well, not uniformly zero, sorry. Outside the cell, the force on a charge carrier is due only to the electric field, which is conservative, and that's why the exact path doesn't matter. D'oh! Melchoir 00:55, 3 May 2006 (UTC)

Ah no, outside the cell (between the cell terminals in fact) there exists a pd (not an emf). An emf is not directly measurable.--Light current 00:59, 3 May 2006 (UTC)

What do you mean, exactly, by "there exists"? Melchoir 01:07, 3 May 2006 (UTC)

You can measure it. Its there! 8-)--Light current 01:11, 3 May 2006 (UTC)

But, see, without more information that's a content-free statement. You can speak of the emf around a given loop or the pd between two given points. Neither emf nor pd is even defined at any given point in space (that's the potential's job). So to say there exists or does not exist an emf or a pd at a point or region of space is just undefined. Melchoir 01:18, 3 May 2006 (UTC)

No sorry, I mean that a pd can be measured between the terminals of the cell with a voltmeter.8-|--Light current 01:22, 3 May 2006 (UTC)

Well, yes, certainly. Melchoir 01:49, 3 May 2006 (UTC)

It seems that there are two distinct 'emf' definitions here. There is the usual closed contour integral definition (emf is the change in energy per unit charge when moved along a closed path). That is, a non-conservative electric field can drive electric charge along a closed path whereas a conservative field cannot. Then, there is the broader definition of emf which is simply the measure of the strength of a some kind of charge 'pump' such as a chemical battery. In this case, there is no non-conservative field. Instead, we have a physical apparatus that can separate charge onto conductive terminals where this charge can then participate in a current in an external circuit. As we have discussed above, the closed contour integral in this case is zero as long as the contact potentials are taken into account.

I agree that emf differs from pd by more than just sign however, the sign of the emf is opposite that of the pd. It must be in order to cancel the pd in the contour integral. Does Steve really think that the article implies that the only difference between emd and pd is sign? Alfred Centauri 15:55, 4 May 2006 (UTC)

citation needed

A citation needed for:

1. Maxwell's 1865 explication of what are now called Maxwell's equations used the term "electromotive force" for what is now called the electric field.
2. The unit of emf is the "energy per unit electric charge" and so the term "force" is misleading.
3. The expansion of the acronym is considered obsolete.
4. The use of the term "emf" is in decline but it is still found in introductory and technical level texts on electricity.

Thank you. 134.193.168.248 13:28, 13 June 2006 (UTC)

concise explanations

Rudolf F. Graf, in the Dictionary of Electronics published by Radio Shack, states that "electromotive force (emf)" is a force which is the fundamental principal for electricity to stream when there is a potential difference between two points. This is in line with Wikipedia:Verifiability. 134.193.168.248 13:30, 13 June 2006 (UTC) (PS., Graf also released "The Encyclopedia of Electronic Circuits")

Yes. Im not sure Radio Shack books are that authoritative. Also, its an incomplete and bad definition using words like 'stream' and electricity. THe correct words are flow and current or charge.

I have no objection to you quoting the ref. but please dont just rewrite the complete lede without discussion! I would be grateful if you would alter it back to what it was, then we can discuss your amendments. THanks --Light current 13:41, 13 June 2006 (UTC)

Rudolf F. Graf ^{google Graf} is an authoritative source. This is the complete definition. I did not use "flow" as that is the term that he used (I did not want to plagiarize), using stream instead. Electricity is movement of electric charge. What is there to discuss? This is what emf is. 134.193.168.248 13:45, 13 June 2006 (UTC)

Flow is the correct term. Stream is confusing and what is 'electricity'?. In my opinion you are making the article worse. Please change the lede back before someone reverts you. 8-(--Light current 15:49, 13 June 2006 (UTC)

Radio Shack is not an authoritative source, especially when it contradicts other sources. I'm comfortable reverting it myself. Melchoir 16:00, 13 June 2006 (UTC)

Rudolf F. Graf is an authoritative source. 204.56.7.1 16:06, 13 June 2006 (UTC)

We already have an article on the electric potential difference between two points: Voltage. Graf is confusing emf with voltage, and so are you. emf is not necessarily electric in nature, and it is not defined between two points, but around a loop. Now, if some people use "emf" to mean voltage, then the article should mention that. But it is senseless to turn this article into a clone of that one. Now, please stop inserting the change. You have been reverted by two people, and no one agrees with you. Melchoir 16:16, 13 June 2006 (UTC)

NPOV is one of the three policies are non-negotiable and cannot be superseded by any other guidelines or by editors consensus. (Wikipedia:Neutral point of view Revision as of 14:52, 12 June 2006). 204.56.7.1 16:21, 13 June 2006 (UTC) (BTW, Voltage cites Rudolf F. Graf too.)

See Wikipedia:Citing sources for rules mandated by Wikipedia:No original research and Wikipedia:Verifiability. 204.56.7.1 16:25, 13 June 2006 (UTC)

The current definition is already cited. And NPOV does not force us to contradict ourselves just because our sources contradict each other. The spirit of NPOV is that we recognize alternate usages for language. If you want to write that some authors treat "emf" as a synonym for "voltage", go ahead. But that's all. Melchoir 16:42, 13 June 2006 (UTC)

Stop reinserting the change! We are having a discussion here. You are trying to win support for a radical change to the article, and you have yet to succeed. Until you do, leave it alone! Melchoir 16:45, 13 June 2006 (UTC)

NPOV states that "representing views fairly and without bias". Get a citation or stop reverting the authortative information!

This isn't radical. It's Encyclopædia Britannica's professional view and the view of Rudolf F Graf ("Modern Dictionary of Electronics"; and "Safe and Simple Electrical Experiments"; and "Encyclopedia of Electronic Circuits"). 134.193.168.235 17:41, 13 June 2006 (UTC)

Will you quit asking for a citation when there's one in the article? "Like the electric potential at a point and the voltage between two points, the emf around a loop is measured in volts. Unlike the first two quantities, the emf is sensitive to non-electrostatic forces, since the force f can include magnetic, chemical, mechanical, and gravitational components." This is from Griffiths p.285. And Griffiths actually goes to the trouble to define what he's talking about in full generality. It's a certain loop integral, which is already stated in the article, also with a citation.

You say "Electromotive force... is, under normal conditions, called voltage". Voltage is defined between two points. Do you have a source that defines the emf between two points? Melchoir 18:07, 13 June 2006 (UTC)

have a source ? John Markus, Neil Sclater, "McGraw-Hill electronics dictionary". New York, McGraw-Hill, Edition 5th ed., international 3rd ed. c1994. ISBN 0071134867 ISBN 0070404348 134.193.168.235 18:16, 13 June 2006 (UTC)

What does it say? Melchoir 18:30, 13 June 2006 (UTC)

Griffiths's POV is that "Electromotive force" a "lousy" term. Through mathematic, not experimental practice, he states that this is "an integral of a force per charge". That's his view and should be referenced. Electromotance also should be included in the article.

The various other valued sources, with more practical experience, have stated what I have put in. 134.193.168.235 18:37, 13 June 2006 (UTC)

Griffiths may not like the terminology, but he is serious and thorough about the concept. Have you considered that your sources come from a narrow-minded POV that doesn't define the things they talk about with due precision or generality? According to Griffiths' definition, an emf arises from every kind of force on whatever charge carrier is involved in a circuit, including for example statistical forces on ions. It isn't confined to potential differences acting on electrons and holes.

I ask what Markus and Sclater say because I would love to see a definition of emf that doesn't require a closed loop. But do they really provide one? How exactly do they define their terms?Melchoir 18:56, 13 June 2006 (UTC)

I know that Griffiths as a sources is from a narrow-minded POV. Griffiths does not like the terminology nor the concept (I just read the pages). He'd rather use "Electromotance" and, to better suit the page with his reference, that is put in the section that refers to him. Markus and Sclater and Graf all state what I have put in and cited. 134.193.168.235 19:12, 13 June 2006 (UTC)

I want quotations involving "two points" and "voltage". What, verbatim, do they actually say? Melchoir 19:28, 13 June 2006 (UTC)

(And to say Griffiths doesn't like the concept when he devotes a section to it is silly.) Melchoir 19:30, 13 June 2006 (UTC)

Another valuable reference (though not cited in the article) says, "Since an EMF is a voltage, it is given by a line integral of the form". See, http://scienceworld.wolfram.com/physics/ElectromotiveForce.html 204.56.7.1 20:04, 13 June 2006 (UTC)

Scienceworld is often wrong; trusting it is little better than a Google search. Are those quotes coming soon? Melchoir 20:20, 13 June 2006 (UTC)

Go look them up. They are at the local library. 204.56.7.1

And all the external articles are wrong,

• University of Alberta, Department of Physics, 1999.
• Cyberspace Chemistry (CaCt), uwaterloo.ca.
• The Columbia Encyclopedia, Sixth Edition. 2001-05.
• IUPAC Compendium of Chemical Terminology 2nd Edition, 1997. (PDF)
• Semiconductor Physics Group, Department of Physics, University of Cambridge, 2006. (PDF)

Just your source is correct, riiiiiight (now that's silly). 204.56.7.1 20:27, 13 June 2006 (UTC)

Hey, if someone asks me for a quote, I'm eager to supply it. Why is it so hard? Don't you want to prove yourself right? Melchoir 20:28, 13 June 2006 (UTC)

They are as they are cited. Look up the book if you want. I don't need to prove myself right. This is what the cited books and the further reading and the external articles state! 204.56.7.1

Those "articles" appear to be written with only one application in mind, and not even the same application between them. They don't present a concrete definition with a formal discussion. It's not that they're necessarily wrong, but they can't be trusted to provide an overview of the topic. This is a perpetual problem when you're writing for Wikipedia: you have to be careful to identify the scope and context of your sources. In some situations, such as for a battery, emf and voltage are similar concepts and often conflated. Sources that discuss batteries exclusively aren't good enough. Melchoir 20:37, 13 June 2006 (UTC)

Only your source can be trusted, every other source is wrong or biased. Riiight. That's silly. 204.56.7.1 20:42, 13 June 2006 (UTC)

Are you saying that your edits are direct quotes? Melchoir 20:38, 13 June 2006 (UTC)

NO. Get a grip, I did not plagiarize (as stated above; see top of this section; I did have stream instead of flow, but because of comments I changed it (but it is not the exact sentence; much like "flow" means to "stream")).

Look up the fricken book, read the articles, etc. ad nauseum. 204.56.7.1

Fine, I'll take a trip to the library. But I have read your external links, and they contradict each other when they get down to details, so I don't know why anyone should trust them. Melchoir 21:05, 13 June 2006 (UTC)

Removal of Questionable Material

I removed the following paragraph:

"Electromotive force is a "force" which is the fundamental principal for electricity to flow when there is a potential difference between two points. Electromotive force has been stated to be the force that has the disposition to produce a circuit's electric current and is, under normal conditions, called voltage. Electromotive force is the force that moves electrons in a conductor. Increases in electromotive force causes a comparable inclination for electrons to proceed from one point to another. Electromotive force can affect "holes" as well as electrons. "

Anyone with any significant electrical engineering training understands that these statements are pure nonsense.

What does 'flow of electricity' mean? Isn't the 'flow of electric charge' the correct phrase? Also, why is a force required for a flow of electric charge? Charge in motion will stay in motion if not acted upon by an external force, right? Recall high school physics - a force accelerates an object - that is, a force changes an objects motion. This all basic stuff here, folks. There isn't any room for discussion. A force does not move electrons - a force accelerates electrons - clear???

The modern usage of the term emf is the work per unit charge associated with moving that charge along a closed a path. In the electric potential field, this quantity is zero by definition. Sure, anyone can find references to the contrary but these references will not be reference texts in the field but will be instead the kind of 'references' one finds at Radio Shack.

Bottom line, the kind of nonsense I removed above does not belong in this article. Hmmm... I better start checking the rest of the recent edits... Alfred Centauri 23:20, 13 June 2006 (UTC)

Yes - A nice summary of the state of affairs! --Light current 23:40, 13 June 2006 (UTC)

Being restored with references. 134.193.168.249 14:00, 14 June 2006 (UTC)

Removed Back electromotive force

I have removed the section entitled 'Back electromotive force' from the article. Below is the removed text:

Back electromotive force (also called "back torque" or "counter electromotive force") an electromotive force that occurs in electric motors and some generators where there is relative motion between the armature of the motor and the external magnetic field. When an electromotive force is applied to the ends of the coils of the motor, in the presence of a magnetic field, they rotate; each part of the coil moves to a different area within the field. The magnetic flux threading through the area between the coils is therefore constantly changing. By Faraday's law of induction, this induces an electromotive force that, by Lenz's law, opposes the motion of rotation; it is a back electromotive force.

The changing flux produces an emf in the coil. This electromotive force is in the opposite direction to the original one, which caused it. This new emf therefore opposes the main current flow in the circuit. If we assume that the motor is 100% efficient with no resistive forces acting, the speed of the armature will increase until the back electromotive force is equal to the applied electromotive force, i.e. there will be no net electromotive force, no current flow and hence, no net force. The armature will spin at a constant rate, of its own accord."

The reason I have removed this section is that a changing magnetic flux produces an emf - period. Not back emf - just emf and not just in motors or the coils of motors. This reminds me of the term deceleration versus acceleration. Acceleration is a change in motion - period. No need for deceleration just as there is no need for back emf. Bottom line, the emf that results from the magnetic flux current in the motor opposes the voltage applied to the terminals of the motor. If two voltage sources are connected together through a resistor, is one of them called the 'back voltage source'? Nonsense... Alfred Centauri 23:44, 13 June 2006 (UTC)

Another thought: A motor spins due to an applied voltage at the motor terminals. Now, with the applied voltage still present, apply a torque to the shaft in the same direction as the rotation of the motor. The emf generated within the motor now drives a current into the voltage source (perhaps a battery that is being charged). Is this emf still a 'back emf'? Or is just the amount that opposes the applied voltage and the amount over and above now a 'forward emf'??? Alfred Centauri 23:58, 13 June 2006 (UTC)

Being restored with references. 134.193.168.249 14:00, 14 June 2006 (UTC)

I think Alfred probably intended to quote the first two paras above as extracts (added by an anon user) from the page as those with which he disagrees. No doubt he should have indicated this and put the quotation in italics using the <blockquote> syntax.--Light current 00:32, 14 June 2006 (UTC)

LC, you are correct. I even left out the opening 'I have removed this section...". My apologies. BTW, apparently I am too senile to figure out the blockquote syntax you suggested. Perhaps you could further enlighten me with an example? Alfred Centauri 02:13, 14 June 2006 (UTC)

Surely! 8-) If I am quoting something I normally use the <blockquote> 'syntax' that indents the quoted para like this:

This is an example of a quoted piece of text. I usually also put it in italics to make it standout even more -although this is not mandatory. BTW, to retain the indenting of the block , you have to make sure that each new para is similarly indented - like this post.

Hope this helps 8-)--Light current 15:51, 14 June 2006 (UTC)

FYI, back emf is obviously not physically different than other emf due to a generator, but it is a technologically useful term because it describes one specific emf source in one situation which is very commonly encountered. It describes the situation where you are powering a motor, and the motor is spinning. In that case, the resistive voltage drop + "back emf" (proportional to speed) = the voltage of the source powering the motor. If you changed the situation and used the motor as a generator, the cause of this emf doesn't change, but you're no longer in the situation described by the term "back emf." Because "back emf" is probably the most common use of the word "emf" in engineering, I don't think it's unreasonable to mention it somewhere in the article. Dreadengineer (talk) 11:25, 21 March 2009 (UTC)

Split the article?

Mabey the article needs to be split into Electromotive force (electrical engineering) and Electromotive force (physics). Anyone? 134.193.168.249 15:12, 14 June 2006 (UTC)

You have yet to show that they are different in essence not just usage--Light current 15:53, 14 June 2006 (UTC)

Read the references. 134.193.168.249 15:55, 14 June 2006 (UTC)

Why remove from Electromotive force article:

1. "Electromotive Force (EMF)". Cyberspace Chemistry (CaCt), uwaterloo.ca
2. Eric W. Weisstein, "Electromotive Force". scienceworld.wolfram.com, 2006.

??? 134.193.168.249 16:32, 14 June 2006 (UTC)

They give a one sided view of emf. emf can be generated either chemically or by electromagnetic induction. Both these methods lead to the same phenomenon. Of course emf cannot be measured directly but its effects can. Also the Wolfram quote is self contradictory-- read it carefully.--Light current 16:38, 14 June 2006 (UTC)

Someone may want to looks at User:Metacomet/Emf page. Has some info that could be incorporated (especially in the "Explanation of electromotive force"); Looks like the user has left wikipedia because of some reason.204.56.7.1 17:50, 14 June 2006 (UTC)

Since when is a battery a source of induced emf? Alfred Centauri 03:09, 15 June 2006 (UTC)

got knocked out

This got knocked out.

Regardless of how it is generated, emf causes an electric current through a closed circuit connected to the terminals of the source. For example, the chemical reaction that separates electric charge onto the two terminals of a battery proceeds as long as there is an external circuit through which electrons can flow from the '+' terminal to the '-' terminal and thereby recombine with the positive ions.

Put back in if necessary. 204.56.7.1 20:02, 14 June 2006 (UTC)

Recent anonymous edits

Many recent anonymous edits to this article are poorly (even strangely) worded and technically incorrect. Further, these edits look more like notes taken for a research paper rather than a serious attempt at editing an encyclopedic article. There is no flow or coherence to these edits. Inserting paraphrased statements from one reference after another is NOT the way to edit a Wikipedia article. I have removed these bizarre edits a second time.

Also, what the heck is electric motor material doing in this article under the title 'Back emf'??? I have removed this out of place material a second time. Alfred Centauri 02:19, 15 June 2006 (UTC)

Please stop removing reference and authoritative material. 134.193.168.250 13:29, 15 June 2006 (UTC)

User:134.193.168.250 you really must try to discuss your changes on the talk page to obtain consensus, or sooner or later, most of them WILL be reverted or removed. 8-(--Light current 13:44, 15 June 2006 (UTC)

try to discuss your changes? When others do not try the same thing? :-t134.193.168.250 13:56, 15 June 2006 (UTC)

We do! THats what were doing now!--Light current 13:58, 15 June 2006 (UTC)

Alfred Centauri doesn't. He's removed the relevant and citable material twice now! 134.193.168.250 14:00, 15 June 2006 (UTC) (PS., the emf disambig page has "electromotive force (voltage)")

THats because you have inserted weirdly worded claims and dubious refs into a relatively stable page without discussion. I suggest you and Alfred address each others concerns on this talk page. Im sure that you two can come to a happy compromise! 8-| --Light current 14:16, 15 June 2006 (UTC)

More emf Citations

From "McGraw-Hill Encylopedia of Physics, Electromotive force (emf)"

"The electromotive force around a closed path in an electric field is the work per unit charge required to carry a small positive charge around the path. It may also be defined as the line integral of the electric intensity around a closed path in the field."

"The term emf is applied to sources of electric energy such as batteries, generators, and inductors in which current is changing."

From the textbook "Field and Wave Electromagnetics, 2nd Edition, Cheng"

"A steady current cannot be maintained in the same direction in a closed circuit by an electrostatic field"

"Consider an electric battery with electrodes 1 and 2... Chemical action creates a cumulation of positive and negative charges at electrodes 1 and 2 respectively. These charges give rise to an electrostatic field intensity E both inside and outside the battery. Inside the battery, E must be equal in magnitude and opposite in direction to the impressed Ei produced by chemical action... The line integral of the impressed field intensity Ei from the negative to the positive electrode inside the battery is customarily called electromotive force (emf) of the battery."

"The emf induced in a stationary loop caused by a time-varying magnetic field is a transformer emf"

"If the moving conductor is a part of a closed circuit, then the emf gnerated around the circuit is ... refered to as a flux cutting emf or motional emf.

From the textbook "Electric Machinery, 5th edition, Fitzgerald, Kingsley, Umans"

"The term electromotive force (emf) is often used instead of induced voltage to represent that component of voltage due to a time-varying flux linkage"

From the textbook "Physics, 2nd Edition, O'Hanian"

"To measure the 'strength' of a source of electric potential energy, we introduce the concept of electromotive force, or emf. The emf of a source of electric potential energy is defined as the amount of electric energy delivered by the source per coulomb of positive charge as this charge passes through the source from the low-potential terminal to the high-potential terminal."

"The work associated with the rod is the work done by the driving force on a hypothetical unit positive charge that passes from the negative end of the rod to the postive. The driving force on a unit positive charge is... called a motional emf because it is generated by the motion of the rod through the magnetic field."

"The induced emf around a closed mathematical path in a magnetic field is equal to the negative of the rate of change of the magnetic flux intercepted by the area within the path."

Thoughts? Alfred Centauri 02:56, 15 June 2006 (UTC)

Alfred without access to most of these references you (or the unknown user) have quoted, could you tell me what you are trying to illustrate by the above listing? Is it that for every ref the anon user has included, you can find one that says the opposite or backs up our case?

Im afraid we have left ourselves open to this sort of critisism by not including proper refs. If you have some to backup your statements, then It would be good to include them in the article (prefereably as in line refs)--Light current 14:02, 15 June 2006 (UTC)

This is why the article needs to be split. 134.193.168.250 14:21, 15 June 2006 (UTC)

How does that conclusion follow from the above? 8-?--Light current 14:22, 15 June 2006 (UTC)

Because one is from a physics view and the other is from a electrical enginnering view. Notice the textbooks that Alfred Centauri, all physics books. The other references are electrical engineering books. Physicsts tend to think that they are absolutely correct, damn the other fields. I would like to see that all the topics and concepts in one article and are included together, but not acknoweldging them all is hideous (such as about questioning the existance of back emf or counter emf, ppl that do that have no idea about generators). IF it's going to lead to a edit war, I don't want any part of it and the articel should be split. 134.193.168.250 14:29, 15 June 2006 (UTC)

Although I share your sentiments somewhat (I am an engineer), we must accept that all engineering is based upon physics. Usually, engineering practice dilutes the physics to make things easier to handle and whilst these approximations make no diff to the engineering, they are in fact not strictly correct. The case of induced/back emf in an inductor comes to mind!. However on this page there is no reason why a definition of emf that is acceptable to both physicists and engineers cannot be hammered out. It will of course reqiure that some of the refs written from a biased ?(one sided POV) cannot be included. I hope that you will want to work with us to achieve this aim! 8-)--Light current 14:40, 15 June 2006 (UTC)

On the contrary, two of the textbooks I referenced are in fact electrical engineering texts (Field and Wave Electromagnetics and Electric Machinery) thus refuting the claim by our anonymous friend. Further, it occurs to me that he or she is confusing the words 'electrical engineering' with 'technical electronics'. Electrical engineering and physics textbooks, as evidenced above, align quite closely. On the other hand. one can find statements like:

"Electromotive force. The electron-moving force in a circuit that pushes and pulls electrons (current) through the circuit." (Schrader, 'Electronic Communication, 4th edition, McGraw-Hill').

While such statements may be helpful to an electronics technology student attending a community college, they are nonetheless misleading and technically incorrect. Such statements do not belong on Wikipedia except perhaps in a section that explicitly points out the errors in such a statement. Thus, all references are not equal.

For what it's worth, I am also an EE, not a physicist. However, I personally don't 'get' the comment about physicists damning other fields (now, mathematicians are a different story!). It appears to me that our friend has an axe to grind and wants to do something about it in this article.

Finally, I do question the existance of so called back-emf. I don't question the existence of the term but that term is used simply as a label just as the phrase 'applied emf' is used. Does the term 'applied emf' deserve a separate section in this article too? Alfred Centauri 15:25, 15 June 2006 (UTC)

Unneeded removal of facts

---

(I have 'broken the rules' and embedded comments within the original comments inserted by User:134.193.168.250|134.193.168.250) Alfred Centauri 22:02, 15 June 2006 (UTC)

The law of electromagnetic induction states that with a changing magnetic flux transverse a circuit, an electromotive force is brought forth, in the general course for restraining that change.

This was removed for the reason that it is redundant and worded in an obscure way that is out of place with the rest of the article. Alfred Centauri 22:02, 15 June 2006 (UTC)

In electrical engineering, the electromotive force is a "force" which is the fundamental principal for electricity to flow when there is a potential difference between two points.

This was removed because it is wrong, wrong, wrong. While it may be a fact that Graf wrote this nonsense, what is written is not a fact. When there is a potential difference between two points, the force on electric charge is due to a conservative electric field that cannot produce an emf. Alfred Centauri 22:02, 15 June 2006 (UTC)

Electromotive force "moves" electrons in a conductor.

Also wrong, wrong, wrong. Emf is a scalar quantity. Which direction does a scalar point? Alfred Centauri 22:02, 15 June 2006 (UTC)

Increases in electromotive force causes a comparable inclination for electrons to proceed from one point to another.

Wrong again and for the same reasons I've state above. Alfred Centauri 22:02, 15 June 2006 (UTC)

Electromotive force can affect "holes" as well as electrons.

Why leave out ions and other charge particles? Is a hole a 'real' particle? Why is this even relevant? Alfred Centauri 22:02, 15 June 2006 (UTC)

For induced emf, the term "induced voltage", or voltage around a network path caused by a changing electromagnetic flux linking the path , is used, though this is in a different sense to the uses of emf in physics. The term "applied voltage" is also used in other instances, though this not as common.

It is??? How are these terms used differently in physics? Alfred Centauri 22:02, 15 June 2006 (UTC)

---

The counter-electromotive force (abbreviated counter emf or CEMF) is the force that runs against the current which induces it, it is caused by a changing electromagnetic field. It's represented by Lenz's Law of electromagnetism. Back electromotive force (also called back torque) is an electromotive force that occurs in electric motors and some generators where there is relative motion between the armature of the motor and the external magnetic field. Counter emf is a voltage developed in an inductor network by a pulsating current or an alternating current. The voltage's polarity is at every moment reverse that of the input voltage

In a generator using a rotating armature and, in the presence of a magnetic flux, the conductors cuts the magnetic lines of force in the magnetic field or changing flux produces an emf in the coil. The voltage opposes the applied voltage. By Faraday's law of induction, this induces an electromotive force that, by Lenz's law, opposes the motion of rotation. It opposes some of the input voltage, which reduces the armature's circuit flow of current. This voltage acts in the opposite direction to applied voltage; therefore, it is called "counter-electromotive force". This new emf therefore opposes the main current flow in the circuit. This electromotive force is in the opposite direction to the original one, which caused it.

If it is assume that a motor is 100% efficient with no resistive forces acting, the speed of the armature will increase until the back electromotive force is equal to the applied electromotive force, i.e. there will be no net electromotive force, no current flow and hence, no net force. The armature will spin at a constant rate, of its own accord.

I removed these three paragraphs mainly because it is not relevant to the article and secondly because the usage of the term 'back-emf' is not limited to motors and generators which this section implies. Alfred Centauri 22:02, 15 June 2006 (UTC)

Reliable sources

134.193.168.250 14:11, 15 June 2006 (UTC)

I believe the correct procedure may have been to copy the offending statements by the unknown user, quote them (in italics maybe) and then reply to them individually as seen fit. However, in this case Im not going to make a fuss because I think we have probably seen the last of 134.193.168.250 8-) --Light current 22:58, 15 June 2006 (UTC)

Note the numerous references to: Gibilisco and Sclater, "Encyclopedia of electronics". Here's an excerpt from a review by Library Journal:

"although the book is part of the publisher's professional reference series, the level is definitely somewhat below that of the professional scientist or engineer. This makes it especially appropriate for the technician, hobbyist, and amateur." (emphasis mine)

Here are some additional scholarly publications by Gibilisco:

"Teach yourself Electricity and Electronics"

"Electricity Demystified"

"Electronics Demystified"

"Meteorology Demystified"

"Alternative Energy Demystified"

"Hot ICs for the Electronics Hobbyist"

Here are some additional scholarly publications by Sclater:

"Electronic Technology Handbook"

"Mechanisms & Mechanical Devices Sourcebook"

"Wire and Cable for Electronics: A User's Handbook

Here are some additional scholarly publications by Rudolf F. Graff, author of "Dictionary of Electronics":

"Home Wiring: Improvement, Extensions, Repairs"

"The Tab Handbook of Hand & Power Tools"

"Directory of Toll Free Numbers"

"The Build-it Book of Electronic Projects"

"The Build-it Book of Fun & Games"

"ABC's of Electronic Test Probes"

"The Safe and Simple Book of Electricity; 101 exciting experiments using common household articles"

Here are some additional scholarly publications by John Markus, author of "McGraw-Hill Dictionary of Electronics"

"Television and Radio Repairing"

"Electronics Projects Ready Reference"

"Popular Circuits Ready Reference"

"Sourcebook of Electronic Circuits"

As I said earlier, our anonymous friend apparantly doesn't 'get' the difference in authority between electrical engineering and/or physics texts and electronic technology texts. Alfred Centauri 03:02, 16 June 2006 (UTC)

Yes I agree that whilst we may all have some of these 'hobby' books in our libraries, they are not really suitable references for WP because they will not necessarily have been peer reviewed to the correct academic level (if they have been reviewed at all). THere are some exceptions of course and I have few Radio Shack books written by professors of engineering, chief engineers of well known electrical and electronics companies etc. So really, its a question of looking both at the authors and the publishers to check for reliable reference! I dont think I would use any of the above as refs without cross checking in another erudite publication. Specifially encyclopedias/handbooks of XYZ can be notoriously unreliable! But this is where long years of experience are useful in determining 'reliable' sources!! 8-)--Light current 18:11, 16 June 2006 (UTC)

While I understand the value of 'Electricity for Dummies' type texts and while I certainly don't mean to berate the technician, hobbyist or amateur (FYI, I repaired TVs and VCRs for a living for over a decade), I also understand (from first hand experience) how hard it is to 'unlearn' some of the misconceptions that are repeated over and over in these texts. Our friend Wjbeaty understands this too. Where the heck is he anyway? I would think he would be interested in this discussion. Alfred Centauri 00:24, 17 June 2006 (UTC)

I agree with these sentiments. Now I am beginning to understand why you understand so much and why you seem to have a breadth of knowledge that many of the so called specialists dont have!. Its cos youre old (like me!) but have kept on learning. BTW where did you learn all your physics? Not on an EE course surely! 8-)--Light current 07:27, 17 June 2006 (UTC)

Removal of text

I reverted the following edit because I believe it serves to confuse rather than to clarify:

Electric fields can be created in only two ways: by changing magnetic fields or by separation of electric charges. Typically, electric charge separation is responsible for the electric field. The charges are separated due to the existence of a region of lower potential. For example, in the Van de Graff, the charges are "mechanically" pushed. In other scenarios (electrochemical emf sources), quantum mechanical principals (Pauli exclusion and the uncertainty principles) are necessary to understand the existence of lower potential regions.

First, there is only one source of the electric field and that is electric charge. Magnetic fields ultimately are due to the motion of electric charge somewhere. Second, to say that separation of charge is responsible for the electric field and then to say that charges are separated due to the existence of a region of lower potential is to say that the electric field is due to the electric field. After all, the electric potential and the (conservative) electric field are not independent. That is, a charge moves to a lower potential region because that is where the (external) electric field pushes it - charge flows downhill. Alfred Centauri 14:02, 26 February 2007 (UTC)

Eugene: While I think I understand what you trying to say in general, there are several things in your response that are either just wrong, or have been expressed incorrectly.

First, to my knowledge, there is no net electric field between the plates of a battery. That this must be so is obvious. The electrolyte between the plates is conductive and so, if there were an electric field between the plates, there would be current within the electrolyte until the net field became zero. Thus, the electric field you must be referring must exist at the boundary between the electrolyte and the plate. It is this field that gives rise to the so-called 'contact potential', right?

Second, what is the overall potential you are refering to? For a potential to exist, there must be an accompanying force field and, to my knowledge, the only macroscopic force fields are the electric and gravitational fields. I don't think that gravity plays a significant role in this problem. Any other potential, e.g., chemical potential, is electric in origin.

Third, charge can easily move against an electric field. Remember, the electric field is responsible for the 'acceleration' of charge. Charge will allways accelerate in the direction of the electric field but that doesn't necessarily imply that charge never moves in the opposite direction of the field. The charge carriers within the electrolyte move in random directions at random speeds. Classically speaking, if a charge carrier (ion) in the electrolyte comes close enough to the electrolyte/plate boundary it may be 'turned back' by the intense electric field at the boundary or may only be slowed depending on its initial momentum. Should the ion find itself within close proximity to the plate molecules, a chemical (electric in origin) reaction may occur resulting in the transfer (or removal) of charge to (from) the plate. This reaction is, of course, driven by electric interactions in a microscopic region where QM comes into play. Bottom line, the motion of the charge carriers are kinetic in nature within the battery with accelerations due to the electric field occuring in very localized regions.

Finally, and this is a nitpick, charge flows not current. Alfred Centauri 01:41, 28 February 2007 (UTC)

OK, you're correct. I removed earlier objections, so as to not look too silly! -- Eugene.

"emf" acronym

I have deleted the sentence that said "electromotive force" is an obsolete term. It's in every electromagnetism textbook I have read. I would say that it is a historical convention such as conventional current.(Kelleycs01 22:43, 13 April 2007 (UTC))

Difference between emf and Potential Difference

My textbook says "Voltage and potential difference are usually not equivalent." I came here to better understand why. If anyone can explain this, it would make a great addition to the page. stemperm 22:55, 12 November 2007 (UTC)

Google = voltage "potential difference" = 1510000

Martin Segers (talk) 20:05, 7 January 2008 (UTC)

I'll try in brief. Make a short circuit. The result is that the potential difference is zero but electric current is stil flowing. EMF is a reason for flowing of electric current. Potential difference is a result of the said flow and is dependent on resisitivity of the circuit. EMF depends just on external process transforming chemical, or mechanical energy onto electric energy. rgds --78.88.154.97 (talk) 23:11, 9 January 2008 (UTC) from Poland.

Did you look up Electromotive force yet? ie The polarity of this measured potential difference is always opposite to that of the generated emf--TreeSmiler (talk) 23:56, 9 January 2008 (UTC)

Potential difference is :..... related to the energy that would be required to move a unit of electrical charge from one point to the other against the electrostatic field that is present. sounds rather similar to emf!--TreeSmiler (talk) 01:12, 10 January 2008 (UTC)

It sounds similar, but in fact, it is OK. You must read it like one side of mathematical equation:

(Work done by electric field) = (work done by You against forces of electric field)

If you have an open emf the current is zero I=0 and potential differece U>0. Emf is a cause that moves charges "+" and "-" apart thus creating the potential difference and electric field. Work done by efm when creating the field and potential difference is equel to work done by the field when the unit of charge is flowing in external circuit back from "+" to "-". Take notice that inside efm tche current flows from "-" to "+", and in external circuit conversly i.e. from "+" to "-", but the work is still the same - see the written and mentioned above equation.

We may also write:

The work done inside emf = work done in connected to emf external circuit

and this related to energy (work is related to energy)

rgds --78.88.154.97 (talk) 18:40, 10 January 2008 (UTC)

Im not trying to be funny but, which part of the Electromotive force and voltage difference para are you having difficulty with?--TreeSmiler (talk) 18:29, 10 January 2008 (UTC)

In my electrical engineering and physics classes, there was never such an insistence that emf and voltage are separable, in the way that this article and discussion try to separate them. I'll try to illustrate with an example from the "Electromotive force and voltage difference" section.

For a circuit as a whole, such as one containing a resistor in series with a voltaic cell, voltage does not contribute to the overall emf, because the voltage difference on going around a circuit is zero. (See Kirchhoff's Law)

This is very misleading. Here is what happens: the voltaic cell will have a voltage difference across its terminals, caused by its internal chemical reaction pushing positive charges onto its positive terminal and negative charges onto its negative terminal. When a resistor is placed between the two terminals, the voltage difference across the resistor causes current to flow through the resistor. Nothing more is happening physically. So, first off, the voltage integrated around the whole circuit is zero, as it always is, but that does not mean voltages across components have nothing to do with current flowing.

It's really unclear what the difference is between calling the battery an emf source versus stating that it produces a voltage difference between its terminals. The only way to have a voltage difference between two points is to have emf between them; the two things have the same physical definition. It can get complicated if there are multiple emf sources in series before you can actually measure a voltage, as in a charging/discharging battery where resistance has effects, but that doesn't change the fact that each emf source corresponds to some voltage difference; if they didn't, they would have no measurable units. So it seems that, in my understanding, there's really no physical way to separate emf from a potential difference, and the insistence on trying is misguided. Am I misunderstanding emf, or the intent of the article? Dreadengineer (talk) 11:04, 21 March 2009 (UTC)

+ or -

${\mathcal {E}}=-L{di \over dt}$

Better: + or -
Martin Segers (talk) 07:49, 6 January 2008 (UTC)

Opening sentence

Currently, the opening sentence is:

"Electromotive force (or potential) of a body is the work done in joules to bring a unit electric charge from infinity to the body."

AFAIK, this is incorrect. A source of EMF drives charge around a closed circuit. The electric potential (work done in joules to bring a unit electric charge from infinity to the body) cannot do this. Any comments before I remove this statement? Alfred Centauri (talk) 02:09, 9 March 2008 (UTC)

I agree 100%. That statement was added in this edit. The definition used before that edit is quite good, and could be put back in almost unchanged if you want. --Steve (talk) 17:20, 9 March 2008 (UTC)

Typo in 'and due to the capacitor'?

A sentence in the voltage difference section looks funny to me (emphasis mine):

"For a circuit consisting of a capacitor that discharges through a resistor, the emf that drives current is solely due to the voltage difference across the resistor, and due to the capacitor."

I'm pretty sure that "and due to the capacitor" is missing a "not" but not sure enough to make the edit myself... if the sentence is technically accurate, I would suggest removing the word "solely".

Sintaur (talk) 16:27, 20 September 2008 (UTC)

I agree --Steve (talk) 16:34, 20 September 2008 (UTC)

Proposed deletion

The section on "Electromotive force and voltage difference" currently contains the following material:

"According to Maxwell, even a potential difference can have the same effect as an emf. Nevertheless, normal usage does not consider a voltage difference as a source of emf.

For a resistor the voltage difference across its ends serves as the sole source of emf.

For a voltaic cell the net emf is the sum of the chemical emf, which always tends to drive current so as to discharge the cell, and the voltage difference emf across its terminals. The combination of the two emfs can drive current in either direction, thus permitting both charge and discharge; in equilibrium, where there is zero current, these two emfs cancel.

For a circuit as a whole, such as one containing a resistor in series with a voltaic cell, voltage does not contribute to the overall emf, because the voltage difference on going around a circuit is zero. (See Kirchhoff's Law)

For a circuit consisting of a capacitor that discharges through a resistor, the emf that drives current is solely due to the voltage difference across the resistor, and due to the capacitor.

If a source of emf is not connected to an external resistor, then an electric current cannot flow through that resistor (Ohm's Law). In this case, between the terminals of the source there must exist a true electric field that produces a voltage difference that exactly cancels the emf of the source.

The source of this true electric field is the electric charge that has been separated by the mechanism generating the emf [6]. For example, the chemical reaction in a voltaic cell stops when the electric field across each electrode is strong enough to stop the reactions at each electrode.

This electric field between the terminals of the battery creates an electric potential difference that can be measured with a voltmeter. The polarity of this measured potential difference is always opposite to that of the generated emf. The value of the emf for the battery (or other source) is the value of this 'open circuit' voltage. When the battery is charging or discharging, the emf itself cannot be measured directly. It can, however, be inferred from a measurement of the current I and voltage difference V, provided that the internal resistance has already been measured: I=( -V)/r.

I do not fully understand what points it was intended to make in these statements, but it appears to me that there are some errors or ambiguities in these statements and that the general lack of clarity means that non-experts have little chance of understanding what is being said. I therefore suggest that this material adds little to the article and that we delete it all. (RGForbes (talk) 18:43, 14 April 2009 (UTC)) (Richard)

I'd take it that the objective here is to distinguish between voltage and EMF. The attack taken is to provide some examples. Unfortunately, the general principle is not enunciated, so the reader cannot apply the principle to the examples, but must infer the principle from the examples. Perhaps instead of deletion, a clear principle could be stated at the outset. The stuff about Maxwell is gratuitous. Brews ohare (talk) 22:27, 16 May 2009 (UTC)

I rewrote some of this and brutally excised some. I hope nothing of value was lost. Brews ohare (talk) 00:32, 17 May 2009 (UTC)

I don't understand the point either. The rewrite is confusing and still essentially unsourced. Most sources I look at don't seem to have any trouble seeing EMF and voltage as interchangeable concepts. What source makes this distinction? Dicklyon (talk) 19:13, 21 June 2009 (UTC)

I've noticed in several articles someone is running around claiming that EMF and voltage are distinct concepts, but no sources are ever provided. Whoever has been doing this has been put on notice that unlikely unsourced claims are not acceptable, and no sources have been provided. It's time for this to go. --Jc3s5h (talk) 15:32, 22 June 2009 (UTC)

Mechanism

This article and all the related articles on batteries, voltaic cells etc. never explain how the chemistry leads to a voltage difference. That is a grievous fault. Other mechanisms also are not mentioned except an aside on Faraday's law. Brews ohare (talk) 15:33, 16 May 2009 (UTC) I have added a section to help fill this omission. Brews ohare (talk) 19:05, 16 May 2009 (UTC)

Dubious definition

The "Formal definition" says "The EMF of a source (electromagnetic, chemical, thermal or otherwise) may be defined as the work done by an external agent, per unit charge, with sign reversed, in bringing a test charge once around a circuit that contains the source and no other source." What is this supposed to mean? In a circuit with a battery, the result would always be zero, unless there's a magnetically induced emf as well. Doesn't the emf of the source need to be defined in terms of its terminals, not a closed path? Dicklyon (talk) 19:03, 21 June 2009 (UTC)

The "seat of EMF" concept as presented is also dubious. All the sources I find apply this concept to the question of where exactly in an electrochemical cell the EMF comes from. They don't say that a battery is a seat of emf, but that it has one, somewhere, in it. Dicklyon (talk) 20:01, 21 June 2009 (UTC)

I taken the dubious bits out now. Please someone review Electromotive_force#Formal_definition_of_electromotive_force for consistency of definition, terminology, and sources. Dicklyon (talk) 20:24, 21 June 2009 (UTC)

I just realized that "Griffiths" was among the old-style ref listings, and that it's searchable on Amazon. The cited page for the loop integral (p.293) or actually the page before, introduces a distinction between different electric forces, such that the electrostatic part cancels out, but there's another part that doesn't, attributable to a source of emf. I don't understand this. Does anyone, who can explain? Dicklyon (talk) 23:09, 21 June 2009 (UTC)

The definition there helps: "the work done, per unit charge, by the source", and says that in some books emf is defined that way. It makes sense because it makes clear that "the source" needs to be identified, and that the emf is a property of the source, not of the closed loop, and that the loop integral doesn't count the work expending in pushing the current through the rest of the loop, for example. In the statement we had, "The EMF of a source (electromagnetic, chemical, thermal or otherwise) may be defined as the work done by an external agent, per unit charge, with sign reversed, in bringing a test charge once around a circuit that contains the source and no other source," the connection between "a source" and "an external agent" was not at all clear, but I see now what it was getting at. But as Griffiths points out, you don't really need to integrate around the loop, just across the source itself, if the source is not a loop (since $f_{s}$ is zero outside the source, it says). It also says "there's some subtlety involved", so it needs more study before we see if we can re-incorporate some of this into the article. Dicklyon (talk) 23:18, 21 June 2009 (UTC)

There could be a distinction in definitions between publications that treat EMF in terms of fundamental phenomenon, such as the net effect of electric fields, and publications that treat EMF in terms of simplified lumped-element circuit analysis. Those with formal university-level education in electricity know that lumped-element circuits are only an approximation, but this is seldom explicitly acknowledged in publications (and in some areas of study, never causes practical problems). --Jc3s5h (talk) 15:39, 22 June 2009 (UTC)

My understanding of "integration around a closed loop" is that the conservative part of the E-field makes no contribution to a closed loop, so all that is left is the EMF contribution. The whole point of EMF is to express how "external agencies" (for example, non-conservative fields and chemical sources) result in charge separation that, in turn, causes electrical potential difference. Its about converting energy in various forms to electrical form. Brews ohare (talk) 16:28, 22 June 2009 (UTC)

But doesn't that integral give zero even when there's a battery in the loop? Most of the sources I looked at had integrals from A to B; the ones with closed loops were all about magnetic effects (transformer emf) unless I missed something. Dicklyon (talk) 21:14, 22 June 2009 (UTC)

Self discharge

The sentence: "However, one might note that even in the open-circuit condition, where no current is drawn, self-discharge occurs due to various secondary reactions." has been deleted from the article.

This sentence with its Wiki link to the topic should be restored for these reasons:

(i) It is a topic that shows up in many discussions of EMF in batteries.

(ii) It is a strength of Wikipedia that a reader can find links to related material; the utility of Wikipedia is thus diminished by elimination of such links. Brews ohare (talk) 16:40, 22 June 2009 (UTC)

I can't really comment without seeing the context of the statement. However, in a general article about EMF, a statement that only applies in particular situations should not be made, lest readers think it is an inherent property of EMF. --Jc3s5h (talk) 16:49, 22 June 2009 (UTC)

The context describes the main chemical reactions occurring in a battery that cause charge separation, and so this sentence serves mainly to caution the reader that other reactions are present as well, and have observable everyday effects. Brews ohare (talk) 16:58, 22 June 2009 (UTC)

The connect to the topic appeared to be tenuous, esp. lacking a source that would make that connection. Putting it back with reference to source discussing self-discharge in the context of battery emf seems like a good idea. Dicklyon (talk) 21:16, 22 June 2009 (UTC)

Formal definition of electromotive force

I find the replacement of the formal definition originally present:

"The EMF of a source (electromagnetic, chemical, thermal or otherwise) may be defined as the work done by an external agent, per unit charge, with sign reversed, in bringing a test charge once around a circuit that contains the source and no other source."

is not equivalent to the present definition:

"The emf along a path between two points A and B is the integral of the electric field aligned with the path."

The problem with the present definition is that the conservative portion of the electric field contributes to this definition, and is not part of the EMF. That is why Griffiths and others use a closed path.

The closed-path definition used later in the same section contradicts the leading formal definition. Brews ohare (talk) 16:50, 22 June 2009 (UTC)

That's exactly what I'm confused about, too. It's not clear to me how the the later one contradicts the first one, but it seems likely that at least in some cases they are incompatible, like for a resistive loop with magnetic induction in it. Definitions in the literature are not mutually consistent. That's why I added the quote in the lead, so that the reader will at least know that there are several conceptions that go under the name emf. If we can include several sourced definitions, even if not mutually consistent, and say how they relate to each other or to various points of view, that might be a good way to increase the value of this article. Or if there's a dominant or standard definition that we can base the article on, and explain variations relative to that, that might be OK, too. So far, I haven't found a description that I'm able to interpret sensibly for both the chemical and transformer types of emf. Dicklyon (talk) 21:23, 22 June 2009 (UTC)

Transformer emf

Transformer emf is also an agency that can cause charge separation, as evidenced in Faraday's law of induction that includes both motional and transformer emf. In transformer emf, for instance, a coil in open circuit configuration subject to a ∂B / ∂t sees a nonconservative E-field (one with a curl) that drives electrons to one end, and leaves a positive charge at the other end of the coil. This is the charge separation that results in a conservative E-field that drives current if the coil is attached to a resistor.

Also, if one integrates around a closed curve to find the work on a test charge, the conservative E-field produces a zero contribution because it contributes the same single-valued electrical potential at the start and at the end of the closed path, while the non-conservative E-field will contribute the negative of the transformer emf. Brews ohare (talk) 20:12, 22 June 2009 (UTC)

Those explanations make sense individually. But in a loop that's not open circuited, there's no charge separation, right? Say a loop of resistors. What does it mean to have an open-circuit loop that you integrate around anyway? And how does the loop integral work with a battery in a circuit with a resistor? Isn't the loop integral zero? Or how do you determine what to integrate in that case? I understand the concept that if you're doing the loop integral you want to separate the energy input mechanisms from the energy output or dissipation mechanisms, but what definition makes that work? Dicklyon (talk) 21:29, 22 June 2009 (UTC)

Let's try a motional emf. A segment of wire moves in a fixed B-field. The Lorentz force produces an emf, and charge separation occurs. If we imagine the build-up of this charge as the motion begins, it steadily increases until the conservative electric field from the charge at the ends of the wire is large enough that the eE force on the electrons inside the wire balances the Lorentz ev × B force. At that point we have the open-circuit voltage at the ends of the wire. If now a resistor is attached across the ends of the wire, a current flows. That current is attempting to restore the charge neutrality condition in the wire. As charge leaves one end of the wire to travel through the resistor it has to be replaced, or the wire ends return to charge neutrality. The motion of the wire tends to maintain the charge difference, but due to the "internal resistance" of the source, the actual voltage between the ends of the wire drops a bit. How much it drops is clearly a balance between how much current the emf can supply and how much the resistor takes away. So, for example, if the moving conductor is a cheese grater, a different amount of current is supplied by the motion of the cheese grater than if it were a solid copper plate. Perhaps the literature on generators or motors has such a discussion somewhere? Brews ohare (talk) 23:29, 22 June 2009 (UTC)

Let's try a battery: initially a chemical reaction occurs driven by the lower energy of the charge separated ionic products. The charge separation creates a E-field, and this field grows until it is no longer energetically favorable for the reaction to continue because the work done against theE-field is equal to the energy reduction due to the reaction.Then the reaction is arrested, except for self-discharge effects. Then we have the open-circuit voltage at the terminals. If a resistor is placed across the terminals, a current flows trying to re-establish charge neutrality on each terminal. The steady-state voltage across the terminals occurs at a value that makes the current supplied by conversion of the reactants equal the current through the resistor. Brews ohare (talk) 23:40, 22 June 2009 (UTC)

Yes, that's all clear, but doesn't answer my question. If we take emf to be a two-terminal path integral, it's OK. But in the loop integral, how do you ever get other than zero for a circuit with a battery? Some authors evidently try to separate fields into parts that associated with emf and parts that are not, but I don't see you doing that. So if the emf in the loop is zero, like KVL, that's OK, but then we still need the two-terminal path integral to say what the emf of the battery is, no? My point is that the loop integral can't be the whole story -- and in many sources, it's not, as they rely primarily on the A to B integral. Dicklyon (talk) 00:03, 23 June 2009 (UTC)

I'd say that within circuit theory you always get zero. The emf doesn't show up but the voltage generated by the emf does. That is how Kirchhoff's laws work: the sum of the IR drops = sum of the voltage from voltage sources. What you are asking, it seems to me, is how do you calculate what the voltage provided by a source of emf is. For a battery that seems to be an exercise at bottom in electrochemical reactions and thermodynamics. For a loop moving in an electromagnetic field, it can be done by using all the Maxwell equations and not just a macroscopic subset like Kirchhoff's laws. Is that the point? Brews ohare (talk) 00:23, 23 June 2009 (UTC)

Yes, sort of; but how to calculate is probably easy, once we have an adequate definition that covers those cases clearly. Dicklyon (talk) 06:52, 23 June 2009 (UTC)

That version of KVL seems like a tautological way to avoid defining emf or sources; I see the KVL article has a patch for transformer emf, but it doesn't say anything about batteries. Of course, that could be because of my revert] of an unfamiliar and unsourced version of the law that sounded like nonsense at the time, when I didn't know people had all these conceptions of emf different from voltage. Dicklyon (talk) 07:02, 23 June 2009 (UTC)

some sources

Sources provided by Brews ohare; comments by others:

• charge separation and emf
• charge separation and emf
• charge separation and emf
• moving rod in magnetic field – good stuff on magnetic emf, yet the indirect definition on p.795 seems to exclude magnetic induction. p.898 introduces "induced emf" without definition. Dicklyon (talk) 16:47, 23 June 2009 (UTC)
• emf as energy conversion – same book as above, the p.795 that I commented on. Dicklyon (talk) 16:55, 23 June 2009 (UTC)
• emf is not voltage – this page acknowledges imprecise use of the term, applies a definition that excludes the magnetic or tranformer emf, and states that confusion does not result; I don't see the word "voltage" on that page. Dicklyon (talk) 16:45, 23 June 2009 (UTC)
• emf as non-electrical energy
• emf as non-electrical energy
• emf and line integral
• emf and contact potential
• emf and contact potential
• emf and contact potential

Brews ohare (talk) 04:22, 23 June 2009 (UTC)

Would you say that these represent a variety of viewpoints on emf? Or a consistent viewpoint? If the former, we should attempt to identify a few to report, and if the latter, we should figure out a good definition that represents both the battery and the closed-loop induction cases. I've looked at a lot of sources already, but am at a bit of a loss to interpret it all. Dicklyon (talk) 06:51, 23 June 2009 (UTC)

Ross quotation

"To some authors it is synonymous with 'voltage.' To others it means the open-circuit voltage of a battery. To a third group of authors it means the open-circuit voltage of any two-terminal device. This use is met most often in connection with Thevenin's theorem in circuit theory. To a fourth group it means the work accounted for by agencies other than differences of the (not measurable) Galvani potentials. Such authors equate the current–resistance product of a circuit branch to the sum of voltage plus e.m.f. A fifth group extends this use to field theory. The authors of this group equate the product of current density and resistivity to the sum of electric-field strength plus an e.m.f. gradient. A sixth group applies the term to electromagnetic induction. These authors define e.m.f. as the spatial line integral of the electric-field strength taken over a complete loop. To them the term 'counter e.m.f.' means something. We therefore think it advisable to avoid the term e.m.f. altogether."

The author's conclusion here obviously is not going to happen: emf is a very strongly established term and is not going to go away. Besides, it is a useful concept. I will argue below that this author is making too big a deal about controversy, and that there is no real division in interpretations of emf. He lists 6 usages:

1. emf = voltage; this is just sloppy usage that crops up because the potential difference generated by a source of emf happens to have the same numerical value as the emf. Voltage refers to a potential difference, and is due to the conservative portion of an E-field. On the other hand, emf is an expression of conversion of energy from some other form (e.g. chemical bonds) to electrical form as a potential difference caused by separated charges. See #some_sources. This confusion in terms was noted in an earlier version of the Wiki article, but this caution was removed as conjectural.
2. emf=open-circuit voltage of a battery; general usage does not restrict emf to apply only to a battery, and even within this restriction, no-one restricts it to the open-circuit voltage. Also, it isn't a voltage.
3. emf = open-circuit voltage of any two-terminal device; not a common case, and emf is not a voltage
4. emf=an extra term in Kirchhoff's voltage law: never heard of such a thing. Kirchhoff's law is an expression of the conservative E-field component.
5. emf as emf gradient: this terminology shows up in the theory of semiconductor devices like diodes, for example. In this context it amounts to the gradient of the quasi-Fermi level. The quasi-Fermi level notion is an attempt to incorporate the non-electrical forces driving a current due to variations in material composition. For example, the built-in voltage in a pn-junction is said to be due to emf differences, the same idea as the charge-separation notion in the customary definition of emf.
6. emf = electromagnetic induction: this is a reference to Faraday's law of induction, which is not a different definition, just a different source of emf

I'd say this author is (i) making a mountain out of a mole hill (ii) has confused sources of emf with emf itself (iii) confuses voltage with emf and (iv) actually has omitted the most common and accepted definition of emf, namely the one that originally appeared in the article:

Electromotive force, or more commonly, an EMF, is a term used to characterize electrical devices that supply electrical energy to a circuit, such as voltaic cells, thermoelectric devices, solar cells, electrical generators and transformers.[1] The EMF of a device is the energy per unit charge provided by that device to the circuit.[2] Thus, for a given device, if an electric charge Q passes through that device, and gains an energy W, the net EMF for that device is the energy gained per unit charge, or W/Q. EMF has SI units of volts, or joules per coulomb.

I would recommend deletion of the misleading Sydney Ross quotation and reversion to the original definition or perhaps some more directly worded version from #some sources. The Sydney Ross quote is only a note to an historical essay by a non-scientist, is not supported by citations or discussion, but only the author's say-so, and is inaccurate. If the supporting citations for the original definition are deemed insufficient, more are found on this talk page at #some_sources. Brews ohare (talk) 13:26, 23 June 2009 (UTC)

Brews, the work you've undone was all based on sources. You can't just remove sourced stuff and replace it with a different interpretation without telling us the source. The discussion above was left hanging, without answers to the questions or sources for the answers. You're asserting a particular POV, not even well defined yet as far as I can tell, and calling sources that dissagree confused. You add orignally added the statement that "Occasionally, EMF is confused with the electrical voltage that it generates" without a clue as to why you said so. The quote from Ross was what I found when I looked for what was behind this statement. Ross rings true, because when I search sources I find a variety of interpretations and definitions, not a uniform POV based on an accepted definition. If I've got this wrong, show we which sources you consider to be definitive, and we can at least mention the others as alternative POVs, and then when we're clear on what the POVs are we can work on figuring out how prominent each is. You've embarked on a string of controversial edits. Slow down and work on getting a consensus, we don't have to reset back to a sourced state. Dicklyon (talk) 15:00, 23 June 2009 (UTC)

In particular, the opening definition, "the external work that must be expended to produce an electric potential difference" doesn't apply to the resistive loop where there are no potential differences, does it? And "The electric potential difference is created by the external agency by separating positive and negative charges, thereby generating an electric field" is also inapplicable in the case of a loop with equal potential and charge density all the way around it. This edit "delete confusing material" removes the move common formula and definition that I found in sources, and didn't say why it's confusing. So let's discuss please. Dicklyon (talk) 15:05, 23 June 2009 (UTC)

Dick: I've removed the Ross quote for reasons given: please indicate what specifically you object to in this list of reasons. I also eliminated the suggestion that the integral of E between two points in an open loop is emf. That is simply wrong (only the non-conservative E contributes) and contradicts other things in the article. I'll discuss it further with you if you like, although I believe the discussion already given demonstrates that fact conclusively and it is sourced.

The definition is not mine, it came from the source. Can you flesh out the "resistive loop" example for me? - I don't get it. Likewise the "loop with equal potential and charge density all the way around it" looks like an exception, but I cannot imagine what it's about. Brews ohare (talk) 15:18, 23 June 2009 (UTC)

It seems that you either do not accept the definition in terms of an agency causing charge separation, or think it is one of many. You have suggested at some stage that Faraday's law of induction is an exception to this definition, and I have explained that it is not. Do you accept that there is no exception here? I am highly skeptical that you can actually come up with any sourced example system that does not fit the definition, although you may be able to find some verbal expression of opinion that is simply the conjecture of a misguided soul like Ross. Brews ohare (talk) 15:30, 23 June 2009 (UTC)

Under some definitions, the integral of E from A to B is not emf; in others it is. Ross is the only source I've found that has surveyed the usage of emf in primary and secondary sources and did an analysis of its various meanings. Why would you dismiss this analysis out of hand? The reasons you give are that you disagree with some of the usages he found; yet he found them, he says, and why would you doubt that? I find them, too (maybe not all 6), which is why it's so hard to write a definition that works for everything. I understand where you're coming from, the emf is just that voltage that represents externally applied energy, but that's not the only definition that's common in sources, and isn't even a definition that has been made clear anywhere, as far as I can find. And you keep saying "emf is not a voltage"; what does that mean, for a quantity measured is volts? Dicklyon (talk) 15:41, 23 June 2009 (UTC)

Hi Dick: I have given my reasons, and asked you to point out specifically where you disagree. Please do that. Please provide me with links to alternative definitions that support Ross's confusion. In doing so, please supply example systems not mere comment, and please avoid the 19th century, which is what Ross is reporting on. (As you know, the notion of emf used by Maxwell, e.g., is not the modern view.)

Your view that Ross "has surveyed the usage of emf in primary and secondary sources and did an analysis of its various meanings" doesn't seem to apply to the cited work by Ross, where no such discussion occurs in his note on emf, and he says explicitly that he has deliberately avoided the use of the term emf throughout his article.

The view that "emf is not a voltage" can be found in #some sources, and is consistent with the sourced definition. As you know any energy divided by e becomes volts. There is no reason to think that means anything measured in volts is a voltage (that is, an electrical potential difference).

Again, please answer my questions about examples that are exceptions to the sourced definition I've cited. Brews ohare (talk) 15:54, 23 June 2009 (UTC)

Here is a source written for translators, specifically describing different uses of the term emf, some of which are voltages. It specifically states that the usage in electrochemistry is a different one from the use in electromagnetics. I'm not saying it's impossible to unify these, but this scholar, at least, didn't see it that way. I was not familiar with the distinction you asserted that, that "voltage" always means "electrical potential difference", but that may be so; if so, it seems to conflict with the definition you provided in the lead, "external work that must be expended to produce an electric potential difference" which appears to define emf as a voltage by that definition. Dicklyon (talk) 16:38, 23 June 2009 (UTC)

Well, Dick you now have an ally in Crispmuncher. Therefore, rather than battle against these odds, I will retire from this effort with these comments to your last communication:

I think you should interpret his intervention as basically a procedural objection, not a support for my content position. If you can work with us via normal procedures, your help will be welcome. But if you won't, then retiring from this article is the right move. As he mentioned, he's had trouble with your procedure before; so have I, and I'm not going to sit idle while you do to more articles what you did to speed of light and others. Dicklyon (talk) 17:24, 23 June 2009 (UTC)

1. It is not only "possible to unify" the E&M definition taken from Faraday's law of induction, this unification has been outlined specifically in the article (at least in some of its forms) and in discussion with you. Your continuing failure to address this point despite requests to do so is peculiar.

Can you say what source supports that unification clearly? I've been doing my best address exactly this point, but you have been failing to follow up my questions, probably because things are just moving too fast. Dicklyon (talk) 17:24, 23 June 2009 (UTC)

2. There is no doubt that some mechanisms (e.g. a battery or an electrical generator) can creates a charge separation that also creates an electrical potential difference. That does not mean the agency creating the separation is a voltage; it means the voltage is the result of the charge separation. That appears to me to be straightforward logic. However, if you doubt it, you can break down and consult the references in #some sources. Brews ohare (talk) 16:57, 23 June 2009 (UTC)

It's unclear to me what this means: agency is not a voltage. I have not introduced the notion of an agency here; what source does that? I think I get your point that the emf represents the work done on charges in a circuit, from external agencies; what I don't see is a good source that explains that in a way that answers my questions about both batteries and magnetics, without tautology, and works in general; that's why I keep asking. What I do see is lots of sources with other interpretations, including tertiary sources explaining that there exist other interpretations. Let's try to represent sources fairly and clearly, so others will be able to verify that what we write is true, or at least backed up by reliable sources, rather than idiosyncratic interpretation. Dicklyon (talk) 17:24, 23 June 2009 (UTC)

It is not for him to justify his actions. He is not the one removing references in favour of unsourced statements, or removing details of disputes and disagreements in favour on one particular arbitrary view. You might want to consider your general editing style: I first encountered you recently on speed of light and now I see that you are doing much the same thing all over the physics articles. Think carefully before making any changes. Don't read something, make some changes and then discuss whether you have understood it correctly. Get the groundwork done beforehand or else even legitimate improvements to an aricle are likely to be reverted. I'll remind you you have already received a warnign for disputive diting - take that on board.

In regards the substance of this article you seem to find abhorent the equation of voltage and emf. Although I know that many people do this, personally I don't disagree with you - it is akin to stating that length and metres are the same thing. However in your recent edits (which I reverted) you seem to equate work and force. That is not really up for debate as they are clearly different. If you understand this then in debating all and sundry on multiple articles at once you inevitably lose precision. Another reason to slow down in favour of more considered edits rather than incoprating whatever it is you have just read on Google with no selectivity whatsoever. CrispMuncher (talk) 16:44, 23 June 2009 (UTC)

To conclude that work and force is equated in what you deleted is just sloppy reading. Please show specifically how you come to that conclusion, which is not implied by any logic that I can think of. Bear in mind that all this material is sourced by citations to textbooks, so be careful. Brews ohare (talk) 16:56, 23 June 2009 (UTC)

Reversion without comment to erroneous conception of emf

Crispmuncher: The material you restored includes statements that are at variance with the literature as found in the section #some sources. How about defending your views on the Talk page instead of intervening in an uniformed manner on the article page? Please provide any exceptions you know of to the very conventional and sourced definition I provided, viz:

In physics, electromotive force, emf (seldom capitalized), or electromotance is the external work that must be expended to produce an electric potential difference. The electric potential difference is created by the external agency by separating positive and negative charges, thereby generating an electric field.

Brews ohare (talk) 16:33, 23 June 2009 (UTC)

BTW you sloppily also failed to proof-read your changes which now include a hodge-podge of undefined notation for the line integral. Brews ohare (talk) 16:41, 23 June 2009 (UTC)

Did it ever occur to you that I was in the process of writing some comments here before you instantly reverted and cried that I had not replied? A detailed rebuttal takes time. This is why I eventually gave up at speed of light. Indeed I have now had to enter all this text three' times as a result of your rapid-fire editing. CrispMuncher (talk) 16:44, 23 June 2009 (UTC)

C, thanks your your help. By the way, when you get an edit conflict, you can usually just copy your text from the lower edit window into the right place in the upper window and save; no need to retype. When you have several comments interspersed it's more complicated, but you can at least copy and save what you've typed, like I just had to do... Dicklyon (talk) 17:02, 23 June 2009 (UTC)

I made no reversion whatsoever. The article remains erroneous and in contradiction with sources at #some sources. The article retains the sloppy editing errors introduced by hasty reversion by Crispmuncher and his/her failure to proof read his/her changes. Brews ohare (talk) 17:40, 23 June 2009 (UTC)

The definitions in the sources are vague and contradictory. For example, one definition says "the external work that must be expended to produce an electric potential difference." So if the emf source is a secondary battery, is it the "external work" the reduction in stored chemical energy? Is it an artificial concept where one models the battery as a Thévenin equivalent circuit and the external work includes both the energy dissipated in the circuit-under-test, and in R_th? Or perhaps it is the pro-rata share of the energy that was used to charge the secondary battery. Some of the sources describe the emf from a transformer winding, but does not seem to include energy dissipated in the resistance of the winding.

I suspect when all is said and done, emf will turn out to be a vague concept for which there is no single rigorous definition. --Jc3s5h (talk) 16:55, 23 June 2009 (UTC)

That seems to be what many of the sources say; I suspect there's also a crisp concept here, corresponding to what Brews has in mind, if we can find an adequate definition and sourcing; there's no reason we can't represent both of these situations, and/or more. Dicklyon (talk) 17:02, 23 June 2009 (UTC)

I do not agree that the sources are vague and contradictory. It seems pretty clear to me that if you separate two opposite charges, work must be done. If that work is done by motion in a magnetic field, or by chemical rearrangement of some ions in a molecule, or by charge transfer across a pn-diode, that agency is an emf that results in conversion of some energy stored one way into energy stored in the separation of charges. There is nothing vague about this. Brews ohare (talk) 17:07, 23 June 2009 (UTC)

So now emf is an agency? What about the resistive loop in magnetic field? There's no charge separation, no variation in charge density, no variation in potential. What definition of emf makes it work for this case? Conceptually it's clear that the emf is equal to the voltage that would drive that amount of current in the loop; but what source provides a definition that leads to the right answer here? Dicklyon (talk) 17:14, 23 June 2009 (UTC)

So an agency creates an emf. Do we need a lawyer here? Brews ohare (talk) 21:05, 23 June 2009 (UTC)

EMF in closed resistive loop

I'd be happy to engage you here, but I'm unsure of the set-up. Am I to move a closed wire loop in a fixed B-field, for example? That is an example of Faraday's law: an emf is generated according to his law. The current that flows as a result of the emf depends of course upon the resistance of the loop.

The example you take can be viewed as a limiting case, I suppose. We can break the loop, obtain a charge separation, find the open circuit voltage. Then we reduce the resistance of the break from infinity to a finite value. The current that flows depends upon the resistance of the break. Of course, this current also flows in the loop, and so generates a voltage drop (a case of what is called internal resistance). That drop reduces the voltage the loop places across the break. The steady-state voltage across the break is the one that results in the same voltage across the break as the open-circuit voltage less the internal IR drop in the loop itself.

That example seems not to cause any difficulties, do you agree? Brews ohare (talk) 17:29, 23 June 2009 (UTC)

By a resistive loop I meant a physical "wire" with distributed resistance, such that the potential is everywhere equal. I can see how one could make a lumped-circuit approximation, breaking the loop into point resistances and extended conductors, such that the voltages induced across the wires would be dropped across the resistors; this would allow separating emfs from voltage drops, measuring each with voltmeters, etc. But the question is this: what sourced definition of emf applies to this (distributed, not lumped) situation? Dicklyon (talk) 17:35, 23 June 2009 (UTC)

I believe the example discussed above fits the description of a "physical wire with distributed resistance". The loop in the example can have any resistance distribution along its length one might like, and the "break" can be tiny. In the limit that the R of the break approaches the R one would like, a closed loop of arbitrary resistance variation results. A lumped circuit approximation is not necessary, although I think such an approximation still would establish the point: the EMF is that of Faraday's law, which applies to any closed loop regardless of its resistance. The EMF of Faraday's law is therefore the same as that of the loop with a tiny break of infinite resistance. In other words, in open circuit, the EMF of Faraday's law is opposed exactly by the open circuit voltage due to charge separation. (That is why no current flows in the open-circuit case.) In closed circuit, the EMF of Faraday's law is reduced by the internal voltage drop due to internal resistance of the loop. Brews ohare (talk) 17:48, 23 June 2009 (UTC)

Quotes about emf

Some ideas at variance with the introduction to the present article: they all say emf causes a voltage difference, and is not itself a voltage difference.

• Cook "Although the definition of ε is very similar to the integral giving a static potential difference ΔV about a closed path and both quantities are measured in volts, ε and ΔV are physically very different; ε is in general not equal to zero and hence arises from nonconservative force fields while ΔV us necessarily always zero and corresponds to a conservative force field." This source oddly is cited as support in the introduction when it actually contradicts the introduction by saying emf is not voltage.

I had actually added that source on the opening sentence in support of the alternative term electromotance, and hadn't really looked at what else it said yet. Now that I look at it, it is the closest I've seen to a workable complete definition to support your preferred definition, as it includes specifically the Faraday induction term and some terms (not very well defined) related to electrochemical and thermal processes. I think we can use this; not that it's the only definition, but it's as close as I've seen to the definition you would want, yes? Dicklyon (talk) 21:39, 23 June 2009 (UTC)

Take a shot at it , Dick. It might work. Some later discussion also needs revision to fit this change. Brews ohare (talk) 22:25, 23 June 2009 (UTC)

• MacDonald "the 'battery-like' EMF must have pushed positive charges in the horizon toward the equator and negative charges toward the pole, until the charge separation created sufficient electric field of its own to counteract the EMF ..."
• Lerner "An electric potential difference is produced by a system that does external work on electric charge. ... The external work per unit charge that must be expended to produce an increase ΔV in the electric potential of the charge is called the electromotive force.
• Halpern & Erlbach "To maintain the steady flow of charges therefore requires an external source of energy that in effect takes charges leaving one end and brings them back to the other... The external source, in effect, maintains a net positive-negative charge separation between the two ends of the conductor, which of course is what causes the potential difference to be maintained. ... The energy per unit charge supplied by the external source in maintaining the voltage is called the EMF ("electromotive force"), and it is the EMF that replenishes the electrical energy lost as the charges flow within the conductor."
• Bhatnagar "In order to maintain continuous flow of current, the positive charge which arrives at the negative terminal must be pushed ... to the positive terminal by some non-electrical forces acting within the seat of emf."
• MeenaKumari "Sources of emf include electric generators, batteries, fuel cells etc., which all convert non-electrical energy into electrical energy." Brews ohare (talk) 18:43, 23 June 2009 (UTC)

I contend that these sources clearly override the unsupported opinion of an historian (discussing the 19th century concepts of Volta) following his personal and undocumented review of whatever (uncited) literature on emf he may have run across. He says so far as he can tell the term is so muddy nobody should use it, which might be taken as fact, or as simply an admission that he never did get the concept. Certainly many modern authors use the concept and have a shared understanding of its meaning. Brews ohare (talk) 19:01, 23 June 2009 (UTC)

Actually, I don't think it can be over-ridden. I can find sources to support at least several of the interpretations he cites; just because there are a lot that do it one way, doesn't mean there aren't other ways. But I'm willing to strike a balance. I don't think we can say "anyone who calls an emf a voltage is simply confused" or anything to that effect; instead, let's state what the alternative interpretations exist, and which main interpretation the article will focus on. It doesn't have to be hard. Dicklyon (talk) 01:23, 24 June 2009 (UTC)

Some others, that at least illustrate that some authors think emf is a voltage: Dicklyon (talk) 02:16, 24 June 2009 (UTC)

• Croft Electromotive force, abbreviated e.m.f., and sometimes called voltage, electric pressure, or difference of potential, is used to designate the "push" that moves or tends to move electrons... (if it's "sometimes called voltage", is he saying it's a voltage? is he saying it's a difference of potential, or is he just saying it's sometimes called that? this book also has lots of illustrations of hydraulic analogy, which someone was looking for for the Ohm's law article.)

• Karmel et al. section ELECTROMOTIVE FORCE (EMF) OR VOLTAGE really does treat the terms as equivalent

• Hann the term orignally stood for electro-motive force and implies the voltage generated by... (definitely he's saying it's a voltage; but not that all voltages are emfs) and a different meaning of emf occurs in the field of Electro-Chemistry (but he doesn't really explain, so may actually be confused in this case) and despite its rather nebulous definition and apparent double-meaning, English-speaking technologists are reluctant to discard the term and emf in favor of simply voltage, ... (gives silly reason).

• Dorf induced voltage or emf (this phrase with which he labels a formulaic result can hardly be consistent with saying that emf is not a voltage)

• Edmonds has a clear definition for the magnetically induced emf: The line integral of the electric field around a given path is called the electromotive force or emf. He later notes it is similar to a voltage change but doesn't say it is one. And as soon as he mentions battery he stops talking about emf, maybe because he realizes that his definition doesn't work for a battery-driven circuit.

Hi Dick: Dorf does not really support the view that voltage=emf; he uses a closed path and says induced voltage = emf, by which he means the voltage from Faraday's law of induction. Likewise, Hann refers to a "near" synonym and says it implies " the voltage generated by a particular chemical or magnetic arrangement." Croft (1917) refers to emf apparently as either voltage or emf from Faraday's law, depending upon the example discussed. Hence, he doesn't really come down on either side. Likewise, Karmel uses a line integral between two points and says "in general the voltage depends upon the path chosen" and later makes the inconsistent statement "In later chapters we will discover the different sources of emf other than batteries which provide potential difference between their terminals as a result of internal chemical reaction." and adds "Eq. 3,21 is 'general enough' (my quotes) in conventional electric circuits." The fact is, you cannot get a battery emf from a line integral through the battery because taking a charge through a battery involves a problem in quantum statistical mechanics to find how the energy of the entire ionic-electronic system changes with position of the charge, and calling that a line integral is sweeping a lot under the rug.

As for Edmonds, I'd say I agree with him that Faraday's law produces emf, and I'd say we have sources that say this case is not different from a battery: an external agency creates a charge separation and the work done in doing so is the emf.

So what have we got here? We have two possible supports for the emf=voltage idea, one rather ambivalent about it (and rather dated) and one (I'd say) far from clear. Neither is a straightforward endorsement of the emf = voltage concept. Both might be explained as simply a bit sloppy. In the Wiki article I'd say that they have to be portrayed as a marginal and ambiguous view. Brews ohare (talk) 02:45, 24 June 2009 (UTC)

I think we can report it as "another view" without necessarily judging it as marginal or ambiguous; we can also balance with sources that specifically say emf is not a voltage. Dicklyon (talk) 03:33, 24 June 2009 (UTC)

Well, I suppose that it can be brought up, but this idea has no merit from either a calculation or intuitive viewpoint, as the idea of voltage handles the conservative part and the "separation of charge" emf handles the rest, leaving no role for the combined form except to increase the level of confusion and ambiguity. Brews ohare (talk) 05:33, 24 June 2009 (UTC)

Rojansky source

In the intro, Rojansky is cited as a supporting source for saying emf is the same as voltage. This source almost supports the article. It does equate emf to integral of E-field between two points, but I'd say that if one feels impelled to mention this view, it should be flagged as a minority opinion (possibly the opinion of one). I say that for these reasons: (i) This view is not compatible with Kirchhoff's law, except in special cases (namely the cases where the emf is zero). (ii) This view is not compatible with the "separation of charge" definition, which is the dominant view. (iii) Rojansky explicitly limits this discussion on p. 187 to the case where there actually is zero emf according to Faraday's law - he suggests a "redefinition" is necessary when a Faraday's law emf is present. (iv) Other sources of emf (besides Faraday's law) don't come up for discussion - since Faraday's law also is excluded by Rojansky himself, his definition of emf actually applies only where other authors would say there was no emf at all. Brews ohare (talk) 20:23, 23 June 2009 (UTC)

Actually what he says is that emfs "are also called voltages" which is not quite as strong as what you said he said. His open-circuit approach certainly does lead to a potential difference equal to the emf, right? But he refers to the electrostatic case as a special case and also introduces the loop integral; I presume the E in that includes magnetic effects, as opposed to being merely the potential gradient? Not clear. Anyway, I find an awful lot of such difficult-to-interpret sources. I'm not really to decide that some of them are just daft. I think it's more like there's no clear definition, so everyone makes up one that suits them, and they vary. Dicklyon (talk) 02:25, 24 June 2009 (UTC)

No, his open circuit approach mixes up the conservative and non-conservative contributions, except his caveats actually exclude any case where real emf's occur. He also disqualifies himself by saying his definition is not general and has to be modified to take Faraday's law into account. He also has not discussed batteries per se.

You may be anxious to adopt the "it's all confusing" stand point to avoid adopting a sharply focused definition that you are not yet confident in. That confidence can be increased by thinking about it more. Or perhaps we have to find an irrefutable source so you don't have to think about it.

The unsourced assertion of confusion ("Occasionally, EMF is confused with the electrical voltage that it generates...") was inserted into the article by you; I took it out. If confusion exists (which it might), then our job is to represent the points of view involved, whether they confuse or not. I'm certainly not looking to adopt an "it's all confusing" stance, just trying to understand the landscape well enough to report it accurately. I'd also like to report a sharply focused definition that works, but so far I'm not sure I've seen it. Your "formal definition" with the loop integral of the force per charge is pretty good if you can define what part of the force per charge it refers to; it seems to require more definitions to motivate a way to separate those forces due to external agencies from those not. How do you do that, e.g. how do you split up the fields or forces in a battery? I find the definitions rather lacking still. Your "vector field F represents the force per unit charge on a charge carrier" didn't even try, and left a definition that gives zero without magnetic effects, and therefore doesn't work for batteries, right? Dicklyon (talk) 03:26, 24 June 2009 (UTC)

If you insist upon presenting the view emf=voltage you are going to have to say that no-one really says it outright, but a small minority of sources appear to imply it is a possibility in some cases, and certainly it is not the generally accepted view. The use of a line integral over a line segment is to be avoided: a closed path is the way to go, and it is best restricted to the Faraday's law case because the line integral cannot be calculated through a battery. Brews ohare (talk) 02:49, 24 June 2009 (UTC)

My view is not emf=voltage. That's just one of the things in the literature that needs to be represented in the article. My personal views don't enter into this (though of course my understanding and interpetation of the literature does, but I'm pretty open minded there, as I feel like there's good stuff to be learned here, historically and usage wise).

I did not intend to say it was your view, but that it was a view you wanted to see represented. I shudder to hear there are other issues as well, but maybe this one can be settled first? Brews ohare (talk) 05:29, 24 June 2009 (UTC)

And why can't a line integral be calculated through a battery? Do you mean as opposed to a closed loop through a battery? Or what? Dicklyon (talk) 03:26, 24 June 2009 (UTC)

Well, I'm inclined to say something in words like "an external agency creates a charge separation and the work done in doing so is the emf", restrict the line integral discussion to the Faraday's law case where it is simple to evaluate because the Lorentz force is simple, and use thermodynamics for the battery. I don't know how to calculate the portion of a line integral of E·dl that sits inside the battery. Some macroscopic simplification like thermodynamics that can handle energy in multiple forms is more tractable, and seems to be the way the battery is most commonly handled. See the article on this. Brews ohare (talk) 05:29, 24 June 2009 (UTC)

The line integral may not be useful as a definition to calculate to, but conceptually it's no problem, I think. The problem comes when you have a circuit with current flowing, and the forces that are part of the emf may be hard to distinguish (via definition) from forces due to resistance (or electrostatics and Ohm's law). The definitions suggest that some part of the force, or field, is supplied by external agencies; but in a loop with e-field integrating to zero, how does one really define or determine what parts of the e-field are from external agencies and what parts aren't? Is just that you have to agree to agree that you know what part is external, part of emf, and what is not? I remain a bit confused about how to clarify, in a way that works for all kinds of emf. Dicklyon (talk) 06:22, 24 June 2009 (UTC)

Needing a break

Brews, take over for a while, now that you have heard my point. Try to keep one main interpretation in focus without excluding the others. And don't do so much at once that nobody can intervene and comment as it goes. Please. Dicklyon (talk) 06:17, 24 June 2009 (UTC)

Revision details

The introductory section contained a lot of repetition and some self-contradictory elements. The introduction now contains the most prevalent view of emf, and other views are summarized in the section Terminology.

The discussion of units is consolidated under the section Notation and units of measurement, which existed before, but was not the only place where this was discussed.

The line integral formulation has been extended using the formula of Cook in the section Formal definitions of emf. The usage of a line integral across a line segment has been deleted, as it contradicts the Cook formulation and is (as far as I know) advocated only by Rojansky, who has at best a minority view. (I'd say a useless view).

The quotation by Ross has been trimmed. As discussed on this Talk page, he rather exaggerates the importance of variations in usage.

Several sources have been added. Brews ohare (talk) 16:57, 24 June 2009 (UTC)

So you punted on the opening definition, making it an open-circuit definition instead of the loop integral definition? Too hard to find a unifying definition? Dicklyon (talk) 03:47, 25 June 2009 (UTC)

I regard the "charge separation" definition as the guts of the matter, and the closed loop integration as a follow-up theorem or corollary. It is presented under Formal definition, which may not be the best title. Brews ohare (talk) 09:42, 25 June 2009 (UTC)

Why the italics on emf? Dicklyon (talk) 06:42, 25 June 2009 (UTC)

There was a mixture of emf and emf so I made them all the same. I guess I felt emf emphasized that it was not a word, but an acronym. Brews ohare (talk) 09:42, 25 June 2009 (UTC)

Why the partial of the Sydney Ross quote, omitting his analysis of the different meanings in the literature, yet keeping his statement of one generalized meaning? Sort of defeats the purpose of quoting a historian. It seems a fair characterization of the variations that we encounter among the sources. Dicklyon (talk) 06:54, 25 June 2009 (UTC)

Ross quotation: reprise

I've made comments earlier about this quote. To summarize, the quote is based upon an aside by Ross to justify his avoidance of the term electromotive force, and is not a scholarly assessment of the history of the term. As such, I don't regard it as an historian's assessment, but as an off-the-cuff summary. You will notice that he does not cite any literature to support his statements, nor does he provide any discussion to indicate he has weighed the merits or importance of the different usages. In fact, I think it is a pot pourri he dredged up, and many of the usages are not significantly employed, and some are equivalent although he has listed them as distinct. The Wiki article does describe the main alternative usages in a balanced manner, and treats the Faraday's law and electrochemical emfs as examples of the same charge-separation definition, rather than following Ross and treating them as different. Quoting the full Ross remarks would then involve the article in historical debate, which I don't think is warranted as Ross does not support his views.

I added a number of sources that make different use of emf than the opining definition. Mostly they are from circuit theory, and are focused on solving circuit problems rather than carefully examining how emf and voltage are related.

I'd be happy to omit Ross altogether and use Graneau, the other source. Brews ohare (talk) 09:42, 25 June 2009 (UTC)

If you find someone to back up your "I don't like it" argument, I'll consider it. Until then, it's in – it's one of the few analysis of usages that we have, and appears at least to be a rather careful analysis of a wide range of literature. Dicklyon (talk) 04:54, 26 June 2009 (UTC)

Solar cells

The solar cell bit had some confusions, including in the lead, so I simplified the statement in the lead and worked on the description of how it works. It's not as different as it was saying. Dicklyon (talk) 08:03, 25 June 2009 (UTC)

I'm no expert on solar cells, and the solar cell article isn't much help. I'm somewhat concerned over whether this example opens a Pandora's box of emf sources that have no connection to the opening definition (or any of the others). The current is driven by diffusion, and the photo emf seems to be incidental. What do you think? Brews ohare (talk) 09:42, 25 June 2009 (UTC)

I am a bit of an expert on photodiodes, usually in reverse bias as a detector, but have passing familiarity with their usage as solar cells as well. There's some diffusion involved in places, like when the photon absorption occurs outside the depletion region, but I think that to say that's what drives the current is quite wrong; the source you had cited for that didn't appear to support it, so I took it out. Dicklyon (talk) 16:07, 25 June 2009 (UTC)

Hi Dick: I wonder if you noticed Eq. 6.17, which contains the diffusion lengths as determining the open circuit voltage of the photocell? That would seem to support the view that diffusion drives the current in this device. See also Short circuit current which appears also to determine the current in general. Brews ohare (talk) 16:27, 25 June 2009 (UTC)

Yes, I saw that. Long diffusion length increases the probability that the carriers will survive to separate, rather than recombine; but he certainly doesn't say that diffusion is what drives it. It's involved in part of the current flow, but is not what makes the emf. Dicklyon (talk) 16:47, 25 June 2009 (UTC)

In other forms of emf like batteries and generators, an open circuit condition is obtained when the developed emf creates a back voltage that arrests the continuation of charge separation. In the solar cell, light is continuously applied, and there is a photo emf. However, it seems that unlike the other cases, in open circuit conditions there is not an arresting of the generation mechanism (for example, light emission does not increase to counter the light absorption). Can you provide some discussion of this point and contrast it with the other cases? Brews ohare (talk) 19:30, 25 June 2009 (UTC)

It can be viewed as a voltage-dependent arrest of the action, perhaps, but more specifically it's a recombination via forward current, like a self-discharge path. Think of it as a current source in parallel with a diode. See solar cell#Equivalent circuit of a solar cell. Dicklyon (talk) 21:49, 25 June 2009 (UTC)

Brews, your change to the "unlike in batteries" statement is an improvement, as it's not as false as what it had said about the terminal voltage, but I'm not convinced that the effect you're trying to describe is really so unlike in batteries. Do you have a source for this contrast? Dicklyon (talk) 16:40, 26 June 2009 (UTC)

Hi DIck: BTW, have you noticed that photo emf appears to refer to the voltage introduced, rather than the work done in creating the charge separation. I'd guess the work/charge done in the charge separation is the photo-voltage because the rest of the photon energy just is dissipated?

Rojansky definition is his own, and is at variance with the remainder of the WIki article on emf

This is a very poor source, and presents a minority view of one that emf is given by a line integral over a segment of path.

${\mathcal {E}}=\int _{A}^{B}{\boldsymbol {E\cdot }}d{\boldsymbol {\ell }}\ ,$

All other sources use a closed path to eliminate the contribution of the conservative E-field. Rojansky's formula based upon a segment keeps the conservative E-field contribution. That may be viewed as simply an error, or as adoption by Rojansky of a definition that is neither potential difference (as done by some texts) nor emf (as defined by work done in charge separation), but the sum of the two. He directly acknowledges this fact in noting that his emf in general is path dependent. Nobody else does that.

The Rojansky equation above contradicts the immediately following equation in the WIki article:

${\mathcal {E}}=\oint _{C}{\boldsymbol {E\cdot }}d{\boldsymbol {\ell }}\ ,$

because the closed loop integral has zero contribution from the potential difference, which is zero for a closed loop. Brews ohare (talk) 10:12, 25 June 2009 (UTC)

I have replaced the misleading view of Rojansky with the correct but much more restricted use of a line integral inside a source of emf due to Griffiths. Brews ohare (talk) 14:53, 25 June 2009 (UTC)

If you define emf as in the lead, as an open-circuit voltage, then this electrostatic integral always gives the right answer, doesn't it? It's the emf associated with a two-terminal device, if evaluated at the terminals. I can find sources beside Rojansky if you like; it doesn't appear to me to be an outlier. Dicklyon (talk) 15:53, 25 June 2009 (UTC)

Dick: The lead-in versions I have used do not define emf as an open-circuit voltage, but as the work done per unit charge in creating such a voltage. The emphasis is upon non-electrical forces, and only then upon the resulting voltage difference due to charge separation. I have carefully extracted the relevant portion of Griffiths to replace the nonsense of Rojansky. Brews ohare (talk) 16:01, 25 June 2009 (UTC)

emf is not a voltage

The revision of 06:38, 25 June replaced:

A voltage difference is considered to be the result of an emf, in most usages of the term. The emf is typically considered to be the work done per unit charge in pushing charge through a battery against the battery's voltage difference, for example. However, it is common in circuit theory, for example, to refer to the voltage created by the emf itself as the emf.

with:

Not every voltage difference is considered to be an emf, in most usages of the term. The emf is typically considered to be the voltage created by a source, or the work done per unit charge in pushing charge through a battery against the battery's voltage difference, for example.

with the comment "try this minor adjustment instead".

In fact this is not a minor adjustment at all, but a complete change of meaning that contradicts the opening definition of the article. In fact the replacement text is itself internally inconsistent, defining emf as a voltage difference, and next saying it is the work done against the voltage difference. The definition adopted in the intro is that emf is the work done per unit charge in creating the voltage difference. The key point in emf is the conversion of energy from other forms to electrical form Brews ohare (talk) 10:24, 25 June 2009 (UTC)

A consequence of Brews ohare's argument is that any electrical device that (1) plugs into a household wall outlet and (2) does not contain, and is not connected to, any transformer, photo cell, or receiving antenna, should be analyzed without reference to the concept of emf. --Jc3s5h (talk) 14:26, 25 June 2009 (UTC)

I'd suggest that to do that analysis without emf, one uses the voltage difference between the prongs of the plug and Kirchhoff's laws (say). If one wishes to inquire as to the origin of the voltage difference across the prongs, then one must go into the details of the source of potential difference: is it a battery, generator or whatever. That is where emf comes up. Brews ohare (talk) 14:58, 25 June 2009 (UTC)

Yes, that is just how to do it. I wonder, though, whether the literature does that, or whether the literature describes the hypothetical voltage source used for analysis as an emf. I checked two circuit analysis books on my shelf, and no variation of the term "emf" is listed in either index. --Jc3s5h (talk) 15:20, 25 June 2009 (UTC)

It appears that circuits texts often use emf as interchangeable with voltage difference due to a battery or generator. That usage is pointed out in the Terminology section (or it used to be before Dicklyon's changes, I'm not sure now). Brews ohare (talk) 15:25, 25 June 2009 (UTC)

The statement referenced the Physics text of Halliday and Resnick, but I didn't see any support for it there, so I took it out. The interpretation of emf as a voltage is pretty widespread, and we should acknowledge it fairly. If you want to clarify, you can find a source that clarifies the relationship and talk about that, too. Dicklyon (talk) 15:55, 25 June 2009 (UTC)

Dick: Fig 28-4 in halliday and resnick indicates the emf Є as the voltage difference caused by the emf. You do not appear to have noticed that two or three other circuit references are cited in the Terminology section to support the notion that emf is used interchangeably with "voltage difference" in this field. Brews ohare (talk) 16:04, 25 June 2009 (UTC)

The chapter is called "Circuits" but the field of the writer is physics; it doesn't mention "circuit theory"; how does this say anything about what field this usage is common in? Dicklyon (talk) 16:34, 25 June 2009 (UTC)

Agreed, I haven't established it is a common usage, although this text is in its 6th edition and is a major player. However, there are many other sources cited in Terminology that make the point that halliday and resnick are not alone. Brews ohare (talk) 16:43, 25 June 2009 (UTC)

Of course they're not alone; this usage is widespread; my objection is to your saying that it a minority usage associated with the circuit theory field. Dicklyon (talk) 16:49, 25 June 2009 (UTC)

Do you have sources outside circuit theory (other than Rojansky, who uses his own definition of emf that is not defined by the endpoints A and B, but is path dependent)? Brews ohare (talk) 16:54, 25 June 2009 (UTC)

Collaboration style

Brews, your style of rapid revision, with dozens of consecutive edits that move in a direction for which there is no concensus and in many cases clear opposition, is making this process very difficult. I don't know what to do. Ideas? Dicklyon (talk) 16:30, 25 June 2009 (UTC)

I don't agree about lack of consensus, exactly. I think the major issue is whether you accept the Griffiths replacement for Rojansky at Electromotive_force#Formal_definitions_of_electromotive_force. Brews ohare (talk) 16:45, 25 June 2009 (UTC)

You've done 33 more consecutive edits to the article today since I complained about your style. I still think WP:CONSENSUS is important. Dicklyon (talk) 21:40, 25 June 2009 (UTC)

Hi Dick: Most of these edits are pretty uninteresting stuff. I think the biggy is the one above: the Griffiths material. It's sourced, it's accurate and it agrees with what else has been said. It does not agree with Rojansky's path-dependent emf definition. Brews ohare (talk) 22:27, 25 June 2009 (UTC)

Hi again Dick: Apparently you have no comment on the Griffiths substitution for Rojansky?? Brews ohare (talk) 06:06, 26 June 2009 (UTC)

Ross quotation again

Attempts to shore up this quote using google to find citations have been unsuccessful. All actually used variants in the use of emf are well documented in the Wiki article, showing Ross to have made some erroneous distinctions, and to have grossly exaggerated the murkiness of the concept. Putting the Ross material into the article only confuses things, and makes the reader wonder why the quote is there when it contradicts the article and no explanation is given.

There are basically two usages of emf. One is the lead-in: work per unit charge in creating an electrostatic back voltage. The other equates emf to the electrostatic voltage itself, rather than its creation. The second usage, which ignores the whole idea of how the back voltage comes about, occurs primarily in circuit analysis, where the origins of the back-voltage are not at question, but only the electrical consequences.

Because the second usage is a synonym with electric potential difference, it is worthy of only a subsidiary discussion in the Terminology section. The first usage comes up in Faraday's law of induction, in batteries, in semiconductor devices, and anywhere where the mechanism of generation of back voltage is of interest.

Ross's plethora of other distinctions and usages are undocumented, undocumentable, and marginal. The quote should be dumped. Brews ohare (talk) 14:41, 26 June 2009 (UTC)

differ in sign?

Brews has cited Griffiths to support his interpretation that different definitions differ in the sign of emf. This is not in Griffiths, and seems exceedingly unlikely. More likely, emf is defined as positive for a device adding work to a circuit, but the conventions on what direction to integrate differ. Can we find a source that actually comments on sign conventions, like for the direction of the loop or path integrals, so that we can clear this up? Dicklyon (talk) 16:57, 26 June 2009 (UTC)

Dick: You are on the wrong track here: notice the minus sign in Griffiths two-terminal equation, which does not appear in the closed-loop equation. Also, notice the term "back voltage" in the quotation: the adjective "back" means the voltage is in the opposite direction to the emf. That also is why Griffiths says there is zero net motance inside the source of emf: the emf is countered by the electrostatic field; that is, the electrical voltage is opposite in sign to the emf itself. This is why zero current flows in the open circuit condition. Brews ohare (talk) 17:10, 26 June 2009 (UTC)

I've taken this out of a few places again. The cited Griffiths page doesn't comment on the relative polarities of emf and voltage. What is the convention? When you have a 1.5 V battery connected to a resistor, putting 1.5 V across the resistor and across the battery, measured from the + side to the minus side, that corresponds to a + emf of the battery, as it's adding work to the circuit, right? This mean you run your test charge around in the same direction as the actual current to do the integral? And if you measure the voltage across the battery by looking around the loop in that direction it will be -1.5 V across the battery; is that what you mean by the opposite sign? But when emf is used to mean voltage, that's not the direction that the measure voltage in. The interpretation that the signs differ is your way of saying that you don't measure the voltage in the conventional way perhaps? The voltage that corresponds to the emf is the voltage that the battery supplies to the circuit. At least in all the stuff I ever saw. But point out where Griffiths explains things and talks about "back" and I'll try to get your point. Dicklyon (talk) 06:22, 28 June 2009 (UTC)

By opposite in sign I meant that inside the battery (as an example source) the electric field due to the charge separation opposes the forces driving the separation. In open circuit condition, then, the net work done in taking a charge from A to B is zero, because what is gained from the separating forces is lost in climbing the potential hill. This is what Griffiths says on p. 293: "the net force on the charges is zero". I take that to mean emf + o.c. voltage = 0. Brews ohare (talk) 14:35, 28 June 2009 (UTC)

Emf in diodes, imrefs, and thermodynamics

Dick: The whole idea of emf in diodes bothers me. It is apparent that charge separation occurs in diodes and the potential drop across the diode is due to this charge separation. During the establishment of this drop, a current flows, driven by the energy difference of electrons in the two materials. That all sounds like an emf at work.

However, there is no continuing supply of energy conversion once the built-in potential is established, and putting a load across the junction will not draw a current. That is not like the emf in a battery. Therefore, the definition of emf is faulty, because it does not distinguish such cases of transient establishment of charge separation from the ability to provide steady-state maintenance of current.

The p.d. in a diode does not cause a current and does not enter Kirchhoff's law. The failure of the diode p.d. to enter Kirchhoff's laws seems to put the lie to the 98% of all sources that equate voltage to electrical potential difference. Some sources Quimby Neamen attribute this failure to the need to account for electrode contact potentials as well as the junction built-in potential. If that is done, all these potentials will add to zero, leaving no contribution to Kirchhoff's law. Although true, I'd say its a cop out, and the real thing to look at is the Fermi level, which is flat.

In the photodiode the potential drop across the diode (which pushes no current) is reduced by the photo-voltage. The current that flows in the load does flow in accord with the photovoltage, like the battery case, but you can't calculate the emf the way the definition suggests as some kind of line integral. The emf appears to be given by a different type of formula, one connecting conversion of light energy to electrical energy.

Thermodynamics may be the way to go, or maybe quasi-Fermi levels. In any event, it is energy conversion, whether static or transient, that is the key, and emf denotes an energy imbalance that pushes charge. What do you think the article should say about all this? Brews ohare (talk) 18:28, 26 June 2009 (UTC)

I think we should say not much besides what we find in sources. I think of it this way: ignore the shunt R in the model, and assume you know what the series R is, and treat the diode as part of the elementary "cell"; then at zero current the terminal voltage is the emf, even in the shorted case in the dark. In the dark, then it appears that the emf is nonzero for the open-circuit cell and zero for the short-circuit cell, with no work being done in either case. In the light, the emf similarly adjusts itself to the load voltage, since the cell can't provide more than 1 electron per converted photon. From the terminal voltage and current in any condition, the emf is easy to calculate. Predicting the terminal I–V curve from light level and cell details is more complex, but there are models. From these models, you could work out how emf varies along the I–V curve. Basically, the extracted energy per photon is varying as the bias changes, being greatest for the largest forward bias, though the conversion efficiency goes down there. It's complicated, but the emf is still the work done per charge delivered. The mode I'm more familiar with is the back-biased detector, in which the absorption of photons causes stored potential energy to be dissipated, so the emf is not the concept I've worked with. Dicklyon (talk) 21:27, 26 June 2009 (UTC)

I've forgotten exactly how to analyze the band-bending and built-in potential relative to Fermi level and terminal voltage. I think the key is that when light is provided, you get carriers in the other band, far from the Fermi level. Maybe this changes it to a quasi Fermi level (does that mean non-equilibrium?). Anyway, I don't think I buy that explanation about the metal–Si junctions providing their own potential differences; isn't that what ohmic junctions are about? Doesn't the diode terminal voltage float to an actual nonzero difference in the absense of any load? I thought it did, but now I'm not sure. Dicklyon (talk) 21:33, 26 June 2009 (UTC)

Caveat on Griffiths formula

I believe that the photo diode can be included in the opening definition and in Griffiths line integral provided the E-field included is only that due to the mechanism causing charge separation (the light). That way the built-in electric field caused by thermodynamic equilibrium between dissimilar solids is ignored, and you get the correct photo-voltage if you know the correct charge distribution induced by the light. Brews ohare (talk) 13:05, 27 June 2009 (UTC)

Recent edits

(the path is taken from the negative terminal to the positive terminal to yield a positive emf, indicated work done on the electrons moving in the circuit when current flows in that direction in the source, or from positive to negative through the external part of a closed circuit; measured in this direction, the electric field is negative and the voltage difference, negative terminal relative to positive terminal, is negative).

Dick: By integrating the total field over a segment external to the source of emf, a contribution from both the conservative and the non-conservative E-field may be picked up (e.g. in cases when a magnetic field is present). Hence, the emf calculated on an open path with a segment external to the source does not produce the correct value. That is why Griffiths spends so much time on this equation. Please read Griffiths' discussion and revert this edit. Brews ohare (talk) 13:35, 28 June 2009 (UTC)

I deleted a portion of this edit that conflicts with the following discussion. I'm happy to leave it like that. Brews ohare (talk) 14:47, 28 June 2009 (UTC)

A transformer coupling two circuits may be considered a source of emf for one of the circuits, just as if it were caused by an electrical generator; this example illustrates the somewhat arbitrary nature of the decision of what is the circuit and what is an external agency.

This example does nothing of the kind. The division of the circuit is not arbitrary. It is an example of why "transformer" emf is called that, and in either of the two transformer-coupled circuits, this is a straightforward application of the definition of a source of emf. Please revert. Brews ohare (talk) 13:35, 28 June 2009 (UTC)

I modified this sentence to leave out the "arbitrary decision" stuff. Brews ohare (talk) 14:49, 28 June 2009 (UTC)

How does one decide what part of a circuit to call "external" when there are mutual inductances? What transformer windings are part of the circuit, and which are external? It all seems pretty arbitrary. How do you decide which voltages are due to emf and which are just interactions of elements of your circuit? Dicklyon (talk) 20:33, 28 June 2009 (UTC)

If an electrical voltage is not connected to an external resistor, then an electric current will not flow through that resistor (Ohm's Law). If an emf is applied across the resistor, between the terminals of the source there must exist a true electric field that produces a voltage difference that exactly matches the IR drop in the resistor.

Why was this deleted? Do you disagree with it, or do you think it is redundant? Brews ohare (talk) 13:35, 28 June 2009 (UTC)

It just doesn't make any sense. Maybe "If an electrical voltage is not connected to an external resistor" was intended to mean "If no electrical voltage is applied across a resistor" or something like that. And what does it mean to apply an emf, especially given your insistence that an emf is not a voltage. And what is a "true electric field" in contrast to? I just couldn't find any sensible interpretation or intent, and the section seemed more coherent without it.

Deleted reference to "Basic Electricity". This reference was included as an example of the use of the term emf as a voltage in circuit theory and was deleted as "not referring to emf": Here is a quote from this source discussion Kirchhoff's voltage law on p. 76:

"Kirchhoff's Voltag Law can be written as an equation as shown below:

$E_{a}+E_{b}+....+E_{n}=0$

where E_a, ... etc are the voltage drops and emfs around any closed circuit loop."

The "voltage drops" in the figure are the IR drops and the emf's are the battery voltages, showing quite clearly the usage that was to be illustrated. This reference should be restored. Brews ohare (talk) 13:35, 28 June 2009 (UTC)

My search of the cited page must have missed that. Sorry if so. Dicklyon (talk) 20:33, 28 June 2009 (UTC)

Electrochemical cell considerations

The electrical circuit through an electrochemical cell may be described as a series circuit of 5 ingredients. They are (1) The negative plate electronic resistance, (2) The negative plate emf value, (3) The electrolyte ion transport resistance factor, (4) The positive plate emf value, and (5) The positive plate electronic resistance. The functioning of the electrochemical conversion process is, of course affected by environmental factors, like temperature, and by electrochemical conversion inhibiting factors, like material depletion and/or passivation. and the design and construction of electrochemical cell has to minimize the detrimental effect of all these factors.WFPM (talk) 14:53, 1 September 2009 (UTC)

The Emf potential of the cells active electrochemical material is the quantitative measure of the level of an opposing voltage which would stop the electrochemical material's electrons from moving. Then if that voltage is lowered, the electrons will move through through the connecting circuit in the direction of the lower voltage potential. In rechargeable cells, an increase in back potential will reverse the electrochemical process and restore the electron supplying properties of the cell's negative and positive plate materialsWFPM (talk) 19:42, 2 September 2009 (UTC)

Introduction Should Include More Casual Language

In the introductory section, before the table of contents, there should be 1-2 sentences with language easily understood by a non-Physicist. That section could also include some disclaimer verbiage such as "approximately" or "like...". It would then go on to the more precise introduction and explanation.

I certainly respect the current wording, and that using simpler language would be imprecise. But if you're not a real techie the current introduction is hard to read.

Something like: "EMF is an electrical property of devices like batteries and motors that is

related to (similar to?) voltage.
There are several different technical definitions of emf.
Even the letters in the abbreviation "e m f" stand for different terms
in some textbooks.  And the letter "F", which usually stands for "force",
doesn't mean the same thing as it usually does in Physics.
Although EMF can be measured in Volts, it doesn't have the same meaning as "voltage"
normally does in electronics.
The actual definitions of emf are rather technical."

My example above uses lots of vague words and is imprecise, so perhaps it's not up to Wikipedia standards, so I'm posting it here instead. Since it's in the introduction, and I think it's clear that it's imprecise, wouldn't that make it OK? Or perhaps somebody can do better while still be as clear? —Preceding unsigned comment added by Ttennebkram (talk • contribs) 07:21, 17 October 2009 (UTC)