Talk:Electromotive force/Archive 2

Archive 1

Archive 2

Archive 3

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)