Talk:Electromotive force

Use of this term is deprecated

According to ISO 80000-6:2008 Quantities and Units - Part 6 Electromagnetism: The name “electromotive force“ with the abbreviation EMF and the symbol E is deprecated. See IEC 60050-131, item 131-12-22.

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? Ttennebkram (talk) 07:27, 17 October 2009 (UTC)

I don't think that's the way, but I did find a sourced "simple" definition to start with.

The lead sentence since this diff (one of 28 Brews ohare edits of June 24, the day I had to take a break) says "emf is the external work expended per unit of charge to produce an electric potential difference across two open-circuited terminals." I'm not surprised you don't understand it, since it doesn't actually make any sense. And it's sourced to two documents that don't support it; the first says nothing about open-circuited terminals and the second is specifically about a closed loop. I think it doesn't actually take any work to maintain a voltage difference across open-circuited terminals, since no charge motion is required, so the work per unit charge to do that seems to be indeterminate. I know what he was trying to do, but the edit summary "(Revision following discussion on Talk page; see Talk page for details) didn't exactly lead to an explanation of why this. When I came back and saw it I wrote on the talk page here "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?". But I guess I didn't see just how bad and unsupported it was, as I never did touch it. Time to try to fix it... Dicklyon (talk) 01:33, 18 October 2009 (UTC)

The term "voltage" is introduced without definition in the lead-in, so that its relevance to emf is obscure. I lack the ability to rectify this. Myron (talk) 17:12, 19 June 2011 (UTC)

Definition

Electromotive force: The driving force which maintains the flow of current in the circuit is called Electromotive force

It is denoted by E. —Preceding unsigned comment added by 202.56.7.148 (talk) 10:23, 26 March 2010 (UTC)

The definition as given is patently absurd; a voltage cannot be defined as an energy. I have changed it to the quotient of energy & charge. A more radical criticism is this: as an engineer, I have no use for emf; it does not differ in any respect from any other voltage, and to suggest that it does can only be misleading and unhelpful to beginners. When a battery is connected to a resistor, the terminal voltages of the battery and of the resistor are absolutely identical and indistinguishable. It is absurd to call one a pd and the other an emf. The fact that one component is a source of energy, and the other a sink or load is a point of distinction between a battery and a resistor - that is between the two devices. It is not a distinction between 2 voltages or types of voltage. Whether a device acts as a source or a load can be distinguished by the direction of the current with respect to the voltage, not by any examination of the voltage alone. I feel that no discussion of "emf" can be complete without these points being made. The term is actually obsolete, and should be abandoned; we wisely make no similar distinction between types of forces. A further point is that in the article the "emf" of a cell is described as its open-circuit voltage, a series equivalent circuit being assumed. But whoever wrote this neglected the fact that a parallel equivalent circuit is equally valid, in which case the open-circuit "emf" would clearly be a "pd" - that is the voltage developed across the internal conductance by the internal current generator. G4oep (talk) 13:25, 30 January 2015 (UTC)

The term "emf" is far from obsolete. The emf generated by a battery differs from the terminal voltage of a battery. With no load connected, the terminal voltage is approximately equal to the emf generated by the battery. When a load is applied, there is an internal voltage drop due to the battery's internal resistance. On load, the terminal voltage will be lower than the generated emf. 103.91.192.53 (talk) 03:06, 7 March 2023 (UTC)

First sentence

...doesn't really make sense. Current is not defined as electrons and ions. Current is a flow of electric charge. The sentence is currently saying "that which tends to cause the flow of electrons to flow." The common saying that current "flows" is erroneous. Current does not flow. —Preceding unsigned comment added by Pseudov00 (talk • contribs) 04:57, 11 April 2010 (UTC)

The "Electromotive Force" is that which enters a persons house through his electric power line connection, and causes activity in his connected and turned on electric appliances. Its intensity value is rated in volts and results in a flow of the electric current carrying constituents of the appliances. Its power or Energy rate delivery characteristic is rated in watts (joules/second), which value is integrated over a time period to arrive at the delivered energy value in Watt hours. Note that since the advent of AC power, there is practically no entry of matter through the power line connection into the house, Just the delivery of the electrical EMF and associated electrical power and energy.WFPM (talk) 18:28, 3 November 2011 (UTC)

what does this mean ?

"For a time-varying magnetic flux linking a loop, the electric potential scalar field is not defined due to circulating electric vector field, but nevertheless an emf does work that can be measured as a virtual electric potential around that loop".

Is this correct ?

"In the case of an electrical generator, a time-varying magnetic field inside the generator creates an electric field via electromagnetic induction, which in turn creates a voltage difference between the generator terminals. Charge separation takes place within the generator, with electrons flowing away from one terminal and toward the other, until, in the open-circuit case, sufficient electric field builds up to make further movement unfavorable. Again the emf is countered by the electrical voltage due to charge separation".

In a DC machine the magnetic flux within the machine can be constant. What does "unfavourable" mean in this context ? Is not the open-circuit terminal voltage simply equal to the induced voltage ? Current does not flow because we are considering an open circuit, and not for any other reason.

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Potential for a loop

For a time-varying magnetic flux linking a loop, the electric potential scalar field is not defined due to circulating electric vector field, but nevertheless an emf does work that can be measured as a virtual electric potential around that loop.[5]

You can define a multi-valued potential for a loop of radius a using the angle:

$V=V_{o}{\frac {\theta }{2\pi }}$

Then the electric field is

$E=-\nabla V={\frac {V_{o}}{2\pi a}}$

as expected.

Variability of emf

The following statement in the article, just above the table of emf for various cells:

"The electromotive force produced by primary (single-use) and secondary (rechargeable) cells is usually of the order of a few volts. The figures quoted below are nominal, because emf varies according to the size of the load and the state of exhaustion of the cell."

is inexplicable. The statement that emf varies with load contradicts the definition of emf, which is potential difference in open circuit, when there is no load. Of course the voltage delivered varies with load. If the internal resistance is r and external resistance is R the cell with emf E will deliver current I = E/(R + r) and the delivered voltage is V = IR = ER/(R+r). V varies with R but E does not.

The statement that E varies with the state of discharge is experimentally correct but it is not explained in the section about the source of emf. This section presents a standard and simple model which predicts that E depends on atomic properties of the atoms and ions participating in the reaction. It is by measuring E that we determine these atomic properties. How can the results depend on the state of discharge of a cell?

Bukovets (talk) 16:59, 25 July 2017 (UTC)

Capitalization

Should we go with the traditional lowercase emf, or all-caps EMF? An anon capped, claiming "typo", I reverted back to the conventional lowercase, and he reverted back saying that "Common usage in this case is neither correct according to any of the manuals of style nor encyclopedic." But that's not true; most style guides do use lowercase emf (and older ones e.m.f.) according to this book search. Dicklyon (talk) 01:19, 10 May 2018 (UTC)

Derivative as counterpart of integral.

The article provides

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

showing the relation between emf and the integral of E. It seems logical that there could be an equivalent/analogous relation between E and the derivative of emf. If so, can it please be added to the article.
—DIV (120.17.127.14 (talk) 09:41, 9 August 2018 (UTC))

Compare also

$\nabla \times \mathbf {E} =-{\frac {\partial \mathbf {B} }{\partial t}}$

from Maxwell's_equations#Macroscopic_formulation with

${\mathcal {E}}=-{\frac {d\Phi _{B}}{dt}}$

from Faraday's_law_of_induction#Quantitative.

—DIV (120.17.127.14 (talk) 10:18, 9 August 2018 (UTC))

"emf" vs. "EMF" - inconsistent?

Why is it "emf" when it is "back EMF"? (Or vice versa, why isn't it "back emf"? --Mortense (talk) 22:59, 21 July 2019 (UTC)

@Mortense: As far as I know, "counter EMF" or "back EMF" is another name for "induced EMF", which is a particular type of EMF, due to electromagnetic induction. Does that answer your question? --Alej27 (talk) 23:25, 29 January 2022 (UTC)

The sign(s) in definition of emf

How come the sign in the definition of emf in a conservative electric field is different from the sign in the definition of the induced emf? 2001:1470:FF80:5:5539:D087:2C3C:17CA (talk) 09:00, 23 December 2021 (UTC)

Difference between voltage and voltage difference

Hi user Constant314. In the recent edit reversion you said that voltage doesn't have to be a difference. As far as I know, it does. Voltage is also called electric potential difference, not just electric potential; even Wikipedia agrees. Don't you agree? If not, what does "voltage" mean to you? Are you saying that voltage is the same as electric potential? --Alej27 (talk) 23:22, 29 January 2022 (UTC)

Hello. Voltage is, unfortunately, a vague term that some people use as being equivalent to potential. However, if you want to replace "votage difference" with "potential difference", that would be fine with me. Constant314 (talk) 23:33, 29 January 2022 (UTC)

Good. I've changed it. I just noticed that this article, before my edit, suggested that voltage was potential difference, in the third paragraph of the lead section, where it reads "the equivalent emf can be measured as the open-circuit potential difference, or voltage, between the two terminals". --Alej27 (talk) 02:45, 30 January 2022 (UTC)

Electromotive force vs. source voltage

As has been noted in anonymous comment above, IEC/ISO has deprecated '"electromotive force" as the standardized term in favor of "source voltage" a few years ago (IEC electropedia). This should be acknowledged in the article in some way. --Jähmefyysikko (talk) 20:37, 27 February 2022 (UTC)

EMF of emf

@Dicklyon: We have had discussions across several articles in the past. Here is part of it [1]. Unfortunately, it doesn't show the conclusion. Here are the "n-gram results" showing an overall perponderence of lower case, although recently that has reversed. Anyway the community consensus was to use lower case. So, I am reverting it back to lowercase, pending further discussion here. Constant314 (talk) 15:33, 4 April 2022 (UTC)

If you look into the books you'll see the EMF is usually for electromagnetic field, and emf for electromotive force, even recently. Dicklyon (talk) 16:24, 4 April 2022 (UTC)

What is the proper ISO name?

The first subject on this talk page says "According to ISO 80000-6:2008 Quantities and Units - Part 6 Electromagnetism: The name “electromotive force“ with the abbreviation EMF and the symbol E is deprecated. See IEC 60050-131, item 131-12-22." In which case it seems to me that this page should be renamed to whatever ISO considers the proper term, and then Electromotive force should simply be a redirect to the properly-named page. EMF isn't a force, so probably shouldn't use the word "force" in it or emF.

But what is the proper name? When I look up "ISO 80000-6:2008 Quantities and Units - Part 6 Electromagnetism" or "IEC 60050-131" online the only results I get are for a book that I have to pay hundreds of dollars for. Em3rgent0rdr (talk) 19:18, 25 September 2022 (UTC)

The standardized ISO/IEC names can be found from IEC's Electropedia. In this case Entry 131-12-22. The name is 'source voltage' or 'source tension'. --2A00:1398:4:9003:82E8:2CFF:FEEC:B393 (talk) 08:09, 19 December 2022 (UTC)

The naming convention for pages on Wikipedia is WP:COMMONNAME. It is not WP:OFFICIALNAMES and it is certainly not IEC official names. Until the IEC deprecation of the term actually starts to become widespread, then there is no reason that Wikipedia should be bound by what they say. There are literally hundreds of books published in the 21st century still using it. I also find the IEC definition of source voltage troubling. For instance, the emf of an inductor is zero when there is no current flowing through it. So an inductor can never have an emf by that definition. Even national standards organisations such as NIST and BSI have not yet felt obliged to purge the term from their publications. SpinningSpark 15:08, 19 December 2022 (UTC)

And why on earth are the IEC attempting to resurrect the old-fashioned term tension. After all, tension is a force so has the same semantic problems as force. SpinningSpark 15:15, 19 December 2022 (UTC)

What is the analogy to water pressure?

Intro says:

> Sometimes an analogy to water pressure is used to describe electromotive force.

But without explaining the analogy, that doesn't tell me anything useful (and actually adds confusion). When I look up the referenced citation for that (https://books.google.com/books?id=OW0SAAAAYAAJ&q=%22electromotive%20force%20is%20that%22&pg=PA172#v=snippet&q=%22electromotive%20force%20is%20that%22&f=false), I find two excerpts which do not really talk about pressure at all (but maybe there is other text outside of the excerpt that google books link provides which explains the analogy?)

My opinion is to delete that entire sentence because as it stands now it harms rather than helps understanding. Unless someone can provide a good simple clear explanation of the analogy. Em3rgent0rdr (talk) 19:28, 25 September 2022 (UTC)

I see that "pressure" in that sentence is surprised hyperlinked to Volt#Water-flow_analogy. But even when clicking on that hyperlink it doesn't explain how emf fits into the analogy. I guess would be external energy applied per unit of water that increases pressure or something?? I still would say just delete that sentence if not providing a clear explanation. Em3rgent0rdr (talk) 18:41, 26 September 2022 (UTC)

I think that the analogy is between EMF and pressure difference rather than pressure. For those who insist that EMF only applies to sources, it would be analogous to the outlet-inlet pressure difference at a pump (assuming incompressible water). Constant314 (talk) 18:44, 26 September 2022 (UTC)

Ok. I'm thinking the analogy is really to the mechanical work of the pumping action on the water producing the change in pressure. I'm also noticing the word "analogy" was a suprise link to Hydraulic analogy...I would just write out the full name of the analogy. So maybe the sentence could be written along the lines of:

In the electronic–hydraulic analogy, EMF is analogous to the mechanical work done by a pump on water that results in a change of water pressure (analogous to voltage).

I see that electronic–hydraulic analogy page does not actually mention EMF...maybe something could be added there. Em3rgent0rdr (talk) 19:17, 26 September 2022 (UTC)

Looks fine to me. Constant314 (talk) 19:58, 26 September 2022 (UTC)

Did Volta make a mistake?

Our article currently states Volta's mistake of labeling it a "force" is a misnomer that persists as a historical relic. Calling Volta "mistaken" is a bit presumptious of us especially as the cited source describes this as neither a mistake nor a misnomer. It's hardly mistaken at a time when force did not have the firm definition it has now. We wouldn't call Isaac Newton's laws of motion "mistaken" and "misnomers" even though Newton's concept of force is at odds with what we now describe as force (Newton's force is what we now call impulse). SpinningSpark 15:57, 4 November 2022 (UTC)

oh good point. I've now edited to simply be:

> However, electromotive force is not actually a force, but the label "force" persists as a historical relic. Em3rgent0rdr (talk) 17:24, 6 November 2022 (UTC)

What you probably mean is "not considered a force according to a very narrow definition used by a small bunch of specialists." However, their jargon does not constrain the general use of the term force to include emf. Constant314 (talk) 21:46, 6 November 2022 (UTC)

Not just that, but the units don't match up. Force in SI base units are 1 kg⋅m⋅s⁻² while volt, the unit of emf, is kg⋅m²⋅s⁻³⋅A⁻¹, which differs by a factor of m⋅s⁻¹⋅A⁻¹.

And what is behind emf is not some separate force but rather the some type of actual force doing work.

EMF is more of an energy transfer to a circuit per unit of charge. Em3rgent0rdr (talk) 17:00, 7 November 2022 (UTC)

The units don't need to match up. Force of love. Force of nature. Force of facts. Military force. Police force. Delta force. Those are all legitimate uses of the word force. It is just a general word for something that causes action. He named it electromotive force. That is its name. No need to make an excuse because some other people came along later and specialized the term force. Constant314 (talk) 18:43, 7 November 2022 (UTC)

This article is part of the electromagnetism wikipedia category which is part of physics, so this use of the word "force" in is specific to the physics term, so I do believe it is necessary to clarify that "electromotive force" is not a physical force. It is not making an excuse, but it is an important thing about the name that needs to be clarified for readers.

Category:Electromagnetism

Em3rgent0rdr (talk) 19:18, 7 November 2022 (UTC)

OK, but kindly leave out the stuff about "historical relic". Constant314 (talk) 20:26, 7 November 2022 (UTC)

ok here is what I've shorten to now:

> This energy conversion is achieved by physical forces applying physical work on electric charges. However, electromotive force itself is not a physical force. Em3rgent0rdr (talk) 20:45, 7 November 2022 (UTC)

Chemistry

What does it mean when the end of a cell is + or - 105.112.190.215 (talk) 23:29, 5 March 2023 (UTC)