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A citation needed for:
Thank you. 134.193.168.248 13:28, 13 June 2006 (UTC)
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")
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)
Rudolf F. Graf is an authoritative source. 204.56.7.1 16:06, 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)
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)
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)
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)
Go look them up. They are at the local library. 204.56.7.1
And all the external articles are wrong,
Just your source is correct, riiiiiight (now that's silly). 204.56.7.1 20:27, 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
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)
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
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. [1] 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. [2] Electromotive force is the force that moves electrons in a conductor. [3] Increases in electromotive force causes a comparable inclination for electrons to proceed from one point to another. [4] Electromotive force can affect " holes" as well as electrons. [5]"
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)
Being restored with references. 134.193.168.249 14:00, 14 June 2006 (UTC)
I have removed the section entitled 'Back electromotive force' from the article. Below is the removed text:
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)
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)
- 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.
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)
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)
Read the references. 134.193.168.249 15:55, 14 June 2006 (UTC)
Why remove from Electromotive force article:
??? 134.193.168.249 16:32, 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)
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)
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)
try to discuss your changes? When others do not try the same thing? :-t 134.193.168.250 13:56, 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)")
From "McGraw-Hill Encylopedia of Physics, Electromotive force (emf)"
From the textbook "Field and Wave Electromagnetics, 2nd Edition, Cheng"
From the textbook "Electric Machinery, 5th edition, Fitzgerald, Kingsley, Umans"
From the textbook "Physics, 2nd Edition, O'Hanian"
Thoughts? Alfred Centauri 02:56, 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:
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)
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. [6]
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. [7]
Electromotive force "moves" electrons in a conductor. [8]
Increases in electromotive force causes a comparable inclination for electrons to proceed from one point to another. [9]
Electromotive force can affect " holes" as well as electrons. [10]
For induced emf, the term "induced voltage", or voltage around a network path caused by a changing electromagnetic flux linking the path [11] , 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.
The counter-electromotive force (abbreviated counter emf or CEMF) [12] 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. [13] The voltage's polarity is at every moment reverse that of the input voltage [14] [15]
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". [16] 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)
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)
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)
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)
I reverted the following edit because I believe it serves to confuse rather than to clarify:
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.
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))
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)
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.
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.
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)
Better: + or -
Martin Segers (
talk)
07:49, 6 January 2008 (UTC)
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)
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)
The section on "Electromotive force and voltage difference" currently contains the following material:
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 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)
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)
This page is an archive of past discussions. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page. |
A citation needed for:
Thank you. 134.193.168.248 13:28, 13 June 2006 (UTC)
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")
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)
Rudolf F. Graf is an authoritative source. 204.56.7.1 16:06, 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)
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)
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)
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)
Go look them up. They are at the local library. 204.56.7.1
And all the external articles are wrong,
Just your source is correct, riiiiiight (now that's silly). 204.56.7.1 20:27, 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
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)
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
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. [1] 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. [2] Electromotive force is the force that moves electrons in a conductor. [3] Increases in electromotive force causes a comparable inclination for electrons to proceed from one point to another. [4] Electromotive force can affect " holes" as well as electrons. [5]"
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)
Being restored with references. 134.193.168.249 14:00, 14 June 2006 (UTC)
I have removed the section entitled 'Back electromotive force' from the article. Below is the removed text:
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)
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)
- 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.
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)
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)
Read the references. 134.193.168.249 15:55, 14 June 2006 (UTC)
Why remove from Electromotive force article:
??? 134.193.168.249 16:32, 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)
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)
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)
try to discuss your changes? When others do not try the same thing? :-t 134.193.168.250 13:56, 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)")
From "McGraw-Hill Encylopedia of Physics, Electromotive force (emf)"
From the textbook "Field and Wave Electromagnetics, 2nd Edition, Cheng"
From the textbook "Electric Machinery, 5th edition, Fitzgerald, Kingsley, Umans"
From the textbook "Physics, 2nd Edition, O'Hanian"
Thoughts? Alfred Centauri 02:56, 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:
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)
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. [6]
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. [7]
Electromotive force "moves" electrons in a conductor. [8]
Increases in electromotive force causes a comparable inclination for electrons to proceed from one point to another. [9]
Electromotive force can affect " holes" as well as electrons. [10]
For induced emf, the term "induced voltage", or voltage around a network path caused by a changing electromagnetic flux linking the path [11] , 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.
The counter-electromotive force (abbreviated counter emf or CEMF) [12] 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. [13] The voltage's polarity is at every moment reverse that of the input voltage [14] [15]
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". [16] 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)
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)
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)
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)
I reverted the following edit because I believe it serves to confuse rather than to clarify:
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.
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))
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)
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.
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.
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)
Better: + or -
Martin Segers (
talk)
07:49, 6 January 2008 (UTC)
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)
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)
The section on "Electromotive force and voltage difference" currently contains the following material:
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 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)
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)