Talk:Electromotive force
Emf is often used as a synonym for potential difference, where the potential difference is induced by an energy source such as a battery. This usage is considered obsolete.
I thought that it is not the same because of the internal resistance in the battery, and that therefore the concept of emf is still needed for a battery. - Patrick 12:09 Feb 4, 2003 (UTC)
What's obsolete?
I believe the expression electromotive force in words is obsolete, but the expression emf in letters is not. The problem is the word force — an emf is not a force, so it should not be called one. Thus I recommend moving this article to EMF (voltage), and writing "The expression EMF used to stand for electromotive force, but the term electromotive force is now obsolete, and EMF is a substitute for it."
Any comments?
Fg2 06:20, Sep 5, 2004 (UTC)
- The term "electromotive force" is still used in the latest edition of Jackson's Classical Electrodynamics, and a literature search of the last 14 years finds at least 250 journal articles that use the term in their titles and/or abstracts. So, I don't think there is any evidence to support the argument that the term is "obsolete" in the sense of "no longer used" or even "no longer used by professionals". Yes, it is a bit of a misnomer, but these things happen in language. —Steven G. Johnson 19:14, Sep 5, 2004 (UTC)
Further editing
I get confused with EMF -- is it the voltage based on the internal resistance of the cell, or is the sum total voltage which is required to support the circuit? Tanmay Altered the wording and also added a physical analogy for voltage as a force. Edit was done by me ElBarto as 203.79.121.174.
- EMF is not based on internal resistance. It is equal in magnitude and opposite in direction to the open circuit potential difference of a source.--Light current 20:58, 27 December 2005 (UTC)
The electrons flow out of the positive terminal into the negative terminal. This is because Ben Franklin guessed wrong when researching this. So as a convention (to stay consistent), we say electrons flow from positive. This is Pasive Sign Convention.
- I don't think that's quite right. Current flows out of the positive terminal and into the negative terminal, but current was defined by Franklin as the direction that positive particles of electricity would flow, before anyone knew that it was actually the electrons, or negative particles, that flow in metal conductors, whereas the protons, or positive particles, are at fixed locations. So in fact, electrons flow out of the negative terminal and into the positive terminal, but since current is defined as positive charge flow, the direction of conventional current is just the opposite.
- The mistake that Franklin made was not in choosing the direction of conventional current flow, it was in not understanding that it is negative charges that flow, whereas the positive charges are fixed in the solid structure of the metal. Of course, there was no way that he could have known at the time, since he lived and worked 100 years before Maxwell derived a complete theory of electromagnetism, 125 years before Thompson discovered the electron, and 150 years before Rutherford probed the structure of the atom and discovered the positively-charged nucleus.
- But, in a twist of historical irony, Franklin's choice of showing current in the direction of positive electrical flow turned out, in the age of semiconductors, more correct than people often realize. In semiconductors, the total current results from the flow of both electrons and mobile holes, which represent the absence of an electron in the crystal lattice and which behave just like positive particles of electricity. So the direction of current flow, which is the sum of the contributions from both electrons and holes, is the same direction as the flow of the holes, and opposite to the flow of the electrons.
- We are actually very fortunate that Franklin made this "mistake" and defined current as flowing in the positive direction, because it makes electrical circuits much easier to understand than if he had chosen the opposite convention. It also eliminates the need for minus signs in all of the circuit equations and laws that govern electrical phenomena. The only hard part is to learn and remember that the direction of current flow is not the direction of electron flow, but rather the direction of positive charge flow.
EMF or emf?
Silly people who abbreviate things in lowercase. :-) mph, rms, emf. When will it end??
I know there's some convention for this, but it doesn't make sense to me. Is there anything in the manual of style about this? - Omegatron 15:38, August 9, 2005 (UTC)
Symbol
Moved here for Stevenj's talk page for a more general discussion: "Nothing, but there is no reason to put it in a large font. In any case, the most common symbol I've seen is a script \mathcal{E}, not a Roman-font E. —Steven G. Johnson". The symbol I added is a script E, and the Unicode character for EMF. If it's showing up as a normal E, then it's something to do with the fonts you have installed on your computer. porges 04:40, 12 October 2005 (UTC)
Can emf be measured directly? Or is it the pd you're measuring?
Can emf be measured directly, or must it always be inferred from the pd presented at the terminals? If it must be inferred, when people are taught about measuring various emfs with a voltmeter or potentiometer etc., they are being given slightly the wrong story. Yes?--Light current 02:50, 26 December 2005 (UTC)
Does nobody know the difference between emf and pd?--Light current 05:49, 25 February 2006 (UTC)
- I am not 100 percent sure, but I think the only difference is a negative sign, and the fact that emf is a bit anachronistic as a terminology. I am not sure why it is necessary or useful, and IMHO, it is easier to use the more precise and well-defined concepts of voltage, potential difference, and electric potential depending on the context. -- Metacomet 19:22, 25 February 2006 (UTC)
Unfortunately, many of the electromagnetic principles (ie Lenz law) use the term 'emf' but the people who discovered these principles could not have measured the emf they were talking about!--Light current 03:11, 26 February 2006 (UTC)
- I am confused. What do you mean when you suggest the emf cannot be measured? Isn't emf simply the voltage that shows up on my portable multi-tester when I place it across two terminals (without electrocuting myself!)? Or actually, times -1 to change the sign? Or is emf more complicated than that? -- Metacomet 04:52, 26 February 2006 (UTC)
Youre measuring the opposing pd with your meter- not the actual emf (theyre the same value of course but opposite polarity) See my first post under this heading.--Light current 04:55, 26 February 2006 (UTC)
- And if I disconnect the two leads and connect them to the opposite terminals, my multitester will read the negative value of the pd, which is the... I am not trying to be difficult, but I just don't see any difference between emf and pd except for the minus sign, which is actually arbitrary in both cases anyway (since it depends on which way you decide to hook up the terminals, and which terminal you choose to call '+'. -- Metacomet 05:00, 26 February 2006 (UTC)
Fortunately, the answer is in the article:
Distinction between emf and p.d.
If an external circuit is not connected to a source of emf, an electric current cannot exist. Thus, between the terminals of the source, there must exist an electric field that exactly cancels the generated emf. The source of this field is the electric charges separated by the mechanism generating the emf. For example, the chemical reaction in the battery proceeds only to the point that the electric field between the separated charges is strong enough to stop the reaction. 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 pd 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. emf itself cannot be measured directly.
I guess we will simply have to agree to disagree. If I connect the red wire of my voltmeter to the + terminal of a 9V battery, and the black wire to the the – terminal of the same battery, the meter will read +8.92 Volts. But if I then connect the red wire to the – terminal and the black wire to the + terminal, the meter reads – 8.92 Volts. So in one case, haven't I measured the pd, and in the other case the emf? -- Metacomet 05:20, 26 February 2006 (UTC)
Not so fast! ;-) If I were to say that emf is a cause and pd is an effect, does that help any? You always need a source of emf in any circuit, but you can only measure its effect as a pd - yes?--Light current 05:23, 26 February 2006 (UTC)
Maxwell's use of the term electromotive force
- It is also interesting to note that, according to the WP article, Maxwell used the term electromotive force to refer to what we would today call the electric field, and not what we would call voltage or potential. Since the electric field is defined as the force per unit charge acting at a point in space on a positive test charge, then Maxwell's is actually using the term force more or less in the correct sense. Electric potential, on the other hand, is really nothing but potential energy per unit charge, and so using the term electromotive force to represent potential energy is a bit of a misnomer. -- Metacomet 19:28, 25 February 2006 (UTC)
- Note that in terms of units of measure, electric field is volts per meter or equivalently newtons per coulomb. And from the Lorentz force law, we have:
- So again, Maxwell's use of electromotive force to represent what we call the electric field makes good sense.