Timeline for Suspicious EMF equation
Current License: CC BY-SA 4.0
20 events
| when toggle format | what | by | license | comment | |
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| Aug 13, 2022 at 11:43 | vote | accept | Edward Henry Brenner | ||
| Jul 27, 2022 at 15:55 | comment | added | Charles Hudgins | Maxwell's equations give us a necessary and sufficient condition for when such a $V$ can exist. We know $dE = -\frac{\partial B}{\partial t}$. By the Poincare lemma (assuming our space is simply connected), $E = -dV$ iff $\frac{\partial B}{\partial t} = 0$. That is, we have a well-defined, path-independent voltage precisely when the magnetic field is static. If there is a time-varying magnetic field, we should speak instead of the EMF to emphasize that the work done in moving a particle along a path will be path-dependent. | |
| Jul 27, 2022 at 15:25 | comment | added | Charles Hudgins | My two cents: The reason people can't distinguish between voltage and EMF is because there's a purely mathematical distinction few are taught. Voltage $V$ and EMF $\epsilon$ are both defined by the equation $V = \epsilon = - \int_\gamma E$, where I view $E$ as a 1-form, and $\gamma$ is the path of integration. When $E$ is exact, i.e. there exists a function $V$ such that $E = -dV$, Stokes' theorem guarantees that $V$ is well-defined up to an overall constant. When $E$ is not exact, the path $\gamma$ matters. We emphasize this by using a different symbol $\epsilon$ and name (EMF). | |
| Jul 27, 2022 at 11:34 | comment | added | Ján Lalinský | @PeterRottengatter Indeed often some people do not distinguish the two and when other people try to learn from the first group, they often get confused. The symbol $\mathscr{E}$ or $\mathcal{E}$ is the standard notation for EMF, check for example Panofsky and Phillips, Classical electricity and magnetism, section 7-2, or Griffiths, Introduction to electrodynamics, section 7.1.2. | |
| Jul 27, 2022 at 10:37 | comment | added | Peter Rottengatter | @Ján Lalinský The symbol you're promoting looks to me like a curved E, imitating the electric field E. If that's what you meant, then I'd argue that pedagogically you're doing worse than what you critisise: U and EMF at least are of same dimension, while E is voltage over length, hence an entirely different qantity. | |
| Jul 27, 2022 at 10:01 | comment | added | Peter Rottengatter | @Ján Lalinský I tend to agree that it may make sense pedagogically, but I can't help noticing that in practice obviously very few people make this distinction. In fact, I can't remember ever having seen the symbol you mentioned being used for EMF. Insofar, I also contest your claim "U and V are traditionally used to denote voltage on an element". I don't believe it's true. | |
| Jul 27, 2022 at 9:16 | comment | added | Ján Lalinský | @PeterRottengatter EMF is sometimes called voltage in practice, but potential difference is also called voltage in practice. This is causing needless confusion between the concepts of potential difference and electromotive force, both of which are important in physics. From pedagogic standpoint it is better to restrict the use of symbol $V$ to potential differences (e.g. when applying KVL) and use the symbol $\mathscr{E}$ for all the various kinds of emf's, including induced EMF. | |
| Jul 27, 2022 at 7:51 | comment | added | Peter Rottengatter | @Ján Lalinský You should realise that electromotive force IS a voltage. U and V are therefore exactly the right symbols. | |
| Jul 27, 2022 at 6:30 | history | edited | Qmechanic♦ | CC BY-SA 4.0 |
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| Jul 27, 2022 at 6:09 | history | edited | Buzz♦ | CC BY-SA 4.0 |
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| Jul 27, 2022 at 6:00 | history | tweeted | twitter.com/StackPhysics/status/1552171940080648192 | ||
| Jul 26, 2022 at 23:10 | history | became hot network question | |||
| Jul 26, 2022 at 18:20 | history | edited | Qmechanic♦ | CC BY-SA 4.0 |
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| Jul 26, 2022 at 18:07 | answer | added | Ján Lalinský | timeline score: 9 | |
| Jul 26, 2022 at 17:53 | comment | added | Ján Lalinský | Never use $U$ or $V$ to refer to electromotive force (emf). Use $\mathscr{E}$ or $\epsilon$ instead. $U$ and $V$ are traditionally used to denote voltage on an element (drop of electric potential in the chosen positive direction). | |
| Jul 26, 2022 at 15:36 | answer | added | Peter Rottengatter | timeline score: 4 | |
| Jul 26, 2022 at 15:17 | answer | added | LPZ | timeline score: 4 | |
| Jul 26, 2022 at 15:15 | comment | added | Edward Henry Brenner | @SeñorO It's $dl$ not $dI$. I think that units match perfectly, try again. | |
| Jul 26, 2022 at 15:10 | comment | added | Señor O | Is that a $dI$ or $dl$? Either way the units don't match the other integrals | |
| Jul 26, 2022 at 14:59 | history | asked | Edward Henry Brenner | CC BY-SA 4.0 |