Timeline for How can magnets be used to pick up pieces of metal when the force from a magnetic field does no work?
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| Dec 21, 2022 at 22:37 | comment | added | Ruggero Turra | "work is defined operationally in purely macroscopic terms". This is false. | |
| Apr 13, 2017 at 12:40 | history | edited | CommunityBot |
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| Apr 2, 2014 at 20:20 | comment | added | Tim Goodman | This paper discusses the quantum mechanical case: academic.csuohio.edu/deissler/PhysRevE_77_036609.pdf | |
| Apr 2, 2014 at 20:18 | comment | added | Tim Goodman | As a simplified (classical) example, consider a loop of current-carrying wire dangling from a string (so it's free to rotate) in an external magnetic field. As the loop rotates to align with the field, it gains rotational kinetic energy, but at the same time there's an induced EMF due to the change in magnetic flux through the loop. This opposes the motion of the circulating charges in the loop, thus robbing them of some kinetic energy. I believe if you do the calculation you'll see the amount of energy removed is the same as the amount of energy added. | |
| Apr 2, 2014 at 20:17 | comment | added | Tim Goodman | This answer doesn't seem quite right to me. When a magnetic field gives kinetic energy to an object, it induces an EMF that cancels out an equal amount of internal energy of the object. So the net work done by the magnetic field is truly zero. | |
| Jun 16, 2013 at 21:27 | comment | added | Larry Harson | @jim I think your link to that paper is very interesting and worth quoting in an answer. | |
| Jun 14, 2013 at 14:32 | vote | accept | sTr8_Struggin | ||
| Jun 13, 2013 at 14:37 | comment | added | Jim | My last comment will be that I think we agree on almost everything, but I want to reiterate that a magnetic field cannot do work. Just like the flux of a magnetic field through a closed surface must be zero by its very nature (not completely related but there is a connection), the total work done on a system by a magnetic field must be zero. I direct anyone to Chapter 6 of Griffiths' Electrodynamics textbook to convince yourself of this for the dipole case. Still, Ben's answer is the most accurate one on the subject I've seen on this site | |
| Jun 13, 2013 at 14:18 | comment | added | Jim | Furthermore, were a magnetic field to accelerate an electron, acceleration is not itself doing work; work requires a net displacement. Once the electron starts moving though, we can agree that the Lorentz force would quickly become the dominant force, as it is now a charged particle with a velocity. You also mentioned that macroscopically, it looks like the magnetic field does work. I agree completely. A magnet does do work on the paperclip. As long as everyone agrees and understands that deeper down this work is being done by induced electric fields in the magnet, I have absolutely no issue | |
| Jun 13, 2013 at 14:12 | comment | added | Jim | Having read your new answer, I can agree with the spirit of it, but some of the wording seems a tad off. When a magnetic field orients a true dipole (which is infinitesimal in size) one cannot really say whether work has been done in the classical sense because nothing has moved a distance, d. When it reorients any other dipole, like a bar magnet or a loop of wire, we agree that at a small scale, there are electric forces doing the actual work. The fact is that the force F in many cases isn't the magnetic force, but the net force imposed when a magnetic field is present..... | |
| Jun 13, 2013 at 13:58 | comment | added | John McAndrew | "The Lorentz force F=qv×B never does work on the particle with charge q". Well, if it manages to accelerate an electron from rest, then it's done work. | |
| Jun 13, 2013 at 4:13 | history | edited | user4552 | CC BY-SA 3.0 |
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| Jun 13, 2013 at 3:25 | history | edited | user4552 | CC BY-SA 3.0 |
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| Jun 13, 2013 at 3:25 | comment | added | user4552 | @JohnMcVirgo: No, I didn't say that magnetic fields never do work. The force in the second paragraph isn't a Lorentz force. For a longer discussion, see physics.stackexchange.com/questions/10565 | |
| Jun 13, 2013 at 3:01 | comment | added | John McAndrew | I'm confused: At the beginning you say magnetic fields never do work because of the Lorentz force, then you say an electron will be accelerated from rest in a magnetic field | |
| Jun 13, 2013 at 1:56 | history | edited | user4552 | CC BY-SA 3.0 |
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| Jun 13, 2013 at 1:39 | history | edited | user4552 | CC BY-SA 3.0 |
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| Jun 12, 2013 at 22:39 | comment | added | user4552 | @sTr8_Struggin: I guess I still don't understand how a magnetic field deflects charges in such a way that an electrical force capable of doing work is created. I've added more detail about the case of the wire moving through the field. Hope that helps. | |
| Jun 12, 2013 at 22:33 | history | edited | user4552 | CC BY-SA 3.0 |
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| Jun 12, 2013 at 22:26 | history | edited | user4552 | CC BY-SA 3.0 |
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| Jun 12, 2013 at 22:17 | history | edited | user4552 | CC BY-SA 3.0 |
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| Jun 12, 2013 at 21:56 | comment | added | user4552 | @Jim: It's hard for me to tell whether we're agreeing or disagreeing, and if so, on what. Now that you've studied up on it, how about writing up an answer so we can see a coherent presentation of your point of view? I can't quite make out your position on whether the magnetic field does work on a fundamental dipole. I'll edit my answer to clarify what I claim is happening in the case of the wire. | |
| Jun 12, 2013 at 19:31 | comment | added | Jim | I took the last couple hours to look this up in more detail. It seems that there is some confusion everywhere. In physics forums (including Physics.SE) it is the general opinion the magnetic fields and forces DO work (mainly based on intuition and anecdotes) and most of the top answers state this. However, in pretty much all official sources (texts, paper, etc) it is that they DON'T do work; it is always the result of induced electric fields/forces or external forces that there is any work done along the direction of the magnetic force. | |
| Jun 12, 2013 at 19:29 | comment | added | Jim | @BenCrowell here's an excellent link to read. A magnetic field does no work period. It turns out to be indirectly caused by it being divergence-less. The magnetic field can redirect and/or carry work but not do work itself. What may seem in some cases to be work done by a magnetic field is actually work done by induced electric fields. | |
| Jun 12, 2013 at 18:31 | comment | added | Jim | but wasn't the point that it doesn't do work on a composite dipole? | |
| Jun 12, 2013 at 18:27 | comment | added | user4552 | @Jim: "that qualifier is used because claims were only made that it did work on the non-elementary dipoles because it was accepted that no work is done on elementary dipoles" Not true. The energy of a magnetic dipole in a magnetic field depends only on the field and the dipole moment. If the field can do work on a composite dipole, it can do work on an elementary dipole. | |
| Jun 12, 2013 at 17:54 | vote | accept | sTr8_Struggin | ||
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| Jun 12, 2013 at 15:25 | comment | added | sTr8_Struggin | So it appears that the magnetic field can do work on dipoles (by changing the dipoles orientation). However when a magnetic field does work on a paperclip or current carrying wire somehow electric forces are responsible for doing the work. I guess I still don't understand how a magnetic field deflects charges in such a way that an electrical force capable of doing work is created. | |
| Jun 12, 2013 at 15:25 | comment | added | Jim | that qualifier is used because claims were only made that it did work on the non-elementary dipoles because it was accepted that no work is done on elementary dipoles | |
| Jun 12, 2013 at 15:22 | comment | added | user4552 | @Jim: Right, note their qualifier "non-elementary." | |
| Jun 12, 2013 at 15:15 | comment | added | Jim | Wikipedia, " It is often claimed that the magnetic force can do work to a non-elementary magnetic dipole, or to charged particles whose motion is constrained by other forces, but this is incorrect[19] because the work in those cases is performed by the electric forces of the charges deflected by the magnetic field." | |
| Jun 12, 2013 at 15:01 | history | answered | user4552 | CC BY-SA 3.0 |