@@krabkrabkrab I mean... the damage could be great, but the misunderstanding is still small. It could've been resolved with one sentence like "Work done by magnetic fields is not in scope of classical electrodynamics".
I think the biggest issue with this kind of thing is how people will take in concepts mentally and then fight and die on those hills. Kinda like when someone misunderstands "fossil fuels" and will fight and argue and die on the hill that is "gasoline is made from dinosaurs." Because once a concept has set root, some people just refuse to ever allow it to be changed
Minor misunderstandings turning into a big problem reminds me of an old Kids in the Hall skit. Never Put Salt In Your Eyes. ruclips.net/video/_83MEuLoz9Y/видео.html
Wow - we didn't have this problem in our classical engineering textbooks. They were quite clear that magnetic forces do no work on _electric_ charge, but magnetic field gradients can do work on magnetic dipoles. And also that electric fields and magnetic fields turn into each other when you change reference frames - so they're kind of the same thing.
@@justinpridham7919 Magnetism is what you get when you take special relativity into account with electricity. Basically, magnetism arises from a moving electrical charge. Why? Because when the electrical charge is moving, it "perceives" spatial dilation of the other electrical charges. What was once neutral is now electrically charged. The moving charge experiences a "magnetic force".
Studied mechanical engineering and currently an engineer.... At no point did I get an explanation of how magnetism does work, but 30 seconds into this video my first thought is "of course magnetism does work, motors and generators exist". Without work via magnetism, we would not be able to make use of just about every modern application of thermodynamics from gas turbines to air conditioning.
I think the most relatable part of this for a fellow physicist is when she says “I’m scared of magnets”. I’m getting flashbacks to the time I dropped a Nd magnet in a particle physics lab and it… exploded. I got a shard of it in my leg, and the resulting shards stuck to every piece of metal equipment, sending stray fields all over the place and f’d with the beam lines for years. I’m scared of magnets.
@@DaylightRobberyCA So this particular lab was a low energy particle lab (slow particles, mostly antimatter), so they had a few devices that were long tubes with copper wire around them (making a kind of magnetic funnel to maximise the number of particles hitting the detectors (basically a kind of lens for the beam)). The problem was the earth's magnetic field needed to be accounted for, so they would set the detector at one end of the table, then use permanent magnets in various places until the signal would not increase any further. When this incident occured, all of that needed to be redone to account not just for the earth's magnetic field, but the random bits of magnetic Nd stuck in various places.
Seeing this video title just made me realize that I had almost completely forgotten that I had ever learned this rule. When I first learned it I immediately thought of two magnets attracting each other and asked my teacher about it. My teacher told me straight away that this "magnetic forces do no work" was too general a statement, and that it would be more accurate to say that "Lorentz forces do no work". I thought for a few seconds and said oh, and then never got it wrong again.
@@vipinx8881 Yes, strictly speaking, the Lorentz force also does work when considering a complete electromagnetic field, with both non-zero E and B, it's just that the original context of this discussion was magnetism and especially static magnetism. When I was first introduced to the concept of the Lorentz force, the scenario talked about was almost always the motion of a point charge in a static magnetic field. At that point, the Lorentz force naturally does no work because of that cross multiplication.
My undergrad taught (teaches?) QM _before_ E&M, so my Physics 240 professor straightaway went, "except we all know that magnets are actually spinny bois" and everyone just nodded along. Except me, who was taking the course as a freshman _before_ taking Quantum in the second semester. So I remember fixating like Angela did and trying to work out the limit as r went to zero without QM like Griffiths' collaborator is doing. That class was a ball and a half, and I don't think I ever felt more meaningfully challenged, before or since, but _damn_ if I wasn't a problem student.
Great video! I really tried to cram a lot of info into my video. I talked _really_ fast and had to include a supplementary point in a pinned comment because I rushed production, so I'm glad you made one where you take your time. I love how thorough it is. Also, IMO, Griffiths making a purely classical textbook was a mistake (pedagogically speaking) for reasons you point out. We can't just forget permanent magnets exist. That's unreasonable.
How does classical EM treat permanent magnets? With tiny current loops? I can't imagine people just didn't realize their amazing complete theory of electromagnetism just didn't actually explain magnets.
The thing is, he actually _doesn't_ ignore quantum mechanics entirely for the exact reason you give - in his section on ferromagnetism (page 278 in the 3rd ed) he mentions that neighbouring dipoles prefer to align for an "essentially quantum mechanical" reason "I shall not endeavour to explain" (he even mentions "electron spins", though I guess he _may_ mean "of tiny current loops"). So it's actually arguably _worse_ than some (admittedly eccentric or, y'know, "a _little_ crazy" :) attempt to produce an entirely classical EM textbook because he's arbitrarily non-classical at other times.
I think it's fine to make a purely classical textbook as long as you just say "Hey, permanent magnets don't quite make sense in classical physics, so we're ignoring them." That's all it takes.
@@Huntracony Sure, but if someone does that, they shouldn't give the zero magnetic work thing so much emphasis. Don't bold it and box it like it's a fundamental principle.
I hadn't realized how lucky I was I got the "sorry, that will go into QM" explanation from the professor way back when I took that course with that book.
In fairness, that's _also_ a solution to Dr Collier's problem - either she had a teacher that didn't provide that very simple answer OR she never asked the question (but I guess then we wouldn't have got this very entertaining video :).
I had to extract that from my physics undergrad professor during office hours. I saw that bolded boxed sentence, looked up when he would be in his office, and was like "I can pick stuff up with magnets, explain yourself." I don't think he had really thought about it in a while, and it was fun to watch him remember more and more as he started explaining.
This reminds me of something that I learned once: "A scientific law needs to contain two parts; a mathematical relation that holds true, and a description *under which conditions it holds true."* This whole thing seems like an issue caused by Griffith forgetting to do that second part. There seem to be struggles all throughout science education that stem from not realizing this wisdom. Most notably students when they miss-apply a scientific law. But it certainly doesn't help if a professor, nay, the author of a textbook is guilty of overlooking this aspect of scientific laws.
I can at least empathize with him. My brain does the same thing where it will just skip over "obvious" parts which is...not good in relation to math and physics lets just say. I was confused why Angela has a problem with this originally because to me it was "so obvious" that he was talking classical mechanically only based on how he pharsed things. I just instantly accepted that without it entering my conscious mind. You're right that it can't just go unstated though. This is why I am a TERRIBLE teacher and a subpar communicator, even in subjects I know decently well. I hate it, but it's very, very hard to manually stop and be explicit with the assumptions made. Imagine how much trouble I had with proofs lol
@@Tinil0It's a matter of practice, which reduces the struggle. At first we'll clumsily forget do a step, then after a number of times practicing, it'll start to become second nature.
Before even playing the video, I pictured a name in my head, “David Griffiths”, who I still partially credit with derailing my understanding and interest in physics around 2004-2005. I seriously had no idea you’d mention the exact same book. We also used his QM book with the stupid cat (1st ed). I just remember reading the same introductory paragraph over and over again, trying to understand what he was saying about an electron, and giving up. I was so excited for those classes, but I just couldn’t make it stick in my brain. The profs were all researchers, and seemed more interested in their work than the students they were forced to teach. They didn't care. If our grades were low at the end of the year, they'd just curve them all up and push us along to the next year. I skirted by and got the hell out of there with a BS and a shit ton of debt.
27:19 For our civilization to work, for our world to work, for us to be able to sleep peacefully in our beds at night... ...someone has to care enough about this topic to get totally bent. Thank you, Angela.
I CANNOT OVERSTATE just how close to home this video hits for me. I have spent so much time wondering about this, googling to no avail, and I never got a satisfactory answer. Thank you for this video, you rock!
This is a classic case where we see that Math can't explain "Natural Physical process". The equation is intended to allow the creation of a graph, on which we intend to plot points to create vectors. The graph arranges the Force axis always perpendicular to the Velocity axis. But that is not what happens natural physics, its only applicable to your graph. The upshot of all this is that because math has been elevated above the understanding of Natural Physics, it then is assumed to be driving natural processes. But this is incorrect. In fact, your math may be totally wrong, because you have developed a wrong equation, because you never understood actual natural processes, or you forgot to include something, or you made too many assumptions. A incorrect equation can still give results that resemble what we observe, so this is a very real trap for those that think that the universe is ruled by math. Its not.
@@everythingisalllies2141it is ruled by math, kinda, but math is complicated. Like in her ramp example how she left out the friction of the ramp and the air and said the force is constant. The universe is math, but it’s a lot of math to do anything. Like, do you model every atom?
Time for a new video I completely and entirely do not understand, but could very easily explain why I enjoy. It makes me feel dumb in a way that doesn’t make me feel bad about being dumb.
@@diggysoze2897 probably makes you (read: us) slightly less dumb in the process too! Assuming you (again, we) learned anything at all over the course of the video 😂
Nice video! And I agree that Griffiths could have been a little less coy about the case of electron spin. But I think many of the commenters are missing the point of the discussion. Griffiths is absolutely correct in classical (i.e., ignoring quantum mechanics) E&M -- the net work done by a magnetic force really is zero. It's just that the mechanical work on the dipole is not the only work being done. There is also (negative) work done on the current flowing in the dipole (classically a current loop) and this is equal and opposite to the mechanical work. So, to maintain the current, a source of emf (power supply) must do extra work equal to the work done on the dipole. The net accounting looks like: power supply does work, magnetic force does zero net work (which comes in two equal and opposite pieces), and the dipole gains gravitational PE equal to the net work done by the supply. The transfer of energy is from the supply to the dipole. The basic reason this changes in QM is that the electron's dipole moment (arising from its spin) is immutable and requires no "power supply" to maintain. There is no motional emf in the electron and no work done by any source, so the mechanical work on the dipole is all there is. But this can't happen in classical E&M, even for very small current loops. The comment by @orangebutnotred invoking Sisyphus is actually quite relevant here :-). Imagine we push a block up a ramp with a *horizontal* force, i.e., parallel to the ground, not the ramp. The block gains gravitational PE, but the horizontal force does not lift it, the normal force does. But the normal force cannot do any net work, for the same reason the magnetic force does none -- the displacement of the block is perpendicular to the force. What happens here is that the horizontal push does some work and the normal force does zero work (the works done by the horizontal and vertical components of N are equal and opposite), with the block acquiring gravitational PE equal to the work done by the horizontal push. This is discussed in Griffiths, including the box on a plane example, in section 5.1.3. It is also discussed very nicely in the Feynman Lectures vol. II-15-14ff.
In the preface of the new 5th version Griffiths says: "I have added some new commentary on subtle issues: ambiguities in the definition of polarization in crystals, problematic aspects of electric and (especially) magnetic field lines, the awkward role of intrinsic spin (a strictly quantum phenomenon) in a classical discussion of magnetic materials." I have not gotten far enough in the book to speak on how he incorporates it in the text, but apparently it is now mentioned!
@@Khronogi They're not all this bad, I promise! There are plenty of fantastic ones, they're just not necessarily the ones that get set. It does take a bit of exposure to discern between the good and bad quickly though. And _books,_ in general? There's an _overwhelming_ variance of prose and tone out there! You are _seriously_ blinkering yourself if you let a few elitists put you off an _entire medium._
My favorite genre of video is when RUclipsrs make videos about esoteric grudges in fields I do not care about. Always so much fun and this channel has so many of them.
@@Alex-js5lg And I'll let you know when I make my counter-rant. I mean, singular they does suck, but I'll argue for it as the best option for English. Alternatively, we can talk it out right now?
a couple of semesters of college physics was enough to get me a lifetime of small grudges on how they could possibly represent concepts. ...fuckin' optics, man.
Let me introduce you to ...rants about Lifting Force in Airfoils! The years of online grudge-rants even resulted in entire contrary textbooks being written, by hated enemy groups in academia. (So, if we cannot convince youtubers to reject our opponents' base heresies, well, instead we can educate entire generations of aerodynamics undergrads. What did Johnathan Swift say about "the best way to crack an egg?" The genocidal Big-Endians want to wipe the Little-endians from the face of the Earth. Or maybe it was the other way 'round.)
@@NZsaltz And I'll let you know when I make my counter rant to your counter rant because I think singular they is actually awesome. My native language doesn't have anything like a singular they and it sucks.
I'm so happy you made this, I've also been haunted by memories of that one line in Griffiths, its shadowy presence has weighed upon me all through my PhD studying magnetic materials and even into my current postdoc. You have exorcised an intro physics demon and I suddenly feel free, thank you (*signs up for patreon)
I don't know about you or anyone else, but I have had to accept throughout my education in Physics that some of the information I am given is just plain wrong, either out of lies by omission, overgeneralization, or professors who are straight up wrong. I now have a thing where whenever I read or hear something in Physics, I have to go, "Ok, but is that *really* true?"
@@MrAnonEMossI’m working on my Physics BS right now, and I’ve really come to realize this recently. I will take your comment to heart and always check multiple sources for any new information, thank you ❤
You are AMAZING! I am a female researcher (not in physics). And I love every single video you posted. Please keep up the wonderful work. Love to see more videos from you!
When I, as an ethnically Jewish atheist, taught high school physics in an Orthodox Jewish school, it was very tempting to apply to physics definition of work to the Sabbath. Like... just put everything back and you've done no work on it. But I didn't think that would be appreciated.
Actually they likely would have appreciated it, as it helps distinguish the English word "work" from the sorts of activities your students would consider prohibited on the Sabbath. In general, if you can restore it back to the way it was, i.e., you haven't transformed its state in a persistent manner, then it's not prohibited "work." So in fact it could have been a useful way to discuss different meanings of the word "work" without needing to get into the religious aspects.
i find it very interesting that Griffiths won’t mention dipole forces because it’s not a classical effect. but a few chapters later he goes on to talk about the gyromagnetic ratio and that due to quantum effects, the actual ratio for the electron is different from the predicted classical value
Likewise (in the chapter after "magnets do no work", in a section on ferromagnetism) he explicitly mentions an "essentially quantum mechanical" reason for neighbouring dipoles to align (though declines to explain it). So he's not averse to mentioning QM, except in the section on work done apparently.
We found an excellent use for the NMR magnet back when I was doing my PhD. This guy kept sticking on his crap heavy metal mix tape in the lab, and wouldn't compromise with allowing anyone else's music. When he was off on his lunch break, one quick rub on the side of the NMR magnet downstairs and all his carefully curated death metal was reduced to blissful white noise.
Lol that’s a great story. But I thought NMR magnets were shielded as to prevent things like that? Or was it an exposed magnet not inside the NMR spectrometer? Those are incredibly strong magnets.
Honestly, this seems like a very roundabout way for Griffiths to say that Classical Electrodynamics can't explain permanent magnets. He could just say it outright instead of making two separate editions of his book, investigate a new theory of electromagnetism and give an interview, all the while just being passive aggressive instead of actually answering the question.
My god, I was instantly teleported back to my EM class in 2009. I had an almost identical experience as you, and this video unlocked a _flood_ of memories that I had neatly ignored for 15 years. Thank you so much for chasing this down and finally closing a decade-and-a-half old question on my brain! Amazingly interesting video, as always!
Really good that you think instead of just trying to make faulty statements work. Feels so good to understand things instead of just applying info without the understanding, youre smart i think, all of those points makes it pleasant to watch your videos :)
Have a like and comment for respecting that yes, I did just want the 5min summary today. Also, have a new sub for informing me of the option and linking it in an approachable way. Thanks for not treating your channel as the end-all-be-all source of information where the goal is to monetize attention, but for what RUclips should be about. Your style is great and I look forward to the next one!
There's a channel about old historical firearms where every year they do a holiday fundraiser and when they meet certain goals they will feed berries to a guinea pig, and they play that same tune: so every time it transitioned in this video I thought about a happy little whistle tater getting a strawberry.
For those curious, Barandes did publish his work on a classical theory of intrinsic dipole moments. I think the paper Griffiths was specifically referring to is "On magnetic forces and work JA Barandes - Foundations of Physics, 2021".
@@JulianSildenLanglo According to himself, he has done it. I don't know what objections Griffiths or other smart physicists would have to his construction. I would need to read the paper and go into all the details to see why the construction may not work as intended. I might do it, and if so post another reply here. I'm actually quite intrigued by the paper, since it is common knowledge (At least I was taught this in the context of the Bohr-Van Leeuwen theorem) that you need quantum mechanics to explain magnetism . If this isn't the case, I would be quite shocked.
@@De2Venner And, well, what would be the point? Yeah I get it, it's interesting, but QM has worked pretty damn well for us. Why do we need to jam its concepts into a less effective theory? Just to retroactively vindicate this dickhead?
arXiv:1911.08890 is what I found, so far, for Barandes. Its title is "Can magnetic forces do work?" Paper submitted 4 Nov 2019, and last revised 26 Jul 2023. I am wondering if this is the same paper that you mention.
I went on a real roller coaster with this one, gradually getting grumpier and thinking things like "but that's not the best definition of work" etc. etc. and then got more and more relieved and/or sheepish as the video went on and you then addressed all of those things and I wondered why I thought you wouldn't.
Thanks for this one. I’d convinced myself that I didn’t really understand work, but didn’t realize that it was the stupid Lorentz equation that was at the bottom of it. The whole thing with induced currents was a cop out since the electromagnet won’t pick up non ferrous ( or at least non ferromagnetic ) stuff. That experiment didn’t even cost much. It also doesn’t really matter if you can’t explain something without quantum mechanics, but denying something doesn’t happen when it is happening is silly. All you need to say is that classical electrodynamics doesn’t explain it. I really appreciate your tenacity on this question. You may take more time to say stuff, but you seem to be pretty good at making sure your explanations are clear. They are also fun, which keeps me from giving up on the harder parts.
“Fucking magnets, how do they….” So the moral of this story is to properly tell your readership what your textbook is about or else they’ll think you are a troll.
I'm reminded of how we were taught simplified versions of biology, chemistry, history, etc in middle school and were just not given the note that "of course, you'll learn more in high school and college that will sometimes completely contradict what you're learning now" Leaving off that footnote was frustrating then, and you're just telling me that it doesn't stop in middle school: they will keep annoying me by leaving off that footnote forever. I guess the assumption is that I'd just know school is like that after the first time, so it's too redundant to keep printing it?
@@leeroyjenkins0 that's all fair, but I just want to note that I didn't say the simplified teaching was a problem, I just found it frustrating how rarely I was told, "of course this model has already been superceded by one you'll learn in a few years" This was incredibly frustrating in history classes where the simplified histories really were deeply misleading rather than merely omitting details.
@@silverharloe That's why I like PhysicsGirl Dianna Cowern- she does a great job of simplifying things without dumbing down or making up analogies that will lead you astray later. I hope she gets well and gets back to making videos. (Angela tends to do things for a much higher level than PhysicsGirl)
I know had a similar mental break when learning high school biology because of things like "Your cells store energy as ATP." OK, but what actually IS ATP, how does your body produce it, how is it moved around to places that need energy, how is it used to supply that energy? Like, I know it would be absurd to expect them to get into all the specifics of the chemistry involved, but you're leaving out basically the entire story when you simplify the entire process to "your cells store energy as ATP" and then never elaborate any further in any way. At that point you're not actually understanding anything, just memorizing a list of phrases. It's like how everyone knows "the mitochondria is the powerhouse of the cell," but ask anyone what that actually MEANS and nobody actually knows anything beyond that one phrase.
A "lie-to-children" as it's sometimes known (coined by Ian Stewart and Jack Cohen but popularised by the inimitable Terry Pratchett). I guess the positive version would be, "All our models are approximations of reality and the higher the level you study at, the better the approximations become [but you'll never get The Truth because that's not really what science does]" (which may also have the advantage that if/when people _do_ find out that, when it works as intended, science is _the_ best way we have of becoming less wrong about the world BUT can _never_ provide 100% certainty, they don't then become disbelieving of the whole enterprise, as seems to be happening increasingly often these days).
@@-tera-3345 Usually textbooks have a rough sketch of the aerobic respiration for Eukaryote cells. All things considered it's somewhat superfluous to get into the exact biochemistry of ATP. Everyone remembers "mitochondria is the powerhouse of the cell" because it's been memed to hell and back but we don't look at that textbook diagram showing how O2 is used.
1:15 "audience of only me" - oh, no, I remember this phrase in this book. We debated it heatedly because it didn't make sense to us. We eventually dropped it because it derailed the class for so long. I've thought about it a couple times since (decades ago) but chalked it up to the book being wrong in an effort for brevity.
And yet instead of adding "in classical electrodynamics" to the 4th edition, he adds a completely new section basically saying the people questioning it are idiots. He might write a good textbook, but he seems rather unsocialized.
it's not even just one book, or one teacher.. it's everywhere! It's one of those oft-repeated science phrases that aren't super helpful but get repeated over and over verbatim, like the one about centrifugal force not being real (although that's admittedly more helpful and less confusing. It's just the only thing I could think of this early).
@@idontwantahandlethough Funny-the only places I ever remember hearing that the centrifugal force doesn't exist is pop culture and _maybe_ high school physics. By the time I was learning classical mechanics, we were *all about* doing the coordinate transformations to derive centrifugal and coriolis forces. Now, was that due to me making the jump to college / major physics? -Definitely- Maybe. But I'd like to think it was because I graduated high school in 2005, right around the time that xkcd came out.
@@idontwantahandlethough except, of course, centrifugal force is real. You can, in fact, measure it and it has consequences. The nomenclature here does a real disservice. Inertial and non-inertial are better terms
I love that you reference the Serway book, my dad worked on the instructor’s manual and student solutions manual for that textbook for many years (but not the main textbook). Nice to see people learned from it and became successful physicists who know a lot. I have met Professor Serway and stayed at his house, he and my dad were co-workers and friends. Went swimming in a lake in his backyard, even.
In the 4th edition of Griffith’s, he does an entire section (6.4.2) about the intrinsically quantum phenomenon of ferromagnetism. On page 288, he even mentions electron spins: “… to align virtually 100% of the unpaired electron spins”. Love this book as an intro E&M text, but I really enjoyed your comments on it
Fun fact: that's _also_ in the 3rd edition (page 278 in mine) so the hypothesis that Griffiths has effectively written a book _as if_ quantum mechanics doesn't exist is false. In fairness, Dr Collier _does_ say that she may have missed it, as an undergrad she didn't meticulously read the entire book etc. And anyway, how hard would it have been to just stick a similar line in the section on magnets and work (or to do as he's now done in the 4th edition, include an explicit mention of where he _does_ explain it) ? (but then, we wouldn't have got this entertaining video :)
A note on the foot-pounds vs pound-foot thing. They're exactly equivalent. Pound-foot is typically associated with the torque equivalent to one pound of force acting perpendicular at a distance of one foot, whereas foot-pounds is typically associated with the moment created by that torque. But they can be used completely interchangably with no issues at all. Myself and most other engineers i've interacted with use foot-pounds for both.
Maybe we should use vector units? Then work would have units of feet-dot-pounds, which is the same as pounds-dot-feet. But torque would have units of feet-cross-pounds, which is not the same as pounds-cross-feet! I feel like we're slipping into geometric algebra here...
Not quite: the torque direction is sort of a co-space, like the space of oriented planes vs directions. Angular stuff (and torque) lives in a universe of oriented planes, while non-angular lives in the land of direftions. We can choose to correspond these spaces, but doing so involves an arbitrary non-physical choice of convention. Except of course the weak force shows up and cares about it!
@@gcewing There isn't such a thing as geometric algebra - you are just being taught regular old algebra in a shitty non standard way. And no, you dont need to care about this as the quantities involved are a scalar in one case and an (axial) vector in another. Things being scalars or vectors or tensors isn't something that you need to invent units for, especially since you want to look at individual components to which you also assign the same units as the overall quantity.
Ok, I'm stopping at 12:32 because this is too 'busy' for my brain! But it seems to me that the ramp solution would involve more work than a vertical lift because there are two components - vertical and horizontal, both requiring work. The ramp is the same as lifting the box and then pushing it horizontally (or vice versa).
I had the EXACT same experience in 2009 in EM class, with the same book. In our case, seven guys just sat in a library having this discussion for hours..
As a dynamicist magnetic fields terrify me and I feel like I'm in too deep at this point to have asked anyone about this irl, so I'm really glad you made this video cause this is honestly so relatable
Reminds me of Feynman at Oak Ridge, embarrassed because he didn't ask them to explain the symbols on the blueprints right away. "Right away would have been okay, but you let it go too long!"
I loved that i was perfectly following along on the discussion on work and energy, then stuff went out there with quantum spin stuff and I was once again humbled.
I did my undergraduate E&M course mearly 3 years ago and I used the new edition of Griffiths, but I was left with the same confusion as you. This bothered me to my core, and the new chapter didn't help me. Griffiths definitely made a mistake by not making his intentions clear; to write a book on purely classical E&M. Thank you for helping me find peace. By the way, I love your energy and I think a lot of physics students will find you very relatable. This video deserves more than a million views.
I never took a college physics course (astronomy survey counts?) and I don't really get the maths but I can enjoy listening to you talk about physics for 40 minutes and I just accept that despite sometimes asking myself, "why?" I don't think you need a disclaimer about such a thing that I stumble over, just that I want to share that feeling. Isn't science is all about that kinda thing that pushes us into getting deeper? Someone explains something in hopes that they are communicating at your level and for the most part, it can be OK. For those whom the explanation is not OK, they dig deeper and maybe move things forward a little too. I like how you are really demonstrating how science is fascinating and that keeps it among my thoughts. Thank you!
I've always understood that they do no work on charged particles (ignoring spin), but yeah I never understood how they do no work on magnetic materials.
Exactly. Evidently magnets do not exist in the formal model of Classical Electromagnetics. His note should have said that the fact of magnetic forces in the model unable to do work is proof of the model's incompleteness.
@@sciptick Of course magnets exist in classical electro _magnetism_ - it'd hardly be widely lauded as _the_ crowning achievement of 19th century physics if it didn't account for well known objects which demonstrate the exact phenomena it's attempting to explain. The classical explanation though involves treating electrons as tiny spinning charges (as in _actually_ spinning, like e.g. planets do, rather than their intrinsically quantum mechanical property "spin") and the _work_ is done by _electric_ forces, as per Coulomb's law (which _doesn't_ act perpendicularly to the direction of motion but just along a vector between the charges).
“Imagine you’re moving because you live in America and you’re renting and rent’s expensive and you have to move every single year so every single year you pack up all your books into little boxes and you carry them down the steps and you put them on a truck and you move them to a second place where you carry them up the steps and you unpack them only to do it again in like 11 months …” If all the physics problems I had to do in high school would have started like this, I’d have been way more prepared for the actual world.
@@imwelshjesus Nah, students use milk crates that they... "borrowed"... from behind the cafeteria. That way you never have to unpack -- just flip em on the side and they become a bookcase, flip em again and you're packed and ready to move! I mean... er... so I've heard.... :P
@@olencone4005I mean milk crates are $10 at Home Depot, and some of us acquired ours legitimately. That doesn't stop some people from assuming you took it right out of dairy fridge in the grocery store, because you happen to use one as your shopping basket because it fits conveniently on your bike's rear rack.
In my high school physics class, my teacher illustrated the non-intuitive character of work. He held a lab stool motionless straight out to the side. He declared that he was doing no work despite the emphatic disagreement coming from his arm and shoulder muscles.
But he is doing work. It's just that the work isn't being done to the stool. The work being done is internal to his body. There's ions being pumped around his muscle tissues, things like that.
TLDR: Magnetic force does no work on PURE ELECTRIC CHARGE, but it does work on MAGNETIC CHARGE and MAGNETIC DIPOLE (and higher multipoles). (1) Magnetic charges (monopoles) we have no evidence of existing in nature. (2) And classically, magnetic dipoles result out of moving electric charges, so something else does the work fundamentally. (3) But quantum mechanically, spin 1/2 particles like electrons have intrinsic magnetic dipole. So lifting stuff using a ferromagnet, the magnetic field does work.
Thanks for making this. Sadly, there are a LOT of similar issues I've encountered in education. Many in the sciences and math, but the phenomena is not exclusive to just those fields. Seems to happen everywhere, actually. I suspect the issue stems from how the human brain works. As our neurons connect, those networks are always relative to some other part of the local network. When we attempt to explain something to others, we are often conveying information stored in one network and have no awareness of the other networks required to contextualize that information. The part I find most interesting is the resistance some people have to making the small update to contextualize the information once it's been pointed out to them. Like you mentioned, homeboy should just add a sentence to clarify that the "work" can be explained by quantum mechanics, and is outside of the field of the classical scope of the book.
“I got a couple of comments on my last video […] that were like ‘why are you scared of magnets?’ And like, why *aren’t* you scared of magnets?” This is why I watch past the outro lol
I hate Griffiths so much for this. All he had to do was say "btw, this doesn't apply to ferromagnets", but he just kept repeating "magnetic forces do no work" as if that's an argument.
Angela picking up the 4th edition at around the 22:00 min mark is like the midpoint of act 2 where things look fine (an explanation, huzzah!) on the surface, but are about to go to shit, plunging to the act 3 break pit of despair
I am a 3rd edition kid. In my undergrad, I had two old professors that had a running game of trying to come up with more and more convoluted examples of magnetic fields apparently doing work that the other would have to solve.
I worked in undergrad with neodymium magnets, and even small ones if you're not careful with them will just shoot off and stick to a metal desk leg and they might break on impact, and as you said, they don't care if you are in the middle. Yeah I'm definitely scared of magnets.
Magnets don't scare me. ELECTRICITY is whats scary. "What do you mean there's 'invisible energy' inside those perfectly normal pieces of elongated copper? And If I touch them the wrong way, I just straight up die? "
I love Andrew Zangwill's explanation of this in section 12.3.1 of his Modern Electrodynamics. It helped me get a nice intuitive explanation of this phenomena in classical E&M
You just blew my mind for no other reason but bc u like hearing urself talk… you are the best science communicator i have ever had the pleasure of knowing
As someone who knows nothing about physics past barely understanding my highschool chemistry class and nearly failing geometry( but ive got basic algebra down i think) i would love a video explaining how exactly magnets work on atomic and/or molecular level. I know so much about molecular structures and because im interested in cooking, computers, and metallurgy ( because of my work at a non-ferrous metals recycler), the things i dont know bother me so much more. Magnets are one of them. I use them everyday, ive killed them with heat, ive charged them back up with a machine. Ive owned so many different kinds. Thank you for your videos and time!
I love it! I took E&M from the same book, and got hung up for years on that sentence! I never understood it until I looped back around and taught E&M out of that same book and realised it was a bit of a “for a spherical chicken in a vaccuum” sort of statement. (If you know that old joke.)
Griffiths is such a hit-or-miss author for undergrad physics. His Quantum book is, generously, trash, but his Elementary Particles book is great. I remember E&M being somewhere in between, with some members of the faculty _seriously considering_ swapping it out with Jackson.
@@GSBarlev Indeed Griffith's quantum is horrible, built a lot of bad habits for fellow grad students. For E&M, Zangwill is basically Jackson for someone who doesn't hate their students.
@@vlix123 I learned from Cohen-Tannoudji, it was brutal and rigorous but it taught me the basics in the most general way possible. Griffiths teaches very special cases and doesn't state all the assumptions made IIRC, which I guess is fine for a single semester class, and it's fun to read, but it seemed to hurt my peers in grad school who used it for undergrad cause they had to unlearn/relearn a lot of it in a more general way to progress to the more advanced topics (addition of angular momenta, WKB theory, 2nd order perturbation theory etc.), whereas I felt way more ready thanks to Tannoudji. Zettili is a happy medium I think.
I used Griffiths for E&M last year. I just started the video and don't know if you mention this later, but he actually spends a few pages explaining this a bit later in the book.
I will say, that same boxed statement is in 4th Ed. This time he mentions to read Chapter 8 for more details. It's not until section 8.3, in the very last paragraph of a 4 page discussion, that Griffith's mentions "The magnetic field *can* do work on these 'intrinsic' dipoles...it is *not* classical electrodynamics...". This after 4 pages of justifying magnetic fields doing no work by analyzing all magnetic dipoles as infinitesimal current loops. I always felt that was a bit of a cop out. EDIT: Aaand I got ahead of myself, haha! 23:00
When I first heard that statement many years ago, my own arrogance kicked in and my snap conclusion was: “that is a pompous, overly reductive, pedantic position to take.” I promptly disregarded the statement as silly and haven’t really questioned that judgment since. Then again, I have that luxury because I went into another field and am not a Physicist! Amen!
I automatically recognized it was from Griffith having just read it over a month ago. This was also something that had been bothering me for a long time despite it making sense mathematically. Can't wait to watch the rest of the video...*excitement*
Hi Angela, I loved the video and the discussion! I want to give my contribution, because I don't think that it's necessary to bring Quantum Mechanics and spin into the discussion to explain why cars are attracted by magnets. For the way I see it, it's not the magnetic force that does the work, but the electric force of the electrons on the nuclei. Electrons move fast and they feel the Lorentz force that deviates them (but they don't accelerate, because Lorentz force does no work): so, as the electrons are deviated towards the magnet, they attract the nuclei, and it is the electric force between electrons and nuclei that does the work and accelerates the nuclei (which results in the car moving). The same thing explains why two wires run by current attract or repel each other, and in that case it's also easier to figure out all the forces. If you read it, tell me if this makes sense to you. Love, and great job for bringing this kind of discussion to youtube!
You think foot-pounds are bad, let me introduce you to the wonderful unit of force known as the kip, short for kilo-pound. It's a standard unit in civil engineering. And it even has derivative units such as ksi (kips per square inch.) Traditionally, different grades of steel are even described by their yield strength in ksi. Good old A36 has a yield strength of 36 ksi. Got to love American engineering!
Fascinating. I thought you had answered the question in the middle of the video where you were discussing that 'you' don't do the work on the box, rather the earth does as the earth is actually providing the potential energy. So when a magnet picks up a box or a car the magnet is not doing work but rather redefining what direction 'work' is performed in given the strength of the magnetic field compared to the gravitational field. By 'you' introducing the magnet into the area, you have given the box/car/paperclip potential energy, and when it moves up the magnet is actually *reducing* the objects potential energy.
I started studying physics in 2005 and graduated in 2008. We used that exact book, and I just checked, and I still have it on my shelf. (Because why would I sell it back to the book store after I finished the class when they would give me what amounts to a rounding error on the price of the textbook.) I always chalked up the "magnetic forces do no work" line to the explanation you give about the work you do on the box: you lifted the box, but Earth "performed work" on the box. So for a crane lifting up the junker, the magnetic fields are more like the "tool" we use to lift the car in the gravitational field of Earth (like your arm), but it was still Earth "performing work" on the car. Thank you for going back and connecting the quantum mechanical aspect of it to explain that, in the "real world," magnetic fields can in fact do work. I also went through quantum mechanics, but never really thought about the "magnetic fields do no work line" because I thought it was not-all-that-useful pedantry when we don't even really use that work attribution to do energy accounting in the real world applications like the junkyard crane. The crane operators have to provide electricity to that crane to lift that car, and the energy carried by that electricity is going to be (more than, accounting for losses) the amount of potential energy the car gains being lifted into the air. Just like when you lift the box, sure, Earth "performs work," but your muscles are providing the energy that the box gains, and you're extracting that energy from the chemical energy in food, so in a sense the box is gaining energy from the food you ate. I'm rambling, but I guess what I'm getting at is, pedagogically, not only is "magnetic forces do no work" misleading since they do in quantum mechanics (and the textbook is called "Introduction to Electrodynamics" not "Classical Electrodynamics"), but the physics concept of work is less often a good representation of energy flow through a system than you would expect given the way it's presented in undergrad.
To be fair to him, I'm an electrical engineer so only got babies first electromagnetics and if I read that I would have thought the same thing. And then if you added well this doesn't account for electron spin I doubt that would have helped me. I'd just be like what the hell does spin have to do with this?
I haven’t done any physics for at least a 5 years now, so I somehow confused myself about the box on a ramp diagram at around 10:00. I thought more work would be done, because the box has also moved horizontally, in addition to the 1m vertically.
when its moving horizontally it doesn't have to move against any force (gravity pulls straight down) so there is no extra contribution. If maybe the wind was blowing down the ramp then there would be an extra term to include in the force description which could make it be a greater amount of work then.
@rufioh that's why she had to mention that the ramp is frictionless, leaving the only thing to overcome being gravity. A frictionless ramp then just becomes the same thing as a vertical lift more slowly, so less force but perfectly balanced by more time, the work stays the same. In the real world with friction, moving in an oblique to gravity does result in more total work.
The net work adds up to zero, to move something horizontally. Sure, you have to provide work to get it moving, but since it also has to slow down to rest, you theoretically can get that work back, assuming the agent can both do work and slow something down in a reversible process. E.g. an ideal motor that works in reverse as an ideal generator. Human bodies can't do work, or slow something down reversibly, plus friction and drag inevitably are there, so that's why your intuition is that it would take more work. The work you do stopping an object in motion with human forces, is ultimately lost to heat.
@@carultch respectfully, I don't think any of that is true. I could be wrong, but here is my thinking. If you accelerate a thing to move a meter, you've put in the force and done work. If you decelerate a moving thing you've done work again. Acceleration and deceleration are the same thing, deceleration is just acceleration in a different reference frame. As Angela explained, when you pick the box up and put it on a shelf, you've done the (positive) work to move the box and the Earth does (negative) work to give it potential energy. Work is a transfer of energy. That thing which is transferring the energy is doing the (positive) work and if that thing is you the human body, the human body is absolutely doing work. Net work being zero I believe is because of conservation of energy, not because people can't do work.
@@seijirou302 "If you decelerate a moving thing you've done work again". The reverse is true. Work is done on you, if you are the agent decelerating something. Suppose instead of a person bringing it to rest with non-conservative human forces, we have it stopped by a bumper made from an ideal spring. The spring bumper is set up so that a clip locks it in its compressed state, after the object comes to rest. Because idealized spring forces are conservative, all the work done on the spring while bringing the object to rest, is stored in the elastic potential energy of the spring in its compressed state. You can get that energy back, when you release the clip. An ideal spring can accelerate the mass back to its original speed. "Acceleration and deceleration are the same thing, deceleration is just acceleration in a different reference frame" The reference frame that matters here, is the object's original reference frame, when it started initially at rest. The fact that an object decelerating at 1 m/s^2 while moving eastbound is also accelerating relative to a westbound car that is speeding up at 2 m/s^2, isn't really relevant. The bottom line is, if you apply a force against an object's motion, you are doing negative work on it in some form or another. Either that work dissipates as heat for a non-conservative force, or it is stored in another form for later use (such as EPE, SPE, GPE, or CPE).
The name of the channel is not ACollierAstro anymore, it is Angela Collier. I did not notice until last time when someone mentioned it in the comments.
Not only does it break the "Work=Force x distance" law, it ALSO BREAKS NEWTON'S 3RD LAW and BREAKS THE LAW OF CONSERVATION OF MOMENTUM for the magnetic force between two moving point charges (See "Classical Mechanics" by John R. Taylor, section 1.5, "The Third Law and Conservation of Momentum"! Newton's 3rd law says forces between two objects must be equal and opposite, which results in the law of conservation of momentum, but the magnetic forces on each point charge on the other are equal in magnitude but 90 degrees to each other, not in opposite directions (180 degrees) as described by Newton's 3rd Law, thus breaking the law of conservation of momentum (AND ANGULAR MOMENTUM, because the forces don't act along a line connecting the two objects)! However, the magnetic force between two LOOPS OF CURRENT (aka, an electro-magnet, a good model for a permanent magnet), DOES obey Newton's third law and the Conservation of Momentum, and therefore shows how magnetic forces DO produce work when referring to currents (such as those in an electro-magnet), just not when referring to forces between free individual point charges (See problem 1.33 in "Classical Mechanics" by John R. Taylor).
maybe LOOPS OF CURRENT (centrifugaly accelerating charge) is a electric (-2d) mass while linear current (without acceleration) is a electic (-1d) magnet and massive objects DOES obey Newtons third law while massless do not. 2d massive electron can cyllindricaly rotate (spinning) while 1d massless magnet (photon) do not. So , photon and electron both have same curvature(quantity) '+' but different dimension(quality) 1 and 2 while electric and normal(photon) magnets both have different curvature '-' and '+' but same dimension 1. To invert curvatures we must turn things inside out like a glove and we call it MOTION. In ihis process quantity INFINITY-1 becomes quantity +1. "infinity is a motion" Aristotel.
"Magnetic forces do no work. Note: This only holds in Classical Electrodynamics. There are exceptions in Quantum Mechanics, however that is beyond the scope of this textbook." All you had to do Griffiths.
Not necessarily. All force has to do is result in an acceleration. As the acceleration is centripetal for a moving electric charge, potential energy isn’t added to the system; thus no work is done even though a force is applied.
As an engineer I was taught that boundary forces do work, but not field forces. Field forces should be accounted for in the energy equation as changes in potential energy. If an object falls to the ground (air resistance negligible) loss of potential energy is balanced by gain in kinetic energy, & no work is done by gravity. If big magnet lifts an object I’d say as it rises it gains gravitational potential energy and loses magnetic potential energy
I really love this style of video. It reminds me of my uni days where one of us would learn something first and be able to explain it to the others as colleagues rather than a strict tutor/student relationship.
I too remember learning this in Griffiths and being confused. That confusion was later made worse by the fact that magnetic work appears explicitly in some thermodynamic calculations such as the cooling process known as adiabatic demagnetization. Interestingly this technique only works because of intrinsic spin (either nuclear, or unpaired electrons) and cannot be understood in terms of small current loops.
I didn't have a problem with this...until I saw your video title! Ha! Thanks for clarifying physics terms for us non-physicists! (well, somewhat clarifying, but thats me). So...I only a month ago realized why 30 years ago I didn't understand why my conception of the "warping" of space didn't match everyone elses, and the "right" way to formulate it! (in undergrad intro to physics class). I was applying a mathematical transformation, where, in my mind, everything would be transformed...space, time, spacetime, forces, masses, everything...so that it was just a mathematically different representation of the exact same situation...so such a "warping" would not be noticed by anyone. I know, its a weird way to think of it...that's why I majored in Biology, not Physics, ha!
I read your title, I was indeed disturbed. I started watching your video. I gave you a like for referring me to a shorter video. I watched the shorter video. I came back (not less confused) to watch the rest of your video. I finally understood, that (and how) the phrase "Magnets do no work" makes sense if you define the word "magnet" in a very weird and specific way not only detached from every day language, but also specific to a specific sub-field of electrodynamics. Hooray. If someone who reads this still struggles to understand, watch Asylums video on magnets have no force (or don't, I'll summarize the important part in this comment). There's electromagnetic force, electric force and magnetic force. Electromagnetic force basically combines the 2 others and when dealing with magnets it's the only real force, because we defined a force to be real only when it's real in every reference frame, but we can always shift the reference frame so that all the magnetic force mathematically turns to electric force and vice versa. But doesn't the electromagnetic field and thus the electromagnetic force belong to the magnet? Like if I lift a cube, did I lift a cube or did my arm lift the cube? Well my arm did, but since that arm belongs to me, so did I. But smart people already defined the magnet's force to be the magnetic force hundred of years before they figured out, that we also should have an electromagnetic force defined aswell. If we also consider the electromagnetic force to be part of the magnet, then we run into some problems with overcounting, so let's just not. And thus was a weird definition for magnet born in which magnets do no work. It's the same nonsense as electric current flowing from + to - or - to + depending on what scientific field you are currently in. Stupid in a way, but also very useful and important to know when you're once again towering on the shoulders of giants.
![Blox Fruits Dragon Rework Update [Full Stream]](http://i.ytimg.com/vi/EqDAp8udhm0/mqdefault.jpg)
One must imagine Sisyphus happy pushing the box with a consistent force up the frictionless ramp
Sisyphus spinning eternally into a magnetic field. But since Sisyphus has a spin he can do work and be happy!
Kermit is hopping is work
+2
Sounds about right for a physics problem. When I took physics, Sisyphus was probably just entered the underworld.
How would he pushes himself up
"Magnets, how do they work? And I don't wanna talk to a scientist, 'cause they'll tell you they don't."
Look at you....are down clown?
Kkkkkkkkk this comment is perfect
Congradulations! That was so good!
"(...) 'cause they'll tell you they don't, and make Angela pissed"
Gotta conserve the rhyme and the metric (both contributing to a sense of rhythm)
Also, it's "Fucking magnets (...)"
making a rant about a minor misunderstanding that could've been easily corrected years ago has to be a universal human experience
but it's not minor. It's done real damage.
Every repeat argument me and my husband have ever had 😂
@@krabkrabkrab I mean... the damage could be great, but the misunderstanding is still small. It could've been resolved with one sentence like "Work done by magnetic fields is not in scope of classical electrodynamics".
I think the biggest issue with this kind of thing is how people will take in concepts mentally and then fight and die on those hills. Kinda like when someone misunderstands "fossil fuels" and will fight and argue and die on the hill that is "gasoline is made from dinosaurs." Because once a concept has set root, some people just refuse to ever allow it to be changed
Minor misunderstandings turning into a big problem reminds me of an old Kids in the Hall skit. Never Put Salt In Your Eyes. ruclips.net/video/_83MEuLoz9Y/видео.html
Wow - we didn't have this problem in our classical engineering textbooks. They were quite clear that magnetic forces do no work on _electric_ charge, but magnetic field gradients can do work on magnetic dipoles. And also that electric fields and magnetic fields turn into each other when you change reference frames - so they're kind of the same thing.
I remember this hahaha
Magnetism in space can become electricity?
@@justinpridham7919 Magnetism is what you get when you take special relativity into account with electricity. Basically, magnetism arises from a moving electrical charge. Why? Because when the electrical charge is moving, it "perceives" spatial dilation of the other electrical charges. What was once neutral is now electrically charged. The moving charge experiences a "magnetic force".
@@EclecticSceptic I was being facetious. There isn't supposed to be electricity in space. I kind of think there might be.
Studied mechanical engineering and currently an engineer.... At no point did I get an explanation of how magnetism does work, but 30 seconds into this video my first thought is "of course magnetism does work, motors and generators exist". Without work via magnetism, we would not be able to make use of just about every modern application of thermodynamics from gas turbines to air conditioning.
I think the most relatable part of this for a fellow physicist is when she says “I’m scared of magnets”.
I’m getting flashbacks to the time I dropped a Nd magnet in a particle physics lab and it… exploded. I got a shard of it in my leg, and the resulting shards stuck to every piece of metal equipment, sending stray fields all over the place and f’d with the beam lines for years.
I’m scared of magnets.
Being scared of magnets is entirely justified. Nd magnets are sold as toys but can definitely take a finger off if you're not careful.
How exactly did the magnets affect the beamlines and how did the beamline scientists respond to the disruption to regain reliable operations?
@@DaylightRobberyCA So this particular lab was a low energy particle lab (slow particles, mostly antimatter), so they had a few devices that were long tubes with copper wire around them (making a kind of magnetic funnel to maximise the number of particles hitting the detectors (basically a kind of lens for the beam)). The problem was the earth's magnetic field needed to be accounted for, so they would set the detector at one end of the table, then use permanent magnets in various places until the signal would not increase any further.
When this incident occured, all of that needed to be redone to account not just for the earth's magnetic field, but the random bits of magnetic Nd stuck in various places.
@@zamnodorszk7898 what a pain in the neck!
I fear ALL magic.
Seeing this video title just made me realize that I had almost completely forgotten that I had ever learned this rule. When I first learned it I immediately thought of two magnets attracting each other and asked my teacher about it. My teacher told me straight away that this "magnetic forces do no work" was too general a statement, and that it would be more accurate to say that "Lorentz forces do no work". I thought for a few seconds and said oh, and then never got it wrong again.
good teacher
okay see that's so much better already
wha.... isnt the lorentz force q(E+vxB)??? the force that accelerates projectiles out of a railgun??
@@vipinx8881 Yes, strictly speaking, the Lorentz force also does work when considering a complete electromagnetic field, with both non-zero E and B, it's just that the original context of this discussion was magnetism and especially static magnetism. When I was first introduced to the concept of the Lorentz force, the scenario talked about was almost always the motion of a point charge in a static magnetic field. At that point, the Lorentz force naturally does no work because of that cross multiplication.
My undergrad taught (teaches?) QM _before_ E&M, so my Physics 240 professor straightaway went, "except we all know that magnets are actually spinny bois" and everyone just nodded along.
Except me, who was taking the course as a freshman _before_ taking Quantum in the second semester. So I remember fixating like Angela did and trying to work out the limit as r went to zero without QM like Griffiths' collaborator is doing.
That class was a ball and a half, and I don't think I ever felt more meaningfully challenged, before or since, but _damn_ if I wasn't a problem student.
Great video! I really tried to cram a lot of info into my video. I talked _really_ fast and had to include a supplementary point in a pinned comment because I rushed production, so I'm glad you made one where you take your time. I love how thorough it is.
Also, IMO, Griffiths making a purely classical textbook was a mistake (pedagogically speaking) for reasons you point out. We can't just forget permanent magnets exist. That's unreasonable.
How does classical EM treat permanent magnets? With tiny current loops?
I can't imagine people just didn't realize their amazing complete theory of electromagnetism just didn't actually explain magnets.
The thing is, he actually _doesn't_ ignore quantum mechanics entirely for the exact reason you give - in his section on ferromagnetism (page 278 in the 3rd ed) he mentions that neighbouring dipoles prefer to align for an "essentially quantum mechanical" reason "I shall not endeavour to explain" (he even mentions "electron spins", though I guess he _may_ mean "of tiny current loops").
So it's actually arguably _worse_ than some (admittedly eccentric or, y'know, "a _little_ crazy" :) attempt to produce an entirely classical EM textbook because he's arbitrarily non-classical at other times.
I think it's fine to make a purely classical textbook as long as you just say "Hey, permanent magnets don't quite make sense in classical physics, so we're ignoring them." That's all it takes.
@@Huntracony Sure, but if someone does that, they shouldn't give the zero magnetic work thing so much emphasis. Don't bold it and box it like it's a fundamental principle.
@@ScienceAsylum Yeah, I agree.
I hadn't realized how lucky I was I got the "sorry, that will go into QM" explanation from the professor way back when I took that course with that book.
In fairness, that's _also_ a solution to Dr Collier's problem - either she had a teacher that didn't provide that very simple answer OR she never asked the question (but I guess then we wouldn't have got this very entertaining video :).
she did say she wasn't paying attention during classes
@@judahmatende3769 Undergrad physics was like drinking from a fire-hose to me. It's understandable that she wasn't fully engaged on every nuance.
I had to extract that from my physics undergrad professor during office hours. I saw that bolded boxed sentence, looked up when he would be in his office, and was like "I can pick stuff up with magnets, explain yourself." I don't think he had really thought about it in a while, and it was fun to watch him remember more and more as he started explaining.
@@nickcrovo9512 lol "office hours" what are those? Our profs didn't give a shit. They were too busy with their research.
This reminds me of something that I learned once:
"A scientific law needs to contain two parts; a mathematical relation that holds true, and a description *under which conditions it holds true."*
This whole thing seems like an issue caused by Griffith forgetting to do that second part.
There seem to be struggles all throughout science education that stem from not realizing this wisdom. Most notably students when they miss-apply a scientific law. But it certainly doesn't help if a professor, nay, the author of a textbook is guilty of overlooking this aspect of scientific laws.
I can at least empathize with him. My brain does the same thing where it will just skip over "obvious" parts which is...not good in relation to math and physics lets just say. I was confused why Angela has a problem with this originally because to me it was "so obvious" that he was talking classical mechanically only based on how he pharsed things. I just instantly accepted that without it entering my conscious mind. You're right that it can't just go unstated though. This is why I am a TERRIBLE teacher and a subpar communicator, even in subjects I know decently well. I hate it, but it's very, very hard to manually stop and be explicit with the assumptions made.
Imagine how much trouble I had with proofs lol
@@Tinil0It's a matter of practice, which reduces the struggle. At first we'll clumsily forget do a step, then after a number of times practicing, it'll start to become second nature.
Before even playing the video, I pictured a name in my head, “David Griffiths”, who I still partially credit with derailing my understanding and interest in physics around 2004-2005. I seriously had no idea you’d mention the exact same book. We also used his QM book with the stupid cat (1st ed). I just remember reading the same introductory paragraph over and over again, trying to understand what he was saying about an electron, and giving up. I was so excited for those classes, but I just couldn’t make it stick in my brain. The profs were all researchers, and seemed more interested in their work than the students they were forced to teach. They didn't care. If our grades were low at the end of the year, they'd just curve them all up and push us along to the next year. I skirted by and got the hell out of there with a BS and a shit ton of debt.
Griffith's QM book is way worse than EM book to me.
27:19 For our civilization to work, for our world to work, for us to be able to sleep peacefully in our beds at night...
...someone has to care enough about this topic to get totally bent.
Thank you, Angela.
I CANNOT OVERSTATE just how close to home this video hits for me. I have spent so much time wondering about this, googling to no avail, and I never got a satisfactory answer. Thank you for this video, you rock!
Don’t try then
This is a classic case where we see that Math can't explain "Natural Physical process". The equation is intended to allow the creation of a graph, on which we intend to plot points to create vectors. The graph arranges the Force axis always perpendicular to the Velocity axis. But that is not what happens natural physics, its only applicable to your graph. The upshot of all this is that because math has been elevated above the understanding of Natural Physics, it then is assumed to be driving natural processes. But this is incorrect. In fact, your math may be totally wrong, because you have developed a wrong equation, because you never understood actual natural processes, or you forgot to include something, or you made too many assumptions. A incorrect equation can still give results that resemble what we observe, so this is a very real trap for those that think that the universe is ruled by math. Its not.
what no its totally explained by quantum mechanics just not classical physics@@everythingisalllies2141
@@everythingisalllies2141it is ruled by math, kinda, but math is complicated. Like in her ramp example how she left out the friction of the ramp and the air and said the force is constant. The universe is math, but it’s a lot of math to do anything. Like, do you model every atom?
@@owensspace Your math is only as good and correct as the hypothesis that was used to develop the equations. Garbage hypothesis = garbage math.
Time for a new video I barely understand yet inexplicably enjoy
Truest statement I've seen all day
Lol same
Time for a new video I completely and entirely do not understand, but could very easily explain why I enjoy.
It makes me feel dumb in a way that doesn’t make me feel bad about being dumb.
Absolutely
@@diggysoze2897 probably makes you (read: us) slightly less dumb in the process too!
Assuming you (again, we) learned anything at all over the course of the video 😂
27:18 is giving big, "That's right. It goes in the square hole," energy.
Nice video! And I agree that Griffiths could have been a little less coy about the case of electron spin. But I think many of the commenters are missing the point of the discussion. Griffiths is absolutely correct in classical (i.e., ignoring quantum mechanics) E&M -- the net work done by a magnetic force really is zero. It's just that the mechanical work on the dipole is not the only work being done. There is also (negative) work done on the current flowing in the dipole (classically a current loop) and this is equal and opposite to the mechanical work. So, to maintain the current, a source of emf (power supply) must do extra work equal to the work done on the dipole. The net accounting looks like: power supply does work, magnetic force does zero net work (which comes in two equal and opposite pieces), and the dipole gains gravitational PE equal to the net work done by the supply. The transfer of energy is from the supply to the dipole.
The basic reason this changes in QM is that the electron's dipole moment (arising from its spin) is immutable and requires no "power supply" to maintain. There is no motional emf in the electron and no work done by any source, so the mechanical work on the dipole is all there is. But this can't happen in classical E&M, even for very small current loops.
The comment by @orangebutnotred invoking Sisyphus is actually quite relevant here :-). Imagine we push a block up a ramp with a *horizontal* force, i.e., parallel to the ground, not the ramp. The block gains gravitational PE, but the horizontal force does not lift it, the normal force does. But the normal force cannot do any net work, for the same reason the magnetic force does none -- the displacement of the block is perpendicular to the force. What happens here is that the horizontal push does some work and the normal force does zero work (the works done by the horizontal and vertical components of N are equal and opposite), with the block acquiring gravitational PE equal to the work done by the horizontal push.
This is discussed in Griffiths, including the box on a plane example, in section 5.1.3. It is also discussed very nicely in the Feynman Lectures vol. II-15-14ff.
In the preface of the new 5th version Griffiths says: "I have added some new commentary on subtle issues: ambiguities in the definition of polarization in crystals, problematic aspects of electric and (especially) magnetic field lines, the awkward role of intrinsic spin (a strictly quantum phenomenon) in a classical discussion of magnetic materials." I have not gotten far enough in the book to speak on how he incorporates it in the text, but apparently it is now mentioned!
This type of text is how I could not continue to read text books. And why I don't read any type of books anymore.
@@Khronogi its a really good book though
I actually despise his use of language. It's like he's a 1700's aristocrat
@@Khronoginot the self report 💀
@@Khronogi They're not all this bad, I promise! There are plenty of fantastic ones, they're just not necessarily the ones that get set. It does take a bit of exposure to discern between the good and bad quickly though.
And _books,_ in general? There's an _overwhelming_ variance of prose and tone out there! You are _seriously_ blinkering yourself if you let a few elitists put you off an _entire medium._
My favorite genre of video is when RUclipsrs make videos about esoteric grudges in fields I do not care about. Always so much fun and this channel has so many of them.
I'll let you know when I make my rant about singular they.
@@Alex-js5lg And I'll let you know when I make my counter-rant. I mean, singular they does suck, but I'll argue for it as the best option for English. Alternatively, we can talk it out right now?
a couple of semesters of college physics was enough to get me a lifetime of small grudges on how they could possibly represent concepts.
...fuckin' optics, man.
Let me introduce you to ...rants about Lifting Force in Airfoils! The years of online grudge-rants even resulted in entire contrary textbooks being written, by hated enemy groups in academia. (So, if we cannot convince youtubers to reject our opponents' base heresies, well, instead we can educate entire generations of aerodynamics undergrads. What did Johnathan Swift say about "the best way to crack an egg?" The genocidal Big-Endians want to wipe the Little-endians from the face of the Earth. Or maybe it was the other way 'round.)
@@NZsaltz And I'll let you know when I make my counter rant to your counter rant because I think singular they is actually awesome. My native language doesn't have anything like a singular they and it sucks.
I'm so happy you made this, I've also been haunted by memories of that one line in Griffiths, its shadowy presence has weighed upon me all through my PhD studying magnetic materials and even into my current postdoc. You have exorcised an intro physics demon and I suddenly feel free, thank you (*signs up for patreon)
I don't know about you or anyone else, but I have had to accept throughout my education in Physics that some of the information I am given is just plain wrong, either out of lies by omission, overgeneralization, or professors who are straight up wrong. I now have a thing where whenever I read or hear something in Physics, I have to go, "Ok, but is that *really* true?"
@@MrAnonEMoss Multiple sources, always. That's how it is with all knowledge, in all areas of life.
@@MrAnonEMossI’m working on my Physics BS right now, and I’ve really come to realize this recently. I will take your comment to heart and always check multiple sources for any new information, thank you ❤
You are AMAZING! I am a female researcher (not in physics). And I love every single video you posted. Please keep up the wonderful work. Love to see more videos from you!
I love hearing you ramble around a subject. It is relatable to how I work through questions and problems.
When I, as an ethnically Jewish atheist, taught high school physics in an Orthodox Jewish school, it was very tempting to apply to physics definition of work to the Sabbath. Like... just put everything back and you've done no work on it.
But I didn't think that would be appreciated.
That's amazing 😂
hahahaha love it
Actually they likely would have appreciated it, as it helps distinguish the English word "work" from the sorts of activities your students would consider prohibited on the Sabbath. In general, if you can restore it back to the way it was, i.e., you haven't transformed its state in a persistent manner, then it's not prohibited "work." So in fact it could have been a useful way to discuss different meanings of the word "work" without needing to get into the religious aspects.
Get out of bed without applying a force over a distance and we'll talk.
@@zeggyiv why don't you just come to bed and we'll talk here?
David Griffiths has been real quiet since this dropped
Cancel culture gone mad.
i find it very interesting that Griffiths won’t mention dipole forces because it’s not a classical effect. but a few chapters later he goes on to talk about the gyromagnetic ratio and that due to quantum effects, the actual ratio for the electron is different from the predicted classical value
Likewise (in the chapter after "magnets do no work", in a section on ferromagnetism) he explicitly mentions an "essentially quantum mechanical" reason for neighbouring dipoles to align (though declines to explain it).
So he's not averse to mentioning QM, except in the section on work done apparently.
We found an excellent use for the NMR magnet back when I was doing my PhD. This guy kept sticking on his crap heavy metal mix tape in the lab, and wouldn't compromise with allowing anyone else's music. When he was off on his lunch break, one quick rub on the side of the NMR magnet downstairs and all his carefully curated death metal was reduced to blissful white noise.
Lol that’s a great story. But I thought NMR magnets were shielded as to prevent things like that? Or was it an exposed magnet not inside the NMR spectrometer? Those are incredibly strong magnets.
Honestly, this seems like a very roundabout way for Griffiths to say that Classical Electrodynamics can't explain permanent magnets. He could just say it outright instead of making two separate editions of his book, investigate a new theory of electromagnetism and give an interview, all the while just being passive aggressive instead of actually answering the question.
That's kinda mean to say to magnetic forces, they've been unemployed since 2021. They've been working on in. They cook, they clean!
Yeah, life is treating them so perpendicularly
That's certainly an interesting _spin_ on the issue.
They're learning to code in Python to upskill. They need encouragement, not taking down a peg!
We should arrest the magnetic forces for loitering. The warden will set them to work filling potholes.
I give zero flux, seen as perverse denying magnetic forces do work, as being perverse or not, is a state of denial, I embrace.
My god, I was instantly teleported back to my EM class in 2009. I had an almost identical experience as you, and this video unlocked a _flood_ of memories that I had neatly ignored for 15 years. Thank you so much for chasing this down and finally closing a decade-and-a-half old question on my brain! Amazingly interesting video, as always!
I'm scared of magnets, and now I'm also scared of hypothetical Tyler.
Sup. Im Tyler and I have probably in the range of 1600 magnets
@@tylerduncan5908 Supervillain origin story.
@@tylerduncan5908 FUKCIGN CONTAIN HIM !!!!!!
"Erasable Pens! Make my head hurt!"
Hello there.
Really good that you think instead of just trying to make faulty statements work.
Feels so good to understand things instead of just applying info without the understanding, youre smart i think, all of those points makes it pleasant to watch your videos :)
Have a like and comment for respecting that yes, I did just want the 5min summary today.
Also, have a new sub for informing me of the option and linking it in an approachable way. Thanks for not treating your channel as the end-all-be-all source of information where the goal is to monetize attention, but for what RUclips should be about.
Your style is great and I look forward to the next one!
I also do no work
Is that written inside a little box?
Hell yeah brother
@@stevepayne3094 on the door to his battery compartment
you are a magnetic force
Me and magnets are similar in that way
The little tune before each section makes me so happy
There's a channel about old historical firearms where every year they do a holiday fundraiser and when they meet certain goals they will feed berries to a guinea pig, and they play that same tune: so every time it transitioned in this video I thought about a happy little whistle tater getting a strawberry.
@@scottwatrous Do they mention the artist(s) over there? :)
@@thomastergaard I wanna say Kevin McLeod
For those curious, Barandes did publish his work on a classical theory of intrinsic dipole moments. I think the paper Griffiths was specifically referring to is "On magnetic forces and work
JA Barandes - Foundations of Physics, 2021".
So, eid he manage to do it?
@@JulianSildenLanglo According to himself, he has done it. I don't know what objections Griffiths or other smart physicists would have to his construction. I would need to read the paper and go into all the details to see why the construction may not work as intended.
I might do it, and if so post another reply here. I'm actually quite intrigued by the paper, since it is common knowledge (At least I was taught this in the context of the Bohr-Van Leeuwen theorem) that you need quantum mechanics to explain magnetism . If this isn't the case, I would be quite shocked.
@@De2Venner And, well, what would be the point? Yeah I get it, it's interesting, but QM has worked pretty damn well for us. Why do we need to jam its concepts into a less effective theory? Just to retroactively vindicate this dickhead?
arXiv:1911.08890 is what I found, so far, for Barandes. Its title is "Can magnetic forces do work?" Paper submitted 4 Nov 2019, and last revised 26 Jul 2023. I am wondering if this is the same paper that you mention.
@@OrdenJust No, the one I mentioned is arXiv:1911.00552. He has done multiple papers on the subject though.
I went on a real roller coaster with this one, gradually getting grumpier and thinking things like "but that's not the best definition of work" etc. etc. and then got more and more relieved and/or sheepish as the video went on and you then addressed all of those things and I wondered why I thought you wouldn't.
Thanks for this one. I’d convinced myself that I didn’t really understand work, but didn’t realize that it was the stupid Lorentz equation that was at the bottom of it. The whole thing with induced currents was a cop out since the electromagnet won’t pick up non ferrous ( or at least non ferromagnetic ) stuff. That experiment didn’t even cost much. It also doesn’t really matter if you can’t explain something without quantum mechanics, but denying something doesn’t happen when it is happening is silly. All you need to say is that classical electrodynamics doesn’t explain it. I really appreciate your tenacity on this question. You may take more time to say stuff, but you seem to be pretty good at making sure your explanations are clear. They are also fun, which keeps me from giving up on the harder parts.
“Fucking magnets, how do they….”
So the moral of this story is to properly tell your readership what your textbook is about or else they’ll think you are a troll.
I only consider someone an expert if their book has a preface, in intro, AND a forward!
💀
I'm reminded of how we were taught simplified versions of biology, chemistry, history, etc in middle school and were just not given the note that "of course, you'll learn more in high school and college that will sometimes completely contradict what you're learning now"
Leaving off that footnote was frustrating then, and you're just telling me that it doesn't stop in middle school: they will keep annoying me by leaving off that footnote forever. I guess the assumption is that I'd just know school is like that after the first time, so it's too redundant to keep printing it?
@@leeroyjenkins0 that's all fair, but I just want to note that I didn't say the simplified teaching was a problem, I just found it frustrating how rarely I was told, "of course this model has already been superceded by one you'll learn in a few years" This was incredibly frustrating in history classes where the simplified histories really were deeply misleading rather than merely omitting details.
@@silverharloe That's why I like PhysicsGirl Dianna Cowern- she does a great job of simplifying things without dumbing down or making up analogies that will lead you astray later. I hope she gets well and gets back to making videos. (Angela tends to do things for a much higher level than PhysicsGirl)
I know had a similar mental break when learning high school biology because of things like "Your cells store energy as ATP." OK, but what actually IS ATP, how does your body produce it, how is it moved around to places that need energy, how is it used to supply that energy? Like, I know it would be absurd to expect them to get into all the specifics of the chemistry involved, but you're leaving out basically the entire story when you simplify the entire process to "your cells store energy as ATP" and then never elaborate any further in any way. At that point you're not actually understanding anything, just memorizing a list of phrases.
It's like how everyone knows "the mitochondria is the powerhouse of the cell," but ask anyone what that actually MEANS and nobody actually knows anything beyond that one phrase.
A "lie-to-children" as it's sometimes known (coined by Ian Stewart and Jack Cohen but popularised by the inimitable Terry Pratchett).
I guess the positive version would be, "All our models are approximations of reality and the higher the level you study at, the better the approximations become [but you'll never get The Truth because that's not really what science does]" (which may also have the advantage that if/when people _do_ find out that, when it works as intended, science is _the_ best way we have of becoming less wrong about the world BUT can _never_ provide 100% certainty, they don't then become disbelieving of the whole enterprise, as seems to be happening increasingly often these days).
@@-tera-3345 Usually textbooks have a rough sketch of the aerobic respiration for Eukaryote cells. All things considered it's somewhat superfluous to get into the exact biochemistry of ATP. Everyone remembers "mitochondria is the powerhouse of the cell" because it's been memed to hell and back but we don't look at that textbook diagram showing how O2 is used.
1:15 "audience of only me" - oh, no, I remember this phrase in this book. We debated it heatedly because it didn't make sense to us. We eventually dropped it because it derailed the class for so long. I've thought about it a couple times since (decades ago) but chalked it up to the book being wrong in an effort for brevity.
And yet instead of adding "in classical electrodynamics" to the 4th edition, he adds a completely new section basically saying the people questioning it are idiots. He might write a good textbook, but he seems rather unsocialized.
it's not even just one book, or one teacher.. it's everywhere! It's one of those oft-repeated science phrases that aren't super helpful but get repeated over and over verbatim, like the one about centrifugal force not being real (although that's admittedly more helpful and less confusing. It's just the only thing I could think of this early).
@@idontwantahandlethough Funny-the only places I ever remember hearing that the centrifugal force doesn't exist is pop culture and _maybe_ high school physics. By the time I was learning classical mechanics, we were *all about* doing the coordinate transformations to derive centrifugal and coriolis forces.
Now, was that due to me making the jump to college / major physics? -Definitely- Maybe. But I'd like to think it was because I graduated high school in 2005, right around the time that xkcd came out.
Is it Classical Electrodynamics by Jackson?
@@idontwantahandlethough except, of course, centrifugal force is real. You can, in fact, measure it and it has consequences. The nomenclature here does a real disservice. Inertial and non-inertial are better terms
I love that you reference the Serway book, my dad worked on the instructor’s manual and student solutions manual for that textbook for many years (but not the main textbook). Nice to see people learned from it and became successful physicists who know a lot. I have met Professor Serway and stayed at his house, he and my dad were co-workers and friends. Went swimming in a lake in his backyard, even.
Love how you systematically take us through your thinking and how righteously annoyed you are when not understanding something. Great video
I don’t know anything about this topic but did I still click on, watch, and remain engaged throughout this entire video? Absolutely.
..."I'm like... I'm not disturbed man... I'm just confused!..." ! ! !
In the 4th edition of Griffith’s, he does an entire section (6.4.2) about the intrinsically quantum phenomenon of ferromagnetism. On page 288, he even mentions electron spins: “… to align virtually 100% of the unpaired electron spins”. Love this book as an intro E&M text, but I really enjoyed your comments on it
Fun fact: that's _also_ in the 3rd edition (page 278 in mine) so the hypothesis that Griffiths has effectively written a book _as if_ quantum mechanics doesn't exist is false.
In fairness, Dr Collier _does_ say that she may have missed it, as an undergrad she didn't meticulously read the entire book etc. And anyway, how hard would it have been to just stick a similar line in the section on magnets and work (or to do as he's now done in the 4th edition, include an explicit mention of where he _does_ explain it) ?
(but then, we wouldn't have got this entertaining video :)
A note on the foot-pounds vs pound-foot thing. They're exactly equivalent. Pound-foot is typically associated with the torque equivalent to one pound of force acting perpendicular at a distance of one foot, whereas foot-pounds is typically associated with the moment created by that torque. But they can be used completely interchangably with no issues at all. Myself and most other engineers i've interacted with use foot-pounds for both.
Maybe we should use vector units? Then work would have units of feet-dot-pounds, which is the same as pounds-dot-feet. But torque would have units of feet-cross-pounds, which is not the same as pounds-cross-feet!
I feel like we're slipping into geometric algebra here...
Not quite: the torque direction is sort of a co-space, like the space of oriented planes vs directions. Angular stuff (and torque) lives in a universe of oriented planes, while non-angular lives in the land of direftions.
We can choose to correspond these spaces, but doing so involves an arbitrary non-physical choice of convention.
Except of course the weak force shows up and cares about it!
Why not use sensible units like those in the metric system?
@@kwarra-an okay, newton-meters and meter-newtons are exactly equivalent.
@@gcewing There isn't such a thing as geometric algebra - you are just being taught regular old algebra in a shitty non standard way.
And no, you dont need to care about this as the quantities involved are a scalar in one case and an (axial) vector in another. Things being scalars or vectors or tensors isn't something that you need to invent units for, especially since you want to look at individual components to which you also assign the same units as the overall quantity.
Ok, I'm stopping at 12:32 because this is too 'busy' for my brain! But it seems to me that the ramp solution would involve more work than a vertical lift because there are two components - vertical and horizontal, both requiring work. The ramp is the same as lifting the box and then pushing it horizontally (or vice versa).
I had the EXACT same experience in 2009 in EM class, with the same book. In our case, seven guys just sat in a library having this discussion for hours..
men love to get together with the homies to discuss magnets for several hours
As a dynamicist magnetic fields terrify me and I feel like I'm in too deep at this point to have asked anyone about this irl, so I'm really glad you made this video cause this is honestly so relatable
Reminds me of Feynman at Oak Ridge, embarrassed because he didn't ask them to explain the symbols on the blueprints right away. "Right away would have been okay, but you let it go too long!"
I loved that i was perfectly following along on the discussion on work and energy, then stuff went out there with quantum spin stuff and I was once again humbled.
I did my undergraduate E&M course mearly 3 years ago and I used the new edition of Griffiths, but I was left with the same confusion as you. This bothered me to my core, and the new chapter didn't help me. Griffiths definitely made a mistake by not making his intentions clear; to write a book on purely classical E&M. Thank you for helping me find peace.
By the way, I love your energy and I think a lot of physics students will find you very relatable. This video deserves more than a million views.
I never took a college physics course (astronomy survey counts?) and I don't really get the maths but I can enjoy listening to you talk about physics for 40 minutes and I just accept that despite sometimes asking myself, "why?" I don't think you need a disclaimer about such a thing that I stumble over, just that I want to share that feeling. Isn't science is all about that kinda thing that pushes us into getting deeper? Someone explains something in hopes that they are communicating at your level and for the most part, it can be OK. For those whom the explanation is not OK, they dig deeper and maybe move things forward a little too. I like how you are really demonstrating how science is fascinating and that keeps it among my thoughts. Thank you!
I've always understood that they do no work on charged particles (ignoring spin), but yeah I never understood how they do no work on magnetic materials.
I'm pretty sure that has nothing to do with what it's supposed to be about.
Exactly. Evidently magnets do not exist in the formal model of Classical Electromagnetics. His note should have said that the fact of magnetic forces in the model unable to do work is proof of the model's incompleteness.
@@sciptick Of course magnets exist in classical electro _magnetism_ - it'd hardly be widely lauded as _the_ crowning achievement of 19th century physics if it didn't account for well known objects which demonstrate the exact phenomena it's attempting to explain.
The classical explanation though involves treating electrons as tiny spinning charges (as in _actually_ spinning, like e.g. planets do, rather than their intrinsically quantum mechanical property "spin") and the _work_ is done by _electric_ forces, as per Coulomb's law (which _doesn't_ act perpendicularly to the direction of motion but just along a vector between the charges).
“Imagine you’re moving because you live in America and you’re renting and rent’s expensive and you have to move every single year so every single year you pack up all your books into little boxes and you carry them down the steps and you put them on a truck and you move them to a second place where you carry them up the steps and you unpack them only to do it again in like 11 months …”
If all the physics problems I had to do in high school would have started like this, I’d have been way more prepared for the actual world.
@@imwelshjesusas a daddy with 3 kids. Fuck.
@@imwelshjesus What is the point of this comment
@@imwelshjesusnope, gotta do it myself :(. That's why we just leave it in boxes until we need something.
@@imwelshjesus Nah, students use milk crates that they... "borrowed"... from behind the cafeteria. That way you never have to unpack -- just flip em on the side and they become a bookcase, flip em again and you're packed and ready to move! I mean... er... so I've heard.... :P
@@olencone4005I mean milk crates are $10 at Home Depot, and some of us acquired ours legitimately. That doesn't stop some people from assuming you took it right out of dairy fridge in the grocery store, because you happen to use one as your shopping basket because it fits conveniently on your bike's rear rack.
In my high school physics class, my teacher illustrated the non-intuitive character of work. He held a lab stool motionless straight out to the side. He declared that he was doing no work despite the emphatic disagreement coming from his arm and shoulder muscles.
But he is doing work. It's just that the work isn't being done to the stool. The work being done is internal to his body. There's ions being pumped around his muscle tissues, things like that.
In general relativity, he's accelerating the lab stool. : /
But magnets create a force that MOVE objects over a distance. I think you missed a little bit in this video
TLDR: Magnetic force does no work on PURE ELECTRIC CHARGE, but it does work on MAGNETIC CHARGE and MAGNETIC DIPOLE (and higher multipoles).
(1) Magnetic charges (monopoles) we have no evidence of existing in nature. (2) And classically, magnetic dipoles result out of moving electric charges, so something else does the work fundamentally. (3) But quantum mechanically, spin 1/2 particles like electrons have intrinsic magnetic dipole. So lifting stuff using a ferromagnet, the magnetic field does work.
Someone finally making sense.
Thanks for making this. Sadly, there are a LOT of similar issues I've encountered in education. Many in the sciences and math, but the phenomena is not exclusive to just those fields. Seems to happen everywhere, actually. I suspect the issue stems from how the human brain works. As our neurons connect, those networks are always relative to some other part of the local network. When we attempt to explain something to others, we are often conveying information stored in one network and have no awareness of the other networks required to contextualize that information.
The part I find most interesting is the resistance some people have to making the small update to contextualize the information once it's been pointed out to them. Like you mentioned, homeboy should just add a sentence to clarify that the "work" can be explained by quantum mechanics, and is outside of the field of the classical scope of the book.
I would just to let you know that I pay attention to all your jump cards and interlude music... I always find them very delightful.
This video is also for me, someone currently retaking undergrad EM with that exact textbook
“I got a couple of comments on my last video […] that were like ‘why are you scared of magnets?’
And like, why *aren’t* you scared of magnets?”
This is why I watch past the outro lol
Descartes didn’t want to use horsepower because it’d be putting Descartes before the horse power.
I hate Griffiths so much for this. All he had to do was say "btw, this doesn't apply to ferromagnets", but he just kept repeating "magnetic forces do no work" as if that's an argument.
Magnetic forces: the ultimate slacker.
Worse-while they do no work, they _divert_ the paths of charged particles. So really, they're *the ultimate trolls.*
Angela picking up the 4th edition at around the 22:00 min mark is like the midpoint of act 2 where things look fine (an explanation, huzzah!) on the surface, but are about to go to shit, plunging to the act 3 break pit of despair
fuckin magnets, how do they work?
they dont
if you are fucking them you're doing it wrong
@@devforfun5618 especially if you get them wet
So cool that now we know the answer! They do no work!
@@devforfun5618 Classic [ally].
I love the subjects and the fun delivery of this channel!
I am a 3rd edition kid. In my undergrad, I had two old professors that had a running game of trying to come up with more and more convoluted examples of magnetic fields apparently doing work that the other would have to solve.
I worked in undergrad with neodymium magnets, and even small ones if you're not careful with them will just shoot off and stick to a metal desk leg and they might break on impact, and as you said, they don't care if you are in the middle. Yeah I'm definitely scared of magnets.
My classmate in Uni had his thumb stuck between two 10cm×10cm×5cm neodymium magnegts. It was properly flattened, but luckily they could fix it.😮
Magnets don't scare me. ELECTRICITY is whats scary.
"What do you mean there's 'invisible energy' inside those perfectly normal pieces of elongated copper? And If I touch them the wrong way, I just straight up die? "
Dying everytime Angela has a strong reaction to magnets being magnetic 😆
I love Andrew Zangwill's explanation of this in section 12.3.1 of his Modern Electrodynamics. It helped me get a nice intuitive explanation of this phenomena in classical E&M
You just blew my mind for no other reason but bc u like hearing urself talk… you are the best science communicator i have ever had the pleasure of knowing
As someone who knows nothing about physics past barely understanding my highschool chemistry class and nearly failing geometry( but ive got basic algebra down i think) i would love a video explaining how exactly magnets work on atomic and/or molecular level. I know so much about molecular structures and because im interested in cooking, computers, and metallurgy ( because of my work at a non-ferrous metals recycler), the things i dont know bother me so much more. Magnets are one of them. I use them everyday, ive killed them with heat, ive charged them back up with a machine. Ive owned so many different kinds.
Thank you for your videos and time!
And hey i wouldnt mind an explaination of superconductors either if theyre connected the way i think they are.
I love it! I took E&M from the same book, and got hung up for years on that sentence! I never understood it until I looped back around and taught E&M out of that same book and realised it was a bit of a “for a spherical chicken in a vaccuum” sort of statement. (If you know that old joke.)
Griffiths is such a hit-or-miss author for undergrad physics. His Quantum book is, generously, trash, but his Elementary Particles book is great.
I remember E&M being somewhere in between, with some members of the faculty _seriously considering_ swapping it out with Jackson.
@@GSBarlev Indeed Griffith's quantum is horrible, built a lot of bad habits for fellow grad students. For E&M, Zangwill is basically Jackson for someone who doesn't hate their students.
@@LavabugOh why is Griffiths Quantum trash? I quite liked it when I learned QM with it. Although, now I’m thinking I haven’t learned QM lol.
@@vlix123 I learned from Cohen-Tannoudji, it was brutal and rigorous but it taught me the basics in the most general way possible. Griffiths teaches very special cases and doesn't state all the assumptions made IIRC, which I guess is fine for a single semester class, and it's fun to read, but it seemed to hurt my peers in grad school who used it for undergrad cause they had to unlearn/relearn a lot of it in a more general way to progress to the more advanced topics (addition of angular momenta, WKB theory, 2nd order perturbation theory etc.), whereas I felt way more ready thanks to Tannoudji. Zettili is a happy medium I think.
I used Griffiths for E&M last year. I just started the video and don't know if you mention this later, but he actually spends a few pages explaining this a bit later in the book.
I will say, that same boxed statement is in 4th Ed. This time he mentions to read Chapter 8 for more details. It's not until section 8.3, in the very last paragraph of a 4 page discussion, that Griffith's mentions "The magnetic field *can* do work on these 'intrinsic' dipoles...it is *not* classical electrodynamics...". This after 4 pages of justifying magnetic fields doing no work by analyzing all magnetic dipoles as infinitesimal current loops. I always felt that was a bit of a cop out.
EDIT: Aaand I got ahead of myself, haha! 23:00
"Teacher, my magnet doesn't work!"
"Exactly."
When I first heard that statement many years ago, my own arrogance kicked in and my snap conclusion was: “that is a pompous, overly reductive, pedantic position to take.” I promptly disregarded the statement as silly and haven’t really questioned that judgment since. Then again, I have that luxury because I went into another field and am not a Physicist! Amen!
I automatically recognized it was from Griffith having just read it over a month ago.
This was also something that had been bothering me for a long time despite it making sense mathematically. Can't wait to watch the rest of the video...*excitement*
I was hooked from the start, this was one classical electrodynamics thriller.
Cheers!
“Oh shit” “what” this had me cracking up
Hi Angela, I loved the video and the discussion! I want to give my contribution, because I don't think that it's necessary to bring Quantum Mechanics and spin into the discussion to explain why cars are attracted by magnets. For the way I see it, it's not the magnetic force that does the work, but the electric force of the electrons on the nuclei. Electrons move fast and they feel the Lorentz force that deviates them (but they don't accelerate, because Lorentz force does no work): so, as the electrons are deviated towards the magnet, they attract the nuclei, and it is the electric force between electrons and nuclei that does the work and accelerates the nuclei (which results in the car moving). The same thing explains why two wires run by current attract or repel each other, and in that case it's also easier to figure out all the forces.
If you read it, tell me if this makes sense to you.
Love, and great job for bringing this kind of discussion to youtube!
The music cracks me up every time. Love it.
Another entertaining and informative video. Thank you.
Mozart's flute concerto in D major.
Oh, you changed your channel name?
@@realGBx64 - You think Angela Collier is harder to remember than acollierastro?
Perfect timing for my EM theory class starting magnetism today
You think foot-pounds are bad, let me introduce you to the wonderful unit of force known as the kip, short for kilo-pound. It's a standard unit in civil engineering. And it even has derivative units such as ksi (kips per square inch.) Traditionally, different grades of steel are even described by their yield strength in ksi. Good old A36 has a yield strength of 36 ksi. Got to love American engineering!
What's wrong with kips, it's a great unit.
Fascinating. I thought you had answered the question in the middle of the video where you were discussing that 'you' don't do the work on the box, rather the earth does as the earth is actually providing the potential energy. So when a magnet picks up a box or a car the magnet is not doing work but rather redefining what direction 'work' is performed in given the strength of the magnetic field compared to the gravitational field. By 'you' introducing the magnet into the area, you have given the box/car/paperclip potential energy, and when it moves up the magnet is actually *reducing* the objects potential energy.
I started studying physics in 2005 and graduated in 2008. We used that exact book, and I just checked, and I still have it on my shelf. (Because why would I sell it back to the book store after I finished the class when they would give me what amounts to a rounding error on the price of the textbook.) I always chalked up the "magnetic forces do no work" line to the explanation you give about the work you do on the box: you lifted the box, but Earth "performed work" on the box. So for a crane lifting up the junker, the magnetic fields are more like the "tool" we use to lift the car in the gravitational field of Earth (like your arm), but it was still Earth "performing work" on the car. Thank you for going back and connecting the quantum mechanical aspect of it to explain that, in the "real world," magnetic fields can in fact do work.
I also went through quantum mechanics, but never really thought about the "magnetic fields do no work line" because I thought it was not-all-that-useful pedantry when we don't even really use that work attribution to do energy accounting in the real world applications like the junkyard crane. The crane operators have to provide electricity to that crane to lift that car, and the energy carried by that electricity is going to be (more than, accounting for losses) the amount of potential energy the car gains being lifted into the air. Just like when you lift the box, sure, Earth "performs work," but your muscles are providing the energy that the box gains, and you're extracting that energy from the chemical energy in food, so in a sense the box is gaining energy from the food you ate.
I'm rambling, but I guess what I'm getting at is, pedagogically, not only is "magnetic forces do no work" misleading since they do in quantum mechanics (and the textbook is called "Introduction to Electrodynamics" not "Classical Electrodynamics"), but the physics concept of work is less often a good representation of energy flow through a system than you would expect given the way it's presented in undergrad.
Yesss Dr Angela uploaded and the thumbnail looks like a rant, let's goooo 🍾🔥🔥👀
To be fair to him, I'm an electrical engineer so only got babies first electromagnetics and if I read that I would have thought the same thing. And then if you added well this doesn't account for electron spin I doubt that would have helped me. I'd just be like what the hell does spin have to do with this?
I haven’t done any physics for at least a 5 years now, so I somehow confused myself about the box on a ramp diagram at around 10:00.
I thought more work would be done, because the box has also moved horizontally, in addition to the 1m vertically.
when its moving horizontally it doesn't have to move against any force (gravity pulls straight down) so there is no extra contribution. If maybe the wind was blowing down the ramp then there would be an extra term to include in the force description which could make it be a greater amount of work then.
@rufioh that's why she had to mention that the ramp is frictionless, leaving the only thing to overcome being gravity. A frictionless ramp then just becomes the same thing as a vertical lift more slowly, so less force but perfectly balanced by more time, the work stays the same.
In the real world with friction, moving in an oblique to gravity does result in more total work.
The net work adds up to zero, to move something horizontally. Sure, you have to provide work to get it moving, but since it also has to slow down to rest, you theoretically can get that work back, assuming the agent can both do work and slow something down in a reversible process. E.g. an ideal motor that works in reverse as an ideal generator.
Human bodies can't do work, or slow something down reversibly, plus friction and drag inevitably are there, so that's why your intuition is that it would take more work. The work you do stopping an object in motion with human forces, is ultimately lost to heat.
@@carultch respectfully, I don't think any of that is true. I could be wrong, but here is my thinking.
If you accelerate a thing to move a meter, you've put in the force and done work. If you decelerate a moving thing you've done work again. Acceleration and deceleration are the same thing, deceleration is just acceleration in a different reference frame.
As Angela explained, when you pick the box up and put it on a shelf, you've done the (positive) work to move the box and the Earth does (negative) work to give it potential energy. Work is a transfer of energy. That thing which is transferring the energy is doing the (positive) work and if that thing is you the human body, the human body is absolutely doing work. Net work being zero I believe is because of conservation of energy, not because people can't do work.
@@seijirou302 "If you decelerate a moving thing you've done work again".
The reverse is true. Work is done on you, if you are the agent decelerating something. Suppose instead of a person bringing it to rest with non-conservative human forces, we have it stopped by a bumper made from an ideal spring. The spring bumper is set up so that a clip locks it in its compressed state, after the object comes to rest.
Because idealized spring forces are conservative, all the work done on the spring while bringing the object to rest, is stored in the elastic potential energy of the spring in its compressed state. You can get that energy back, when you release the clip. An ideal spring can accelerate the mass back to its original speed.
"Acceleration and deceleration are the same thing, deceleration is just acceleration in a different reference frame"
The reference frame that matters here, is the object's original reference frame, when it started initially at rest. The fact that an object decelerating at 1 m/s^2 while moving eastbound is also accelerating relative to a westbound car that is speeding up at 2 m/s^2, isn't really relevant.
The bottom line is, if you apply a force against an object's motion, you are doing negative work on it in some form or another. Either that work dissipates as heat for a non-conservative force, or it is stored in another form for later use (such as EPE, SPE, GPE, or CPE).
Great video as always!
1 small thing, could you make the video a little louder? when an ad comes on it's super loud relative to the video
I watched the Asylum video first, and I have to say you made worthwhile additions to the topic. Thank you.
Ahhh, sunday morning coffee and a ACollierAstro video. Perfect combo!
*checks to make sure its monday... yep have to go to work...
@@jjsanchez2 Ahh holiday for me. It feels sunday!
Reading this on a monday night, knowing full well it was posted a few hours ago, I wondered for a second if I know anything about timezones after all
The name of the channel is not ACollierAstro anymore, it is Angela Collier. I did not notice until last time when someone mentioned it in the comments.
Not only does it break the "Work=Force x distance" law, it ALSO BREAKS NEWTON'S 3RD LAW and BREAKS THE LAW OF CONSERVATION OF MOMENTUM for the magnetic force between two moving point charges (See "Classical Mechanics" by John R. Taylor, section 1.5, "The Third Law and Conservation of Momentum"!
Newton's 3rd law says forces between two objects must be equal and opposite, which results in the law of conservation of momentum, but the magnetic forces on each point charge on the other are equal in magnitude but 90 degrees to each other, not in opposite directions (180 degrees) as described by Newton's 3rd Law, thus breaking the law of conservation of momentum (AND ANGULAR MOMENTUM, because the forces don't act along a line connecting the two objects)!
However, the magnetic force between two LOOPS OF CURRENT (aka, an electro-magnet, a good model for a permanent magnet), DOES obey Newton's third law and the Conservation of Momentum, and therefore shows how magnetic forces DO produce work when referring to currents (such as those in an electro-magnet), just not when referring to forces between free individual point charges (See problem 1.33 in "Classical Mechanics" by John R. Taylor).
maybe LOOPS OF CURRENT (centrifugaly accelerating charge) is a electric (-2d) mass while linear current (without acceleration) is a electic (-1d) magnet
and massive objects DOES obey Newtons third law while massless do not.
2d massive electron can cyllindricaly rotate (spinning) while 1d massless magnet (photon) do not.
So ,
photon and electron both have same curvature(quantity) '+' but different dimension(quality) 1 and 2
while
electric and normal(photon) magnets both have different curvature '-' and '+' but same dimension 1.
To invert curvatures we must turn things inside out like a glove and we call it MOTION.
In ihis process quantity INFINITY-1 becomes quantity +1.
"infinity is a motion" Aristotel.
"Magnetic forces do no work.
Note: This only holds in Classical Electrodynamics. There are exceptions in Quantum Mechanics, however that is beyond the scope of this textbook."
All you had to do Griffiths.
If your going to claim that something doesn't "do work" then calling it a force is the main point of failure.
Not necessarily. All force has to do is result in an acceleration. As the acceleration is centripetal for a moving electric charge, potential energy isn’t added to the system; thus no work is done even though a force is applied.
As an engineer I was taught that boundary forces do work, but not field forces. Field forces should be accounted for in the energy equation as changes in potential energy. If an object falls to the ground (air resistance negligible) loss of potential energy is balanced by gain in kinetic energy, & no work is done by gravity. If big magnet lifts an object I’d say as it rises it gains gravitational potential energy and loses magnetic potential energy
I really love this style of video. It reminds me of my uni days where one of us would learn something first and be able to explain it to the others as colleagues rather than a strict tutor/student relationship.
I too remember learning this in Griffiths and being confused. That confusion was later made worse by the fact that magnetic work appears explicitly in some thermodynamic calculations such as the cooling process known as adiabatic demagnetization. Interestingly this technique only works because of intrinsic spin (either nuclear, or unpaired electrons) and cannot be understood in terms of small current loops.
This is literally perfect timing, I have an electrodynamics exam tmr lol
Dr. Collier: This video is just for me and I don’t care who else watches it.
Me: Excellent!
I didn't have a problem with this...until I saw your video title! Ha! Thanks for clarifying physics terms for us non-physicists! (well, somewhat clarifying, but thats me). So...I only a month ago realized why 30 years ago I didn't understand why my conception of the "warping" of space didn't match everyone elses, and the "right" way to formulate it! (in undergrad intro to physics class). I was applying a mathematical transformation, where, in my mind, everything would be transformed...space, time, spacetime, forces, masses, everything...so that it was just a mathematically different representation of the exact same situation...so such a "warping" would not be noticed by anyone. I know, its a weird way to think of it...that's why I majored in Biology, not Physics, ha!
I read your title, I was indeed disturbed. I started watching your video. I gave you a like for referring me to a shorter video. I watched the shorter video. I came back (not less confused) to watch the rest of your video. I finally understood, that (and how) the phrase "Magnets do no work" makes sense if you define the word "magnet" in a very weird and specific way not only detached from every day language, but also specific to a specific sub-field of electrodynamics. Hooray.
If someone who reads this still struggles to understand, watch Asylums video on magnets have no force (or don't, I'll summarize the important part in this comment). There's electromagnetic force, electric force and magnetic force. Electromagnetic force basically combines the 2 others and when dealing with magnets it's the only real force, because we defined a force to be real only when it's real in every reference frame, but we can always shift the reference frame so that all the magnetic force mathematically turns to electric force and vice versa.
But doesn't the electromagnetic field and thus the electromagnetic force belong to the magnet?
Like if I lift a cube, did I lift a cube or did my arm lift the cube? Well my arm did, but since that arm belongs to me, so did I.
But smart people already defined the magnet's force to be the magnetic force hundred of years before they figured out, that we also should have an electromagnetic force defined aswell. If we also consider the electromagnetic force to be part of the magnet, then we run into some problems with overcounting, so let's just not. And thus was a weird definition for magnet born in which magnets do no work. It's the same nonsense as electric current flowing from + to - or - to + depending on what scientific field you are currently in. Stupid in a way, but also very useful and important to know when you're once again towering on the shoulders of giants.