Thank Sir, I had never learnt of applying amperes law when the closed loop passed through different media, I learnt only the case where it passed through one medium. Thanks a lot for teaching me that.
I have a general question, Dr. Lewin. What exactly about the electrons of a material makes it magnetic? Does it have to do with its spin quantum state? In my university general physics class, we did not fully delve into the atomic level of electricity and magnetism, and we barely talked about quantum mechanics. We only briefly talked about quantum mechanics and we summarized the relationship between electricity and magnetism with Maxwell's equations.
thanks prof Lewin for the explanation. I got a question about the direction of the magnetic field inside the ring. Why the field is symmetric around the ring? In the problem, the current goes around a small portion of the ring not the whole ring. Can it still be symmetric? Thanks sir and have a great day
I got lazy and did not run through the numbers. But here is some ancient memories. At what I might call middle school physics we already had to learn the Right Hand Rule for the magnetic field direction. You curve your right hand fingers to show the current in the coil and spread the thumb to show the magnetic field direction. All directions counted from the palm to the finger and thumb tips. I hope this describes it well enough. Later on we had to learn the Left Hand Rule. Point the index finger horizontally (field), other fingers down (current flowing like water down) and the thumb pointing towards yourself (the Force is HERE!). Finally, at the Electrical Engineering studies we performed the air gap calculations over and over. So much so, that I got the order of magnitude effect of the air gap off the top of my head. Also, we had to learn order of magnitude numbers for iron/steel saturation to be used as sanity check. Grain oriented iron as much as 1.8 T, but don’t use that much when designing a transformer, because the inrush current will blow primary fuse every time. You might use about 1.5 T and blow the fuse only sometimes. For ordinary steel laminations, maybe 1.2 T. And solid steel rods, stay below 1 T, due to Eddy current losses. I can relate to why your numbers were picked to produce the 0.91 T. Anyway, thanks for the education that we need, some of us as reminders, some for the first time ever. But always valuable and highly appreciated. I hope you don’t take the fluctuation of the reader numbers as a sign that nobody cares any more!
Question Dr Lewin : it's well known that light slows down when it travels through glass. but why it speeds up agin when it leaves the glass? and isn't that a violation of the holiest law in physics ?
the energy of a photon E=hf h is Planck's constant f is the frequency. When light enters glass (or leaves glass) or any other substance f DOES NOT change!!! thus the energy does not change. The speed changes and thus the wavelength changes because wavelength= speed/f. but E does not change.
nice as always, but the key word what is missed is why do we need an air gap :) People may wonder why do we break magnetic "circuit". In electrical circuit every resistor(air gap equivalent) is dissipating a power, as well it does not help in transformers..but in flyback converter it helps...
Greetings Sir! I've got a question here. The current is traversing a circular path through the windings but since the windings have some thickness the current is also going forward a bit after each revolution. So if we consider the forward current, can we say that there is also a Magnetic field which passes through the centre of the toroid? Thank You!
@@lecturesbywalterlewin.they9259 Greetings sir! Sorry to bother you again. I have a very small doubt. In a non ideal solenoid of a certain length it is known that the Magnetic field at the centre is more than that at the ends which implies that B-field is not homogenous . But when we join the ends of the same solenoid to form a toroid the B-field becomes homogenous. What is the reason for this? Is this because in the latter case the B-field emerges and ends inside the solenoid and nothing escapes in the air? Thank you!
Antareep, That is because in the case of the finite length solenoid the return flux must flow in the space OUTSIDE of the solenoid in the space around it whereas in the case of the toroid, the flux never has to pass into the space outside of the interior of the solenoid to complete the flux path. Magnetic flux lines ALWAYS appear as closed paths since the divergence of the B-field is ZERO. GH
@@georgehnatiuk5806 That's exactly what i thought. Thanks! Haven't worked with divergence yet though. I am still in school. Btw I was wondering if you have the answer to my first question.
Sir, if a current carrying coil of a LARGE diameter has a ferromagnetic rod of finite length and having a small diameter placed at the centre of the coil, then, is the effective area of the inductor(consider as a transformer with single set of winding) so formed, almost equal to the area of cross section of the ferromagnetic core?
I am trying to say that, in case of a transformer, the peak induced e.m.f. is given as, E=BNAw, B= magnetic field at saturation N=# of turns w= 2πf = angular frequency A= effective area of cross section. I was talking about this "A".
Hoiii sir I want to pursue phy honrs after 12 can u plzz tell me something about it what and where should I can work after the course means what's its scope ..plzz tell if possible
Ahem sir! From just a little playing around with magnets or electromagnets it would be intuitive - not counter-intuitive - that a gap will reduce the total magnetic flux. It would also be intuitive that the potency of the gap to reduce total magnetic flux is dependent on the strength of the magnets or strength of an induced magnetic field via current in the coils. Just a little magnet play demonstrates how quickly magnetic field is reduced by distance between poles, which is the inverse square law although one does not need to know the name to be aware of it's properties. I suppose for those who have never played with magnets the significant effect of the air gap might be counter-intuitive. But on the other had I would invoke something like an Occam's-razor rule and ask why would one not expect a reduction in total flux if the toroid is broken....I mean...it's broken.
Love you sir, please I’m curious. Did you study physics? And what aspect of physics did you study. And also, do you have a. PHD, I’d be honored if we could know what your thesis was about. I love you so much and I am inspired by you everyday.
I mean can I use "the magnetic field vector encircles the current carrying wire" in the statement of Ampere's circuital law, instead of saying that "the magnetic field encloses the current" ?
Hello sir , now I'm starting to read you book "for the love of physics ". 😀 I have order it from online website. I'm have waiting for 1 month 😂😂 it took so long time for shiped to thailand🇹🇭🇹🇭🇹🇭
More things need discussing. First, you say symmetry is needed. No it isn't! As soon as you state (assume) the flux is confined to the steel path (and its gap), the path could be any shape whatsoever! It could be square, rectangular, oval, any closed "curve". The only constraint is that the area be large enough (for gap fringing to be insignificant) and uniform (so flux density everywhere is the same) and that its length is 0.5 meter. (Actually, your problem as stated is NOT symmetrical. The coil turns would need to "penetrate" central to the circle, or be distributed uniformly about the toroid, and the air gap distributed about the toroid in order for that to be true). But it doesn't matter, as long as flux leakage from the toroid's surface is low relative to that inside. You could have presented problem with square core 1/8th meter on a side, area >= 5sq cm, and answer would be the same (0.91Tesla). Second, about the "penetration of the surface by the current". In electrical engineering, we talk of the current being LINKED with the magnetic path, like 2 connected chain links. There is no confusion whether a current "counts" or not. (A current is "linked" when it can't be removed from the setup without cutting it or the magnetic path.) Winding direction: the hand of the winding helix has no effect whatsoever. The turns could even be wound directly on top of one another or zigzagged back and forth (most common in real-world coils). The ONLY thing that counts is the direction of traverse of the current "linking" or "penetrating the surface". When a steel core is inserted with a high permeability (and leakage fluxes are small), the magnetic FIELDS problem is reduced to a magnetic CIRCUITS problem. Symmetry is no longer needed. If you look at 60Hz power magnetics (motors, transformers), the paths have all sorts of "crazy shapes" (such as square or rectangular). Because symmetry is not required, simplicity of fabrication is what determines the shape. (It also makes calculating magnetic fields a piece of cake.) Most transformer cores are rectangular for this reason. You should also have explained that the area doesn't matter as long as it is large enough and UNIFORM.
Rajesh Chourishi, The magnetic flux, Φ, must be the same in both medium since it is a series circuit. Φ = Β∙𝐴 and A is the same everywhere. Therefore also B is the same. (Compare with an electrical serial circuit where the current I must be the same). Watch 8.02 as professor Lewin propose.
Rajesh Chourishi, This is because the cross sectional area of the air gap and the core material are assumed to be the same. In a series magnetic circuit, the magnetic FLUX is continuous just like the current in an electrical series circuit is the same in each circuit element. Since the flux Φ in the core and the gap must be same, then the B-fields must also be the same because Φ = B*A and the areas are the same (no fringing assumed). Bc = Φc / Ac and Bg = Φg/Ag Series Magnetic circuit: Φc = Φg = Φ Bc = Φ/Ac and Φ/Ag If Ac = Ag = A, then Bc = Φ/A and Bg = Φ/A, so Bc = Bg = Φ/A. QED GH
Hi Mr. Lewin. Sorry to ask you dumb issues. The question was not mine. I just have to convince a 6th grade geography teacher that no one would say translation as a perfect example for the movement Earth does around the Sun. I tried in several ways, and I thought that maybe your mind could come up with something that would be impossible to argue against. Yes, I did pass on both your courses 801 and 802 with 90% grade, but as you must know, some people are pretty difficult to convince the obvious…..But I have to, since they are teaching the young generation in the wrong way. Don’t you get upset with that ? I do…..Physics is beautiful and it cannot be taught with misinformation. If you can help me, I would appreciate a lot, and you would also be helping hundreds of other students, if not thousands if we can change her mind…What do you say ? If you prefer I can give you my email and we can talk from there…
As a former student of yours from 801 and 802, having aced several exams along both MOOCs and also one of the 5 top participating students at the discussion forum, I would appreciate if you could just answer this question: Why is it not trivial (I know it is not! ) to use the word translation for curved paths ?............and to prove I was on those MOOC’s, there was a Guy named GROVE among us. You got to remember him ,since he posted close to 2,000 posts. He got your autographed book, if I am not mistaken....Me, Grove and some 3 more were the most participating students at the Discussion Forum. I believe I posted around 700 posts…..
Where should i get a good college for masters in physics?? I am very disappointed by the colleges i get here in India. The teachers are criminals here...they don't know a thing about physics. Reservation spoils everything in India. I need good teachers. I want to study healthy physics. Plz tell me a way??
@@lecturesbywalterlewin.they9259 how to get the admission there?? Should i drop my year for preparing the entrances and applying there?? How to get admission? Money is also a problem. Plz tell some way. I need good place. I need a university where they don't care about my history. They just see my passion. I need good labs. Plzz tell me some good universities where i have a higher chance of getting admission. Also it must not be too costly.
@@Rishabhkumar-gp9tg I cannot help you - You should read the application websites of the Universities which you think might accept you based on your present grades.
