8.03 - Lect 13 - Electromagnetic Waves, Solutions to Maxwell's Equations, Polarization

[@emeraldeyes9565]

An interesting historical fact is that Maxwell's formulation was composed of 20 equations in 20 variables which Heaviside showed could be reduced to just 4 equations in 4 variables. These are the ones taught today.

[@schmetterling4477]

One can write the whole thing in tensorial form in a single equation.

[@kyle5555]

@@schmetterling4477 fuckkkk.. lol I’m struggling with the first one still

[@keisi1574]

@@schmetterling4477 I'm working on being able to express it with .33 of an equation.

[@alirezanabavian771]

@@keisi1574 only if humans lost 8 fingers then your research would be useful

[@splat752]

I am not sure Wikipedia would agree with you

[@nomejodasmas]

Very good vibes from this teacher, I don't know much English but I interpret technical questions perfectly. I am not a student, I am just a madman who invents things in the small laboratory at home and steals a little knowledge of the internet and the good people who provide it on the net. Thank you very much teacher.

[@alirezanabavian771]

You're very smart and unique

[@dc1049]

All these lecture series are a gold mine, youtube is a more effective method of teaching than sitting in class ever will be. Simply because of the luxury of the rewind button... I wish the academic infrastructure acknowledged that fact and stopped throwing money away on poor lecturers (who might know what they're talking about but have such poor delivery skills). In the words of Feynman: "It’s impossible to learn very much by simply sitting in a lecture, or even by simply doing problems that are assigned." This is why I believe class time should be used strictly for questions and discussion... a two way road, not a one way force feed of information. The feeding of information can be done at home through tools such as KZbin and amazing lecturers such as Walter Lewin who perfected the craft and put their heart and soul into it. As opposed to tenured professors who sit on their research and could care less about the students they need to teach, or the quality of their lectures.

[@lecturesbywalterlewin.they9259]

+D C I know many tenured Professor (I also had tenure) who do care about teaching. However, few are talented teachers.

[@ms-uj3qe]

+D C Really? Let`s stop all physics classes, because walter lewin has done it better on youtube... Are you serious? Just because you study on a shit university that doesn`t mean that there are no good, passionate professors out there.

[@ms-uj3qe]

+Man, I don`t even have an opinion *Because you think your university is shit* is more like what I was trying to say.

[@dc1049]

Man, I don`t even have an opinion You are quite correct, perhaps I was too harsh with my opinion of some professors. My main problem is with the modern academic structure as a whole sir. I do not think the system of sitting in class while listening to a professor monologue through a discussion is very effective. As I stated before, I think Richard Feynman already pointed out exactly what the problem is.

[@das250250]

D C yes indeed , being a good teacher and a good professor aren't necessarily the same ..teaching is a real skill

[@efeguleroglu]

Thank you for sharing your lectures with us. You're a great teacher Walter Lewin.

[@aibi-rv2mm]

These students have been so lucky to have attended your classes. And I still enjoy learning from your dynamic classes. They are just fantastic. Thank you, Professor Walter Lewin!

[@lecturesbywalterlewin.they9259]

Glad you like them!

[@saulsavelis575]

@@lecturesbywalterlewin.they9259 Maxwell did not postulate, he heard and even cited Faraday who guest correctly that light is EM phenomenon

[@jacobvandijk6525]

Walter, it's a shame there isn't a Nobel Prize for Teaching.

[@outrospection4all]

or at least made an FRS

[@srabanisamanta1599]

Dont worry great person dose not need nobel prize.

[@hrkalita159]

Noble prize is unlucky as these person (Hawking's, lewin) don't touch it.

[@mohammadmursalin6817]

All Jeff Bezos would have to do would be to create the Bezos prize for Teaching and Educating and things would be fine............but nooooo...................Dr Evil had to piss all that money on his space mission.

[@munirkofa4783]

Jacob van Dijk sincerely speaking

[@deathtotruthers1]

How is it possible for Lewin to make these esoteric, ridiculously difficult concepts understandable? Shame he retired - though he deserves it. He was a brilliant educator.

[@sidereal6296]

Mr Lewin, I’m a freshman at MIT right now taking this class, on this lecture. I love my professor but the way you teach, connect everything together, and give meaningful historical context, is simply inspiring. Thank you for uploading these to KZbin

[@CurrentlyObsessively]

Congrats on graduation!

[@DJRobbie54]

That was A Nice Lecture on Electromagnetic Waves Polarization, Professor Walter Lewin. The Math was a bit hard on me, but I began to see it in a spiritual Realm of Quantum Entanglement in the Cosmos of the Universe. This is the first time I have seen any of your lectures, Very Well Done. I look foreword to watching more of your videos, to learn about Electromagnetic Waves. I Like your style of teaching us this amazing subject. It was a lot of fun to watch, and learn, and see your demonstrations that you perform to your students, they had fun to. I like teachers like that, who can have fun, but, be serious at the same time of what he or she is trying to teach the class, that makes learning easier. That encourages students to want more. I also know this was an old Video that was uploaded back in 2015. To me it is a breath of fresh air of learning of a subject I did not understand when I was a young man back in the day. I hope you're still in good Health and still teaching your students. I have become one of them, at 68 years old. Which, go's to say, your never to old to learn. Thank You Professor. January 11th 2023. PSS, Check out Royal Raymond Rife's Discoveries, Back in the 1920's, I think you'll find it very enlightening.

[@misspandora432]

Dear Professor Lewin, At first, I would like to thank you for all the works you made available on KZbin with your lessons. It's terrific. I learnt physics 30 years ago where I was quite successful, and for some reasons, I didn't make a career in Physics. Looking at the way you have done this marvellous job at the MIT, I am almost able to quit my job and everything, and to come back to the University, even to the MIT! Second, in your lecture #13 from MIT 8.03 video, you talk about the way that a human (you !) can see the polarized light and how to recognize it. Could you be kind enough to light my brain about this subject? Dear professor, thank you very much for what you have done in the way you did it, it is "extraordinaire". Best regards, Marc André. Paris, France.

[@pranjaldubey3384]

we read electromagnetic waves today in class and im happy to have found this lecture here today~

[@pad_52]

Dear Sir Lewin, First I want to express my gratitude in sharing for free, those pieces of art that are your Physics Lectures. Your lessons helped me more than once in my engineering career and never stops to help: not just for understanding the argument, but also thanks to your teaching enthusiasm that is so contagious that my learning enthusiasm benefits. I can’t believe in my eyes when all your lectures disappeared from the Internet! Thanks for putting them online again. I’ve a little question: in 1:09:54 you said that you are able to teach a human to see a polarized light like a bee :) Will you ever do a lesson about this? I wish you all the best, Kind Regards. Paolo

[@lecturesbywalterlewin.they9259]

google Haidinger's brush

[@GuilherHast]

Thank you for the question. Thank you for the answer.

[@promethialcronus]

​@@lecturesbywalterlewin.they9259 Does maxwells equation have it all.Can there be more than just the EM radiation .with just 4 equations solved you can make yhe whole world feel the awe and not ask another question just for the sake of readers or whatever or critics maybe.

[@emmanueloverrated]

I wish I had this professor when doing this course... I had this guy (professor?) who was more interrested in research than to teach and made me hate this matter... Better than not failing the course, I would probably loved electromagnetism and been pretty good at it.

[@Leptus87]

Great lecture, conducted with briliancy and passion as well!

[@danielkinyanjui5296]

Professor Lewen, you show such a deep understanding of the subject matter that goes beyond rote. You would be perfect for Africa! Thank you.

[@technocrat711]

Had it about 40 years ago. Enjoy the lecture. Light bulb burning is something few can forget.

[@Solar-vb6zr]

This lectures are the real gold. Thank you Professor.

[@lecturesbywalterlewin.they9259]

Glad you think so!

[@naturalscience2022]

This video inspired me personally to do a demo E&M wave for my grade 12 students in Cambodia. Physics is magic and Beauty.

[@lecturesbywalterlewin.they9259]

super, keep it up!

[@HenryAronofsky]

Dear Professor Lewin, I hope this finds you well. I am a senior in high school and absolutely love your lectures. I have been following your talks and KZbin page for over a year now and, as it’s my last year of high school, have embarked on an ambitious task. I am endeavoring to reconstruct the electromagnetic wave transmitter/receiver demonstration you present at 1:00:07 in this video. I have exhaustively studied the theory and searched the far reaches of the internet and my school for answers on assembly methods, but have yet to find a satisfactory idea or construction methodology. I now turn to you for any guidance you have the time to give me. Any crumb of information would be greatly appreciated, I thank you for all you have taught me and all I have to learn from you. My best, -Henry

[@lecturesbywalterlewin.they9259]

I can't help you as I am not an engineer

[@sumansaurav1945]

This is one of the best lecture given by you amazingly super

[@lecturesbywalterlewin.they9259]

:)

[@spdas5942]

Excellent teaching , sir . Hats off !

[@lecturesbywalterlewin.they9259]

Keep watching

[@MarkFunderburk]

I meant to go to bed 2 hours ago but I can't stop watching this. Thank you sir for keeping me loving physics. I was getting dragged down during physics 2 this past semester and these lectures have helped turn that feeling around.

[@lecturesbywalterlewin.they9259]

:)

[@sundaranarasimhan58]

Excellent quality of presentations. I wish I was your student. Love from India.

[@arkadiptaghosh99]

you have just opened my eyes...I am seeing what I was not able to see it before...Thank you professor..:)) Respect

[@akshatparwani4961]

i was struglling with college level physics. your lecture made it very easy

[@mobile2]

interesting lecture. studied electronic engineering, work as cellular radio network engineer. Since GSM1800 in 1997, cellular antenna has used linear +45/-45 slant polarization as people hold cell phone in slant orienatation

[@federicoborsotti8947]

The representation at 26:12 is a classical representation of EM waves. But if the strength of the electric field is c (speed of light) times stronger than the magnetic field, and if the magnitude of the vectors in the representation represents the strength of the respective fields, does that mean that in a more "faithful" (though clearly unmanageable) representation the arrows used to represent the magnetic field should be c times shorter than the arrows used to represent the electric field?

[@kingg283]

I have seen some youtube lectures also from Yale. Like this it is extraordinary. Think about if only masterpieces like this will be used for lecture. It is meant globalisation of knowledge then. What ist local school then? Will they die? Thanks anyway to Walter for this passionating Lecture.

[@RD2564]

One of the beautiful things about this is how WL does not take any shortcuts with notation, which is helpful for viewers learning the subject. PS: "Don't even THINK of using a left handed coordinate system." Awesome performance.

[@perfectlyimperfect6849]

Sir your lectures give us real feel of physics Thankyou sooo much for these gems Just enjoyed your lecture🙏😊

[@lecturesbywalterlewin.they9259]

It's my pleasure

[@Andreasskaliontas]

17:05 that phone call is written in the history of Physics Lectures by this great video

[@anon6514]

Maxwell surely was a genius. I always preferred to remember them in their integral form. I remember you covered that in 8.02 Although, in the differential form, it's easier to see how a wave emerges. Had to pause the video to process the maths at times but I understand the subject much better now. Thank you. Amazing work.

[@lecturesbywalterlewin.they9259]

In 8.03 I use the differential form to derive the EM plane waves and the speed of light!

[@davegeorge7094]

He draws a dotting line like the best one handed drum roll! He's second to none with hand speed, anyone disagree?

[@Sixalienasa]

it is in fact the way the board works, if you press the chalk at a certain pressure and direction is bounces on the board giving that effect.

[@davegeorge7094]

No, I think it's a skill. watch it again.

[@Sixalienasa]

I have watched several people do it, and yes there is skill attached too.

[@lecturesbywalterlewin.they9259]

Watch this video kzbin.info/www/bejne/qJLYo59qqJWjoLc

[@lecturesbywalterlewin.they9259]

watch this video kzbin.info/www/bejne/qJLYo59qqJWjoLc

[@das250250]

At the end they did not applaud but they looked tired and uninterested , i find this incredible , one of the most enthused lecturer's i have ever seen on here , i wish i had such a teacher at my uni. I would have applauded ..to thank him

[@puneethprahalad7386]

Walter Lewin and H.C.Verma are legends who make you fall in love with physics

[@srikanthtupurani6316]

It is so beautiful. When I look at the maxwells equations I feel the best test for the correctness of any theory in physics is beauty. If a theory is beautiful then there is a high chance that the theory is correct.

[@odal6770]

I have the impression that Maxwell's Laws serve a unique goal, and that is to give empty space all the properties of matter, except visibility and physical substance. In other words, it is the ether formulated mathematically.

[@saskiavanhoutert3190]

Polarized vetor-rotation can be used for rotors of planes, to avoid birdstrikes perhaps, just a notion, thanks for lectures

[@lecturesbywalterlewin.they9259]

:)

[@p0k7lm]

Thanks for the lecture, this will supplement my two Maxwell's volumes to for a better understanding of the E and B fields.🎓📝📚📐

[@kevinradl1462]

There is something I don't quite understand about EM waves: My professor told us that electromagnetic waves are caused by accelarating charges and the Wave can be understood as the 'information' of the changing electric and magnetic field traveling through space. How can we detect them over such long distances and why don't they lose intensity proportional to 1/r^2 ?

[@lecturesbywalterlewin.they9259]

they spead out like a spherical balloon and YES the intensity goes down with 1/r^2. The EM intensity of the Sun goes down as 1/r^2.

[@anisand]

What about the long-known adverse biological effects of electromagnetic waves? A vital caveat to any lecture on this subject surely?

[@geminijake7398]

1:36 but isnt an electron and/or a proton a magnetic monopole? Im so fascinated with this subject matter, just trying to wrap my head around it. Awesome stuff

[@lecturesbywalterlewin.they9259]

they are electric monopoles. Magnetic monopoles do not exist.

[@fisicaematematicacomjean]

Even isolated eletrons seems to present an intrinsic magnetic dipole.

[@inthenameofjustice8811]

His last words in this cracked me up. "Who wants a black eye? See you next Tuesday." Or, put another way, 'Who wants a black eye? C.U.Next.Tuesday.' Though I am sure this was an accident I still laughed like a drain.

[@studywithjosh5109]

I have thought about physical situations where there would be a pattern moving faster than the speed of light before I knew any concrete physics and now I understand I was thinking about phase velocity. It makes a lot of sense now!!!!!! I was so confused before because I thought that nothing could move faster than light.

[@anassbensaid9197]

I always like the physics and I'll thanks for this video :) sir

[@fatihorhan9355]

Great lecture! Thank you for your great uploads.

[@Saptarshi.Sarkar]

What happens to the oscillating magnetic field after light passes through a polarizer? Is it unaffected?

[@lecturesbywalterlewin.they9259]

B field will be affected as the B field in the EM wave will be perpendicular to the E-field.

[@sanjoykumarrouth6410]

After viewing these wonderful lectures every student is going to enjoy physics more.

[@tjcanno]

Thank you for this, Dr. Lewin. Very helpful. Please, if you would , explain to us how we know that the E field and the B field are in phase? I expected the peak/trough of the E field to align with the B field crossing through the zero line, and then the same with the E field crossing through zero where the B field was at a maximum/minimum. Could they not be out of phase by pi/2?

[@lecturesbywalterlewin.they9259]

they have to obey Maxwell's eqs. However, there are solutions to M's eqs which allow for different phase differences

[@Mechomittencaterpillar]

Fun fact about the lecture: this was filmed 17 years ago

[@gbmillergb]

I might get a few test questions right in his class if they are True/False or multiple guess... I mean choice.

[@dwjs_]

Hi sir, I have just finished watching your 8.02x course. In that course you derived integral form Maxwell equations. In this video you used differential form Maxwell equations. I want to know whether you have a video which explained differential form Maxwell equations in details?

[@lecturesbywalterlewin.they9259]

Look up the differential form online.

[@sunyue1909]

Dear Prof. Lewin, at 42:20, the line you draw via the origin looks more perpendicular to the k vector, doesn't it? On this line, k = 0, because it is on the origin, then k dot r = 0 (not k perpendilular to r).

[@babuj6949]

sir, at 38.03 minutes: as of my understanding. EM wave is moving in K- direction (because k- represents propagation direction), then what r- represents. i mean certainly wave is not moving in r- direction. wave is moving in k- direction. what way i can understand about r-direction?

[@lecturesbywalterlewin.they9259]

General form: direction is in an arbitrary direction r.

[@rgudduu]

E field at r (location or position), of a wave moving in k direction. E.g. E(x,y,z,t)= xCap E0 cos(wt-bz) Then say, we want to find E at (x,y,z)=(2,1,5), of this wave travelling in z direction at some time t0. Here r=(2,1,5), k= z direction.

[@asbargathi]

PERFECT. thank you sir, I'm really happy for this good job

[@lecturesbywalterlewin.they9259]

You are welcome!

[@atruety]

I just love you sir 😍😍😍😍😍😍😍 Love from India 🇮🇳🇮🇳

[@KunalSingh-my5nd]

Sir, what is the way which you have learnt to tell wheather a light is linearly polarized? You refer to that at 1:09:50 into this 8.03, no 13 lecture. Please share I am keen!!

[@lecturesbywalterlewin.they9259]

I use my linear polarizer. Google Haidinger's Brush.

[@TheKansi2]

Sir I learned that we use electromagnetic waves to communicate wirelessly and electric field exist due to location of charge and magnetic field due to movement of charge. What I don't understand is that how can we communicate someone in free space ? According to web. there are about 10 atoms per cubic centi-meter. Doesn't that mean there are way less charge for E. and B. fields to do their thing therefore wave to propagate ? what did i miss or misunderstand ?

[@lecturesbywalterlewin.they9259]

There are frequency limitations which is what all radio astronomers know. use google

[@srikanththota648]

At 1:03:26 the hand absorbed the electromagnetic radiation. That's very interesting. How does this happen? And, what wavelengths of radiation can the hand can pass through it? In General, if the electromagnetic wave hits an object, based on what characteristics of the object can we say if the radiation can go through it or not? And, lastly is this explained in any of the lectures?

[@asbargathi]

water can greatly weaken the microwave, like the rain destroy the satellite signal.

[@sohammukherjee8877]

I always had a question when writing the expression for E field we took it along x direction and the wave in z direction. How do we know from the Maxwell's equation that E and B will be perpendicular and the wave will be travelling in the direction of E×B

[@lecturesbywalterlewin.they9259]

I cover all this in my 8.03 lectures.

[@Bless-the-Name]

When he said, "Instead of taking the curl of the curl of E, you can take the curl of the curl of B," I wondered: What about the curl of the curl of C? In fact ... why is there no curl of the curl C mentioned? What happened to the curl of the curl A? Do these curls of the curl go up to Z? Then I realise ... this is not cricket.

[@lecturesbywalterlewin.they9259]

how many minutes into the video?

[@Bless-the-Name]

@@lecturesbywalterlewin.they9259 I see what you did there. Genius.

[@abhiksasmal3304]

Professor, when we write Maxwell's Equations in terms of line integral and surface integrals, we talk about E and B fields that penetrate through a surface. Or in other words, we think like- what will be the line integral of E along the edges of 'this' surface if 'that much' flux changes through the surface. But if we write the equations in terms of curls and divergence, we come to know how the E and B fields are related at every point in space. Am I correct ? Is it true that Maxwell's Equations in curl and divergence form describe these fields at every point in space but same Equations written in integral form can't do the same ? Thank you for teaching us with these amazing lectures

[@lecturesbywalterlewin.they9259]

hyperphysics.phy-astr.gsu.edu/hbase/electric/maxeq.html

[@otiebrown9999]

Good logical presentation.

[@oningsingson5672]

Excellent Teacher🎉

[@lecturesbywalterlewin.they9259]

Glad you think so!

[@gioegiogcbr]

Why is the antenna of the receiver longher than the transmitter, shouldn’t they be same to capture that specific wavelength?

[@soumyamishraCrazyZaynster]

this is an out of context question but sir would you please explain how the useful power of the AM wave lies in its side bands and not in the modulated carrier?

[@lecturesbywalterlewin.they9259]

+Soumya Mishra I don't know. PLease look it up on the web. Google!!

[@stargazer7644]

The carrier isn’t modulated. The sidebands contain the modulation. If you remove the carrier, you can put that “wasted” power into the sidebands. Thats how SSB and DSB works.

[@AnjanakushwahaKushwaha-qj4oq]

Sir , the length of wave in y direction /wavelength multiply by c , this equation I give to the chatgpt but it say it is "wrong ".

[@lecturesbywalterlewin.they9259]

what I wrote and said is correct

[@AnjanakushwahaKushwaha-qj4oq]

Explain it in better

[@AnjanakushwahaKushwaha-qj4oq]

​@@lecturesbywalterlewin.they9259 sir please

[@dhruvinvekariya975]

Whoaaaaaaa Your speed is too good as compare to current videos Your sponteinity is decreasing professor

[@Lucifer-pj8vc]

Sir pls enable caption for this video

[@lovesharesucceed4736]

I think the academic curriculum must be reframed to incorporate these online lectures.

[@ostora94]

why does the electric (or magnetic ) field vary spatially only with the direction of propagation of the wave (i.e. if the wave propagates in the "z" direction "E" is only a function of (z,t) ) ?

[@lecturesbywalterlewin.they9259]

like water waves, they propagate and thus E is also a function of the direction of propagation.

[@shubhamtalks9718]

professor, how can humans recognise polarised light and the direction of polarization under ideal conditions?

[@lecturesbywalterlewin.they9259]

google Haidinger's Brush

[@YannisAlepidis]

At 1:07:30 why the average value of cos2(x) and not the integral of cos2(x) from 0 tο 2π?

[@pavankalyan-zi6ei]

Thank you, Professor, for making my life ambiguous😅.

[@xo.komalpareek]

Sir i'm from india preaparing for jee and your videos are extremely helpful ❤ and can you share some tips for jee exam?

[@lecturesbywalterlewin.they9259]

you have 2 options option 1: eat yogurt every day but *never on Fridays* option 2: Watch all my 94 MIT course lectures. Start with 8.01, then 8.02, then 8.03. Do all the homework and take all my exams. *I guarantee you that you will then do very well on the Physics portion of any freshman college or JEE exam* You will find all information you need on this channel in three playlists "Homework, Exam, SolutionsY & Lecture Notes". 8.01 & 8.02 will each take about 200 hours, 8.03 about 250 hours.

[@williambarrett6527]

Toward the end, professor Lewin claimed that Kirchoff's circuit theory did not apply to the situation in which a changing magnetic field affecting a loop in the circuit. Fair enough, except that every electrical engineer should have had a course in magnetic devices, including a configuration like the one he used. His circuit diagram should have included one "winding" of a transformer - which was effectively the loop in the circuit, with the primary the other coil and switch. Such a device is commonly included in any such circuit, and can be configured as a circuit element to correctly predict the impulse current that Lewin demonstrates. His effect does not demonstrate that a Kirchoff diagram is invalidate, only that he did not include a transformer element, typically shown as two curly windings near each other. An implied magnetic coupling between the two, along with their inductance and wire resistance, can accurately predict the signals seen in Lewin's demonstration.

[@lecturesbywalterlewin.they9259]

Hello William, I have know idea who you have in mind when your write *his diagram*. In any case here are some facts. *I demonstrated at the end of my lecture #16 of my 8.02 E&M course at MIT that two identical voltmeters attached to the same 2 points in a circuit can show very different values.* kzbin.info/www/bejne/pHi0k3Ron9yejas *The reason is that in the case of an induced EMF (Faraday's Law) potential differences are no longer determined; they depend on the path. This also applies to the secondary windings of transformers as the EMF in the closed loop of secondary windings is induced.* kzbin.info/www/bejne/mGjMY6iDrZyaftE *Of course, in cases where Kirchhoff's loop rule (KVL) applies, 2 voltmeters attached to the same 2 points in a circuit will always show the same value.* *My demo was first suggested and published by Romer in December 1982 in the American Journal of Physics.* *This demo has now become a classic; it's done all over the world at many colleges and universities.* *Kirchhoff's original text can be found in the following link, pages 497-514:* books.google.de/books?id=Ig8t8yIz20UC. *Clearly he was fully aware of the prerequisite for his "loop rule". KVL is a special case of Faraday's Law. That's why Faraday's Law is one of Maxwell's equations and KVL is not.* *By teaching students that KVL always works without telling them when it does not work, makes many believe that the closed loop integral of E dot dL is always zero. ElectroBOOM and Dirk Van Meirvenne therefore believe that 2 voltmeters attached to the same 2 points in a circuit must always show the same value which is not true as demonstrated in my lectures.* *They each posted a video (see @ footnote below) on their channel in which they claim to have proof for their wrong ideas which violate Maxwell's equations.* *Apparently they do not know, or do not understand, that in the case of an induced EMF potential differences are no longer determined; they depend on the path.* *MIT students who took my 8.02 course (Electricity and Magnetism) would not make this mistake!* *I therefore believe that to introduce a "modern version" of KVL and then teach students that KVL always holds is not advisable as you may set them up for making the same embarrassing mistake that both Dirk Van Meirvenne and ElectroBOOM made.* *Also read Professor John Belcher's thoughts on Faraday's Law and on KVL. Professor Belcher is my former colleague at MIT.* freepdfhosting.com/0813df09f5.pdf *(@) ElectroBOOM insulted me in his video by mentioning that the reason why my 2 voltmeters read different values was due to "bad probing". He stated that if I had done the demo correctly I would have found that both voltmeters would have read the same value. He could not have been more wrong* This video may also help to digest why 2 Voltmeters attached to the same 2 points in a circuit can give very different readings. kzbin.info/www/bejne/ime2ZomJZZirkMU

[@lecturesbywalterlewin.they9259]

Hello William, I have no idea whom you had in mind when your wrote *his diagram*. In any case, here are some facts *I demonstrated at the end of my lecture #16 of my 8.02 E&M course at MIT that two identical voltmeters attached to the same 2 points in a circuit can show very different values.* kzbin.info/www/bejne/pHi0k3Ron9yejas *The reason is that in the case of an induced EMF (Faraday's Law) potential differences are no longer determined; they depend on the path. This also applies to the secondary windings of transformers as the EMF in the closed loop of secondary windings is induced.* kzbin.info/www/bejne/mGjMY6iDrZyaftE *Of course, in cases where Kirchhoff's loop rule (KVL) applies, 2 voltmeters attached to the same 2 points in a circuit will always show the same value.* *My demo was first suggested and published by Romer in December 1982 in the American Journal of Physics.* *This demo has now become a classic; it's done all over the world at many colleges and universities.* *Kirchhoff's original text can be found in the following link, pages 497-514:* books.google.de/books?id=Ig8t8yIz20UC. *Clearly he was fully aware of the prerequisite for his "loop rule". KVL is a special case of Faraday's Law. That's why Faraday's Law is one of Maxwell's equations and KVL is not.* *By teaching students that KVL always works without telling them when it does not work, makes many believe that the closed loop integral of E dot dL is always zero. ElectroBOOM and Dirk Van Meirvenne therefore believe that 2 voltmeters attached to the same 2 points in a circuit must always show the same value which is not true as demonstrated in my lectures.* *They each posted a video (see @ footnote below) on their channel in which they claim to have proof for their wrong ideas which violate Maxwell's equations.* *Apparently they do not know, or do not understand, that in the case of an induced EMF potential differences are no longer determined; they depend on the path.* *MIT students who took my 8.02 course (Electricity and Magnetism) would not make this mistake!* *I therefore believe that to introduce a "modern version" of KVL and then teach students that KVL always holds is not advisable as you may set them up for making the same embarrassing mistake that both Dirk Van Meirvenne and ElectroBOOM made.* *Also read Professor John Belcher's thoughts on Faraday's Law and on KVL. Professor Belcher is my former colleague at MIT.* freepdfhosting.com/0813df09f5.pdf *(@) ElectroBOOM insulted me in his video by mentioning that the reason why my 2 voltmeters read different values was due to "bad probing". He stated that if I had done the demo correctly I would have found that both voltmeters would have read the same value. He could not have been more wrong* This video may also shows why 2 Voltmeters attached to the same 2 points in a circuit can give very different readings. kzbin.info/www/bejne/ime2ZomJZZirkMU

[@abhiksasmal3304]

Professor, you calculated curl of curl of E and arrived at wave equation.This equation is derived from Maxwell's equations and is therefore valid for every time-dependent Electric Field in vacuum. Then the equation implies that it's solution must be a travelling wave. So does it mean that all time-dependent Electric or Magnetic fields that exist in vacuum must be travelling waves ?

[@lecturesbywalterlewin.they9259]

yes every electrric spark that you produce at home produces EM radiation. You can hear it on your radio.

[@abhiksasmal3304]

@@lecturesbywalterlewin.they9259 Professor, I'v one more question. If I hold a charged object in my hand and then move my hand randomly , its electric field varies with time. So, in that case will the Electric field be a travelling wave ? And what If I move a magnet randomly ? Will the Magnetic field be a travelling wave ? Thank you.

[@studywithjosh5109]

@@abhiksasmal3304 yes the first case will be a traveling em wave

[@powertube5671]

Hi professor Lewin. I love your videos. I am taking meticulous notes. Here's a question. What would be the wave function of the chalk when you make those dotted lines. It's obviously alternating stress and release of the the chalk with just the correct angle of the chalk. :-)

[@lecturesbywalterlewin.they9259]

there is no wave function

[@ruudvansteenis]

That must have been a lot of power that is transmitted on 80 MHz! It even interferes with his wireless microphone.

[@fupa_lvr]

What a Masterclass

[@marcelinobartoletti3259]

Hello sir, if the electromagnetic wave is consists of two fields (electric field and magnetic field) that is perpendicular to each other, then why is the receiving antenna with a light bulb can't receive if it is in horizontal position opposite to the transmitter's antenna? (Is it only the electric field induced a current from the receiver and not the magnetic field?)

[@lecturesbywalterlewin.they9259]

the answer is Maxwell's eqs.

[@marcelinobartoletti3259]

Thank you very much sir Walter Lewin 🥰

[@abhiksasmal3304]

Professor, I have watched all the 8.02 lectures of you. I have a question- Maxwell's equation tells us that- closed loop line integral of B = u × ( conduction current + displacement current) through the surface formed by that closed loop . Let us think of an infinitely long wire that carries alternating current in vaccum. This will produce alternating magnetic field, thereby EM wave will be produced. But as we go further from the wire, the amplitudeude of B field should decrease because we move to a larger closed loop and the amplitude of both Currents do not change. This is contradictory to em wave where the amplitude of B field or E field cannot change !!!! It should not fall with increase in distance from the wire !!! Sir, please help me to understand what I'm missing.

[@lecturesbywalterlewin.they9259]

>>>Let us think of an *infinitely long wire* >>>> But as we go further from the wire, the amplitudeude of B field should decrease because we move to a larger closed loop and the amplitude of both Currents do not change. ====> B-field will not decrease if the wire is *infinitely long*. If the wire length is finite, ofcoz B-field willl decrease

[@abhiksasmal3304]

But professor, ∮B⃗.dl̂= μ(I₁+I₂) where I₁ and I₂ are the conduction and displacement currents respectively. => B×2πr= μ(I₁+I₂) => B= μ(I₁+I₂)/ 2πr Where `r` is the distance of the observer (me) from the wire( infinitely long). This means that- B should fall with 1/r as r increases !! It would be very helpful if you kindly explain once more. Thank you sir.

[@Deedeed33]

KZbin= Free education

[@lecturesbywalterlewin.they9259]

:)

[@mastipathsala912]

Sir if a capacitor connected to a battery and capacitor charged fully then what displacement current will be generates? ?

[@lecturesbywalterlewin.they9259]

watch my 8.02 lecture in which I cover displacement current as a function of time

[@TheKansi2]

Sir, I have an idea and it's just a thought..so my question is; Can we create controled electromagnetic field to change the characteristics and behaviours of a specific area of earth in a certain moment so any incoming earthquake lose some amplitude ?

[@lecturesbywalterlewin.they9259]

How would you do that? use Google and ask Quora.

[@davidmendizabal9892]

33:14 at this point you got my undivided attention. OMG

[@boshooda1941]

are there high res versions of these lectures that can be posted?

[@CurrentlyObsessively]

These are recorded in the 90s. So I would assume no. This IS high def for that era of cameras. I remember them well...

[@siddheswarmondal7764]

Nice... Sir thanku so much 🙏🏻🙏🏻

[@nobutterinhell]

great scientist hair

[@nobutterinhell]

and why are humans on earth? to defy express de zzzzzs

[@suniljoshi5315]

Sir 53:18, when u said theta goes to 0 Lx becomes infinite.. can u specify why it happens in bit more detail? as i didnt catch it with what u said in the lecture sir.... please sir.....

[@lecturesbywalterlewin.they9259]

I can not add to the clarity of this lecture - watch it again

[@suniljoshi5315]

@@lecturesbywalterlewin.they9259 ok sir thank u

[@outrospection4all]

Professor Lewin, a question (if I may) @ 42:20 - " I draw another line perpendicular to r ". The line looks perpendicular to k rather than to r. Also, it's not clear to me why you chose to draw a line at this position. I'm struggling to grasp on the intuition behind this step and its subsequent results, i.e. k.r = 2pi, 4pi...etc. Any clarification would be very appreciated

[@studywithjosh5109]

he chose this point as a point where E is a max so he can show us the wavelength of the wave.

[@JensenPlaysMC]

as commenter above said. thes can be any distance, he was just showing that along this plane at any r vector value, the e values on that plane are the same (for freespace solutions) he chose arbitrarily the distances in wavelengths which are then maximus

[@robelabera8770]

Am I the only one who thinks experiments on these lectures have a vibe of a magic show?

[@huonghuongnuquy7272]

what a great lecture ! Thank you very much.

[@jpdemont]

Awesome teacher.

[@seongeunkim5801]

About polarization, does the magnetic field induce unpolarized electric field, after its been polarized? Because the polarizer only polarizes the electric field, so there're still magnetic field that isn't polarized, right?

[@lecturesbywalterlewin.they9259]

I don't understand your question. Tell me first how your magnetic field induces an electric field.

[@seongeunkim5801]

Sorry, I think i expressed myself wrong. An EM wave propagates by continuous induction. If you use a polarizer, does only the electric field get polarized? What happens to the magnetic field then? Does it follow the path of propagation? Or does it vanish?

[@seongeunkim5801]

Doesn't the magnetic field also need to be polarized? Otherwise it can induce an unpolarized electric field..

[@lecturesbywalterlewin.they9259]

Start with linearly polarized light. Maxwell's eqs will then tell you about the B-field (it's all in my 8.03 lectures) they will answer your questions. Don't be lazy.

[@MsSujoy]

I wanted to say that according to maxwell electric field and magnetic field exists simultaneously, but in electrostatic only electric field exists.. Why?

[@lecturesbywalterlewin.they9259]

Maxwell's eqs are the way they are. That's the way the world ticks. Why they are the way they are is a very different issue which can best be addressed by religions.

[@paulgagnier7360]

Due to potential force within a closed system

[@moe531]

Suppose there is a guy moves with velocity as same as the charge’s velocity, what he observes is an electric field coming from that charge, but what we will observe is a magnetic field, however, you and the charge at rest both are in the same frame, thus you will observe an electric field.

[@gerontius1726]

That was the problem solved by Einstein when he discovered Special Relativity. Two electrons travelling parallel in the same direction repel each other slightly more than they do their own frame of rest, owing to Lorenz contraction and time dilation. Going in opposite directions they repel slightly less than they are attracted by the stationary protons (say in parallel wires, which seem to attract together) . Electromagnetism is nothing but our name for the effect we observe on the macroscopic scale, and permanent magnetism is what we experience when the electrons in the atoms of two lumps of iron are spinning in the same direction.

[@lecturesbywalterlewin.they9259]

@@gerontius1726 >>>electrons in the atoms of two lumps of iron are spinning in the same direction.>>> that's too simple a picture. It requires QM to properly explain magnetism. Magnetic dipole moments can not be explain with Ampere's Law. Thus they cannot be explained with classical physics.

[@babyknifexd5307]

Sir how much voltage was induced by receiver? and what kind of bulb should i use to demonstrate the existence of EM wave? I mean the wattage of bulb, rated voltage etc(12v or 24v etc). Tnx sir.

[@NuVids2025]

Did you ever find the answer?

[@BadBoy-ls5vn]

Sir walter...after the break i didn't understand the phase velocity derivation from the geometry at all..and I've been struggling with it for more than a week now...please help...thank you

[@lecturesbywalterlewin.they9259]

I cannot add to the clarity of this lecture. I suggest you also google "phase veloocity" What is phase and group velocity? Waves can be in a group and such groups are called wave packets, so the velocity with which a wave packet travels is called group velocity. The velocity with which the phase of a wave travels is called phase velocity.

[@lecturesbywalterlewin.they9259]

en.wikipedia.org/wiki/Phase_velocity

[@BadBoy-ls5vn]

Thanka you sir...i understood perfectly the meaning...the only problem i had is with the relationship between what you said comes directly from geometry of the angels Ly/lambda times C =Vpy and so on...i know it looks trivial...bu as you said before...i can have sleepless nights even when i don't get such a simple thing...hope I'm not asking a stupid question...

[@utkarsh22smart]

professor, how did you built that antenna. I wanted to build one by myself. if you have any paper/document that exactly makes it, sir please do share. I am a student of Electronics and communication (Bachelors) than a student of Aerospace (AVDC- Cranfield University). thankfully your old student who is admiring and seeing your lectures from 4 years.

[@lecturesbywalterlewin.they9259]

I did not build it. Andy can send you all details. [e-mail]

[@utkarsh22smart]

@@lecturesbywalterlewin.they9259 thank you SIR, for the reply. i surely will contact him.

[@pakistan010203]

WHAT IS NAME OF THE TRANSMITTER WHICH IS CONNECTED TO THE DIPOLE ANTENNA ? CAN ANYONE PLEASE TELL ?