8.02x - Lect 17 - Motional EMF, Dynamos, Eddy Currents, Magnetic Braking

Motional EMF, Dynamos, Eddy Currents, Magnetic Braking
Assignment Lecture 17, 18 and 19: freepdfhosting.com/885b570f3b.pdf
Solutions Lecture 17, 18 and 19: freepdfhosting.com/f55c880397.pdf

[@nidhisolanki8337]

Professor I am studying physics again. I studied it in high school and graduation and didn't really understand anything. I have been constantly watching your videos and it has indeed changed the way I used to perceive Physics. Now it is so beautiful that if I am not well, i watch these lectures or study Physics. Huge respect for you. I wish I could meet you someday.

[@NuVids2025]

Nobody meets the professor until they solve the Boolean Pythagorean Triples problem.

[@mewsicman9541]

​@@NuVids2025What the hell is that?

[@lecturesbywalterlewin.they9259]

This website contains all my 94 course lectures (8.01, 8.02 and 8.03) with improved resolution. They also include all my homework problem sets, my exams and the solutions. Also included are lecture notes and 143 short videos in which I discuss basic problems. ENJOY!

[@physl2787]

Lectures by Walter Lewin. They will make you ♥ Physics. Sir you say that a changing magnetic flux linked with the coil produce current in the coil. Sir classical physics says that it is due to the inertia of the coil. The coil does not want to change the magnetic flux linked with it. That's why it produced current to oppose the changing magnetic flux. Sir how the reason given by the classical physics is not correct.

[@lecturesbywalterlewin.they9259]

Faraday's Law is classical Physics and *it is 100% correct!!!!* It runs our entire economy. There can not be any confusion. If this is not enough for you, consult google.

[@Experts_top_5]

@@physl2787 this kind of inertia is also called as the self or mutual inductance of coil, which can be verified by both Faraday's or Lenz's law..

[@shivaramanshukla5013]

Sir can u mathematically explain WHY EARTH BEHAVES LIKE BAR MAGNET? BIG FAN SIR.

[@lecturesbywalterlewin.they9259]

@@shivaramanshukla5013 en.wikipedia.org/wiki/Earth%27s_magnetic_field cosmosmagazine.com/geoscience/what-creates-earth-s-magnetic-field

[@keatoru]

I'm so glad I found your videos Professor. I'm always trying to find things to learn more about these kinds of subjects and your lectures are better than anything that I've been able to find. Thank you so much!

[@lecturesbywalterlewin.they9259]

My pleasure

[@DhanasekaranT-de4wz]

I am 60 now. And addicted to Walter's lectures and binge watching all his videos. I loved the Dynamo demo in this video. In my younger days I used to see small Dynamo that can be attached to the backwheel of the Bicycle. A light bulb in the front connected to the dynamo serves as light source during night time riding in the dark. When dynamo rotator attaches to the wheel it's a bit more difficult to pedal the Bicycle wheel (Lenz's law).

[@PauloConstantino167]

"You can't even do it man!" :) That was so funny hahahah! I loved it :)

[@natashalenski7367]

Thanks to this lecture (part about eddy currents and mag braking) I finally understand why a magnet falls so slowly when we let it fall through a hollow conducting cylinder :)

[@obayev]

Thank you, professor! I get so much knowledge from these lectures and I appreciate it a lot!

[@ian6083]

All of your lectures are fascinating and dense with information. I especially enjoy 8.02, but would recommend all of your lectures as required learning for anyone interested in physics!

[@YommiOfficial]

Very captivating. Thank you for these.

[@Nandini_Dwivedi]

I actually got to learn the importance of eddy current as It has many technological uses ,thanks pro

[@michaelinglis4520]

Interesting to note that the shaft in the Indian Point power station that I visited that drives the dynamo is .8m in diameter and about 60m long and made of solid steel. The torque on the shaft causes the shaft to twist 270 degrees our of true when generating electricity! Real work is happening! Thanks for the lectures

[@Anne-tq5zq]

I was supposed to have nice timw with Netflix but ended up here with full lecture.........i love it ,couldn't able to leave....

[@aapddd]

Yes his lectures are addictive. Great teacher.

[@kaustabc7562]

WL: *breaks a lightbulb* Also WL: "Now that, runs our economy!"

[@BrunoAnselmo1]

you are a great man, i'm doing research on an application that uses ZVS and i found you ! good job!

[@bittolas]

Hello professor Walter. Im a Physics entusiast, i dont have any grade or college education. Im just making this coment to thank you. You thought me everything i know about Physics!

[@cricworld6797]

After the professor the cameraman is the one who pursue the most knowledge in physics 😁😁

[@Experts_top_5]

The lecture was remarkable, even today Micheal Faraday is not alive, but by watching this lecture and all experiments that you have done, I found Micheal Faraday is still there.. especially experiments helped me a lot.. thank you.

[@lecturesbywalterlewin.they9259]

:)

[@Experts_top_5]

Sir, In eddy current experiment, what kind of material (conductor) you have used? Magnetic or non- magnetic conductor..?

[@lecturesbywalterlewin.they9259]

The material I used was NOT a permanent magnet.

[@wallaceyang7971]

Thank you Prof. Lewin for your fabulous lectures! In your demonstration with the copper pendulum being slowed by the solenoids, you used power conservation arguments to explain the dampening. I am having trouble picturing how the dampening force (the direction especially) is created using F=qvXB and Faraday's law. Could you please comment? Thank you in advance.

[@lecturesbywalterlewin.they9259]

+Wallace Yang Eddy currents are not easy to model. Yet, it is clear that they consume energy (ohmic heat). Where does that energy come from? It has to be gravitational potential energy since that's the only energy source available. If you moved a metal plate through a magnetic field you will have to do work. You will have to push! Your work emerges as heat in the metal produced by eddy currents. There is a q(vXB) force on your metal that you have to overcome. You move the metal with velocity v. The free electrons in the conductor start moving.

[@user-uj8ts7pi9y]

Helped me pass electrodynamics. Great lectures

[@sandiptodas5740]

Thank you sir... your lectures is helping me in Neet preparation

[@naund293]

I can not appreciate your lectures enough. I'm doing my Master's in Germany and working on Magnetic field influence on Electrochemical process. I was interested in your take on the Magnetic field effect on a charge carrier of an ion conductor (or electrolytes). Thank you! God Bless you, Sir.

[@lecturesbywalterlewin.they9259]

That's great!

[@naund293]

@@lecturesbywalterlewin.they9259 Thank you, Sir ;)

[@michealdavis9884]

dear professor you are the best person I have ever heard,i can understand more from 10min than any one I ever heard not MIT must have lost there mind, you're an inspiration you are my hero you give me inspiration and give me great ideals in anti gravity,people call me a nut job but I have some success in small experiments (it's not really gravity that we are defeating) the earth is a big magnet 😁

[@lecturesbywalterlewin.they9259]

thanks, Michael, for your kind words.

[@mahjoubahmed1287]

Thank you so much professor lewin

[@gentleman3258]

Wish I would have been in that class and enjoyed the live show!

[@lecturesbywalterlewin.they9259]

:)

[@ahmedhaddad5300]

Great thanks sir I want to participate in Ipho this year or next and Ur lectures are really helpful But I have a question: When U talked about the motional EMF u just took the case of a rod what will be the situation if had a conducting sphere or any other shape I hope u answer and thanks!

[@alperurey4492]

Prof. You rotate the loop in x axis which the magnetic flux would not change. You should have rotated in y Axis in order angle teta and also flux to change.

[@jeanpierrerabil]

Great lectures!

[@kimballmarlow4661]

Awesome lecture on magnetic braking. I'm pretty sure metals like aluminum that respond with such vigor to electromagnets do so, because they produce a greater number of eddies than copper, and it possible an alloy out there somewhere would be even more responsive. I was working on a problem in my head using magnetic braking, and thinking I can't have it correct. All that kinetic energy coming to a halt, but where's the equalization. Then you said it generates heat, and the balance of the universe was once again as it should be. I get focused on something and over look a basic function of objects doing work.

[@jatinghaitilak]

Hi. What will happen if we use an electric conductor that is also a magnetic substance during the damping?? how will it work in this case?

[@user-um9sl1kj6u]

Have we worked enough with graphene and copper or iron composites (grains along with graphene oxide) to make a dent in better conductors/magnets?

[@seharirfan204]

Sir u explained very well . ☺

[@hahahahaha651]

If only i had a lecturer with this high quality, i would be a physicist.. unfortunately i only got formulas in learning physics just to calculate without knowing the concepts... and it makes physics is a mystery and boring.. thank you very much Prof, u rock!

[@NuVids2025]

Nobody meets the professor until they solve the Boolean Pythagorean Triples problem.

[@ballifitness1527]

great sir you made me love physics

[@tylerrussell7560]

When discussing the eddy currents, you say the current must pass through the air, but why doesn't the induced current just travel on the surface of the conductor (at 39:24)? Any help would be greatly appreciated!

[@seandafny]

Thank u so much Feynman

[@lecturesbywalterlewin.they9259]

My name is Feywin

[@mamduarahman2665]

@@lecturesbywalterlewin.they9259 sir you are teaching us as like as Feynman did ....😍

[@PluetoeInc.]

@@mamduarahman2665 he is teaching us as lewin did

[@ahmadeldesokey9844]

Sir, at 5:20 how will you be able to rotate that loop with a constant angular velocity? If you were applying a constant torque , the induced current will develope an opposing torque which is also sinusoidal and therefore the resultant torque applied to the loop is not zero then , how the angular velocity will be constant ?

[@Raphael_NYC]

Thank you Dr. Lewin. raphael santore

[@legend-uj2dp]

Hello Professor Lewin or anyone who can answer this question for me. In the demonstration with the copper pendulum, was it the opposing Lorentz force that slowed the pendulum down on the KE being converted to heat, or a combination of both?

[@sweetmae7265]

Very interesting Prof.

[@dishakhurana6739]

Belated happy teacher's day sir!! Sorry its too late!! (In India we celebrate it on 5th Sept.) Thank you very much sir for making us love physics as you do!!!😊😊🙏😀

[@erichjogstream65]

I have a question. The magnetic braking means that a force is slowing down the conductor right? But in a previous lecture he said that a magnetic field never can do work on a charge because the lorenz force is always perpendicular to the velocity so the speed can not be increased or decreased it can only change direction. But here we obviously have a change in speed when we talk about magnetic braking? So what am I missing?

[@hikguru]

I strongly suspect my teachers themselves didn’t understand what they were teaching. Just regurgitating what’s in text books is not teaching and that’s what they did. Everything was geared towards doing well in exams and not stimulating student interest. This is a completely different approach!

[@BruinChang]

I am trying to visualize the geometry of the open surface with a closed loop in Faraday's law, and what I have done is using a helicoid frame (shorturl.at/cxSW3) to match the relationship between the open surface and its associated closed loop. However, I still can not make it. For example, In this helicoid mentioned above, the surfaces which extend from different portions of the frame seem to intersect. Does that matter? Or I went to wrong direction to approach this problem? Any help is appreciated, thanks a lot.

[@solo2309]

Wait a second ! When you lit the light bulb by swiftly moving the coil between the magnetic field, didn't you feel a magnetic braking ?

[@adarshchaturvedi3498]

really nice Sir.... :-)

[@nileshbenz1]

6:50 is it Maxwell tells you the change in magnetic field that matters ? or is he Faraday ?, i think both is okey.

[@sheriefhenry321]

Perfect explanation

[@visheshmittal468]

Sir u are awesome😀😀😀

[@ribb4200]

When the power grid becomes overloaded, generators slow down slightly (frequency drops from 60Hz) and motors slow down and do less work over time. Generation and load are balanced by AC line frequency. Underfrequency relays automatically drop load at about 59.5Hz to prevent generator damage and customer damage. When you turn on a light bulb in Boston, the frequency drops a very small amount on the entire east coast until generation speeds up a very small amount to return toward 60Hz. Time keeping is corrected after midnight when load drops and there is excess generation available. The amount of power a generator adds to the power grid is controlled by it's generating turbine speed (frequency) and not by voltage as most people expect. I really doubt that Walter Lewin camping flashlight (27:46) generates 120 Watts. More likely 1 Watt. or less. Most people can generate about 60 Watts with a real struggle on a bicycle. Thanks Walter, I do love physics and your lectures.

[@lecturesbywalterlewin.they9259]

>>>I really doubt that Walter Lewin camping flashlight (27:46) generates 120 Watts. >>> *what made you think so??? I never said that.* Watch the video again. My dynamo would probably produce 5-10 W if I work very hard!.

[@ribb4200]

(27:46) "This is a 120 Watt light bulb and I keep it going all the time". Either way, you have a great way of personally connecting with your students and that was one example. Thank you.

[@lecturesbywalterlewin.they9259]

ha ha ha ha ha ha ha didn't you realize that that was a joke! Wasn't that obvious!

[@ribb4200]

I thought the joke was you being in the Boston mountains and doing a lot of mountaineering (27:25).

[@mariocalio7623]

Hello professor, I loved this lesson but I was wondering why you didn't talk about the situation of a wire running thought a magnetic field which seems to be a major part of my high school physics exercises

[@g1ntok147]

He told..bro...motional emf

[@gemacabero6482]

Hi! Thanks for the videos. I had a question: in the last example, the one of the loop that has a hole, how can the current jump over the gap?? Why is there current going through the gap? Thank you!!

[@lecturesbywalterlewin.they9259]

how many minutes into the video?

[@gemacabero6482]

@@lecturesbywalterlewin.they9259 46:31 Thanks!

[@PRIYANSHUKUMAR-rg1xy]

mindblowing...

[@fate_map1592]

Around 32.50, while deriving an expression for motional emf, ultimately, we arrive at the conclusion that be it by Faraday's or by work considerations we arrive at the same expression for E. However, imagine a scenario where the magnetic field doesn't pervade the entire area of the loop. We start by pulling the moving arm outwards from an initial point where the moving arm isn't touched by the magnetic field. In such a case how would we explain the scenario from an energy standpoint?

[@kimballmarlow4661]

If you were dealing with iron you would have a weakly magnetic pole depending on the direction of the current you'd have positive or negative. I'm not sure what you'd have using a more resistant semiconductor material. Could be you wouldn't have anything unless the velocity was high enough to overcome the resistance. I may have completely misunderstood what you were asking.

[@pesalasrinivas2439]

Dear Sir, if any student any where in the world doesn't understand your 8.01, 8.02 and 8.03 not grade pass/fail but alteast general ideas/concepts, they are not eligible to pursue their engineering .. that should be made global law.. as without these basics no research could be done.. by those graduating students.. take a bow Sir from india.. and in any univestity engineering disciple.. this should be order of course structre. 1) math series all from calculus to differential equeations .. 2) your phyiscs/nature series.. ........ rest all doesnt matter and student can fully appreciate and understand any discipline..

[@ArvindKumar-wh6nn]

sir your lectures are the very beautiful way of learning the laws of nature. sir, I have a question that can we relate or is there any relation between faraday law with the Lorentz force?

[@lecturesbywalterlewin.they9259]

:)

[@manos1779]

sir , at 38.00 you said that the lorentz force opposes the motion as the current comes out of the blackcoard but there must be lorentz force on the other side too.... right where the current goes into the blackboard but here the lorentz force will aid the motion...seems confusing that the net force would be zero acting on the plate . please explain me this ....

[@JensenPlaysMC]

Amazing that you can measure the current due to the magmetic field of the Earth

[@sahil3735]

Does anything work against when something rotates a loop is a magnetic field? If not it would mean we can just increase magnetic field going through the loop and get a higher voltage.

[@lecturesbywalterlewin.they9259]

If you rotate a loop in a magn field you have to do work. Transformers can increase the voltage by factors of 100 and more, but Energy is conserved. No transformer can create more power than what goes into it (at the primary coil).

[@yash29210]

When you were finding direction of Lorentz force on the conductor in case of Magnetic breaking, there were 2 possible directions of Lorentz force......if we consider the solid disc perpendicular to the page then once current will come out of page and once current will go into page but you found Lorentz force for one direction only....(38:02)????

[@lecturesbywalterlewin.they9259]

Lorentz force will reverse when dphi/dt changes from + to minus. In both cases heat is produced inside the disc. Thus the disc will slow down as it enters the B-field and also as it leaves the B-field. That's called magnetic breaking.

[@nileshbenz1]

At ground 5:30 was confused between angular frequency and angular velocity. But I think both are just fine

[@itachu.]

tehy ar same.

[@tomsaxton9534]

when I watched this I thought about Niagara falls and how many turbines to generate all that current.the force would be gallons per minute producing some amount of turns in the turbines per minute.But what if the flow varied due to climate variations

[@user-vi5ip1hg2d]

Electricity without cables.... Thank you for this video

[@mohamedismail6273]

Almost all electricity that we use come from rotatting loop it's wiref

[@physl2787]

love you sir

[@lauroflorin]

You speak about conservation of energy. What do you think about red-shifting light due to what seems like the Universe stretching? With our current models, it seems that their energy is lost (but then again, there's the "dark" stuff and possibly other things we can't explain yet so this part may be incomplete)

[@lecturesbywalterlewin.they9259]

+lauroflorin Energy is conserved. cds.cern.ch/record/777175/files/0407077.pdf

[@Merilix2]

@@lecturesbywalterlewin.they9259 Very thank you for that link. It saves my life or at least helps me arguing pro energy conservation.

[@pradoprado9993]

At minute 38:00, the Lorentz force from the current in the side that Mr. Lewin pointed to, goes in the left direction and opposes the motion as he said. But what if we look at the current from the other side of the eddy loop? The force would be in the right direction, so why is there a magnetic breaking? Shouldn't forces cancel?

[@lecturesbywalterlewin.they9259]

no

[@kishorsonawane5990]

Where can I find intro for AC CURRENT

[@tanaykumar3493]

Can you please explain why will the disk slow down when the Eddy Currents are induced?

[@lecturesbywalterlewin.they9259]

I cannot add to the clarity of my lecture. If you did not understand my explanation, please use google

[@Abraham21121]

What the relation between open surface and close loop? I couldn't understand properly the concept.

[@lecturesbywalterlewin.they9259]

Faraday's Law

[@benjuliebenjulie7414]

my question is with magnetic damping...the kninetic energy of the moving object is reduced due to the force from the eddy currents and the applied magnetic field. This energy is dissipated in the form of I^2R losses..correct? But what if the object is superconducting, shouldn't there still be an opposing force due to eddy currents and the kinetic energy of the object reduced? where did the energy go if there can be no I^R losses in a superconductor???

[@lecturesbywalterlewin.they9259]

Super conductors will repel any magnetic field. en.wikipedia.org/wiki/Meissner_effect. Thus there will be no energy dissipation in the super conductor. However, electric currents can flow inside super conductors. That's how the LHC powers it's magnets!

[@sahilchavan5173]

Sir I doubt will humans ever be able to fly given the conditions are favourable being Human a good conductor ,and increased magnetic field power balancing the gravitational pull ? And also how can we manage the heat produced by eddy currents ? Can the atmosphere above will do conduction to dissipate fair enough to live while levitated up above in the sky ?

[@bpark10001]

Do you explain in any other lectures why iron cores in motors, transformers, and generators are laminated?

[@lecturesbywalterlewin.they9259]

ask google

[@lecturesbywalterlewin.they9259]

energy conservation - suppression of Eddy currents

[@jianliangge2174]

Fantastic!

[@lecturesbywalterlewin.they9259]

Many thanks!

[@Mariadesoija2560]

Good afternoon sir, you lectures have helped me a lot on understanding the subject, l am experiments with making a generator , but could not produce electricity, l suppose l should keep rotating a lot more. , Which l will try again

[@lecturesbywalterlewin.they9259]

Keep it up

[@ethanwan2605]

May I please ask a question: if the magnet enters the aluminum block at different speeds, will the generated magnetic braking force be different as well? Say more braking force at higher speed?

[@lecturesbywalterlewin.they9259]

how many minutes into the lecture?

[@ansh2880]

Thanks prof for these great lectures , but I confused at one point ,which is difference between eddy current and induced current as both are produced by change in magnetic flux as one gives emf and other produces heat

[@lecturesbywalterlewin.they9259]

Eddy currents are produced by magnet flux changes (thus Faraday's Law). Induced fields are produced by external E-fields.

[@lecturesbywalterlewin.they9259]

Eddy currents are produced by magnet flux changes (thus Faraday's Law). Induced E-fields are produced by external E-fields., not by Faraday's Law.

[@ansh2880]

ok thanks prof .

[@InventTwig]

evening professor :) if we move a metallicc plate in a constant magnetic field ,will there be any induced emf ?

[@lecturesbywalterlewin.they9259]

+InventTwig The metal plate will tumble. Thus the magnetic flux through the plate will vary in time. Thus there will be an EMF inside the metal => Eddy currents (Faraday's Law)

[@InventTwig]

+Lectures by Walter Lewin. They will make you ♥ Physics. Thank you :)

[@dishakhurana6739]

Sir you said that in ac circuit(making a bulb glow) bulb still glows(as heating effect of tungsten still remains) even when current is 0 Sir but what happens if we turn off the lights.. It stops instantly... Why sir?

[@lecturesbywalterlewin.they9259]

>>>It stops instantly>> *NO all have a decay time.* Incandescent lamps much longer than fluorescent lamps and use LDEs - use google.

[@dishakhurana6739]

😊😅Thank You sir!!

[@lecturesbywalterlewin.they9259]

>>>It stops instantly>> NO all have a decay time. Incandescent lamps much longer than fluorescent lamps and use LDEs - The color of the light and the intensity of an incandescent light depends exclusively on on the temperature of the filament. When you turn the light off, the flamnets cool. - use google.

[@dishakhurana6739]

Lectures by Walter Lewin. They will make you ♥ Physics. Ok sir! 😊😅

[@kuniulya8165]

I came to this video long after it is posted, so I am wondering if Prof. Lewin is still available. I am trying to access the assignment, but the link didnt work out, can somebody help me? thank you in advance!

[@surendrakverma555]

Excellent lecture Sir 🙏🙏🙏🙏

[@lecturesbywalterlewin.they9259]

Keep watching

[@physl2787]

Sir suppose a wire is thrown from top of the aeroplane to the earth surface. Sir how the magnetic flux is changing linked with which surfacewith the falling straight wire.

[@lecturesbywalterlewin.they9259]

question unclear and ill-defined - use google or quora

[@sselinnnk]

Dear sir, What happens if we replace the magnet and metal in the eddy current systems? What if we try to push a magnet through two conductors? Where does the eddy current form this time? Also, is there any way to measure the current?

[@lecturesbywalterlewin.they9259]

as long as there is a magnetic flux change in your metal sheets there will be Eddy currents in both. The exact location and dimensions are not easy to calculate. There may be algorithms for it.

[@sselinnnk]

Thank you very much for your answer sir. I will be experimenting on this topic, and your videos have been very helpful...

[@kishoreraghuuma6692]

Thanks a lot professor 🙏

[@lecturesbywalterlewin.they9259]

:)

[@Deathbaker]

Is there any difference between faraday's law of induction and motional emf?

[@lecturesbywalterlewin.they9259]

motional emf is the result of Faraday's Law

[@akanshakukreti2570]

Sir I am in 10th standard. I have a doubt in one question. I hope you will help me. Q student boys water in an electric kettle for 20 minutes .Using the same main supply he wants to reduce the boiling time of water. To do so should he increase or decrease the length of the heating element?

[@sandeeppatidar1106]

Thanks professor

[@immanueljzv6832]

At 33:30 you say when we change the velocity in opposite direction the current flows in square wire in opposite direction won't that flow of current now contribute to the existing magnetic field causing Lenz law now obsolete ??? I know this not the case just need a clarification on how the change of current affects the already existing b field

[@lecturesbywalterlewin.they9259]

question unclear

[@lecturesbywalterlewin.they9259]

@@matheworman6308 there is no paradox - of course, you are entitled to believe what you want to believe. But you have a more interesting option and that is to become educated

[@benjuliebenjulie7414]

is it fair to say that the magnetic field is kind of like a traffic cop directing energy from one form to another??

[@lecturesbywalterlewin.they9259]

It's fair but wrong

[@05afnanabdullah87]

Induced magnetic field is non conservative then how can the inductor store energy in the form of magnetic potential energy? Can work done against non conservative forces get stored in the form of potential energy? Please help me out.

[@lecturesbywalterlewin.they9259]

The integral of E dot dl through an ideal inductor is zero. But a current can go through an inductor and that produces a B field of energy 0.5*L*I^2. Pleas watch kzbin.info/www/bejne/pHi0k3Ron9yejas

[@subhankarpaira8662]

Sir if we use two turns very closely, will the emf be twice

[@lecturesbywalterlewin.they9259]

a coil with N turns will have an N times larger EMF than a coil (with same radius) with one turn.

[@nefereous9082]

38:58 46:19 Great demonstration on Eddy currents, I enjoyed it. But it brought a nagging question to mind. At this time signature, a current is passing an air gap. I was under the impression that current could only flow through air at or above the breakdown voltage of 3 MV via corona discharge. How is it that electrons pass this air gap?

[@lecturesbywalterlewin.they9259]

+Webb Telescope Dry air has a high resistivity but it is not zero. Humid air can have a low resistivity..

[@RobertSzasz]

Current isn't passing through the air gap. Each tooth breaks up the maximum size of the current loop so the total braking is much less.

[@lecturesbywalterlewin.they9259]

That is correct!

[@mauriciobarda]

I was wondering the same, what about the ring with an air gap? Does the current pass over the air? Does the resistance of the air produce heat? min 48:00

[@RobertSzasz]

Mauricio Barda in the ring with the gap the current doesn't flow along the entire ring, rether it flows in tiny loops the thickness of the rings wall. Since the force generated scales with the size of the loop there is almost no braking force. (Voltages generated are small, but fairly straightforward to calculate if you know the dimensions of the ring and the field strength)

[@shankyxyz]

Dear Professor, can we calculate the magnetic field the induced current generates using Amperes law? I guess if rate of change is slow, we can even neglect the Amperes law correction, just use Curl H = J where J=sigma E, where E is the EMF given by solving Curl E = -dB/dt? I always confuse myself on this concept.

[@lecturesbywalterlewin.they9259]

how many minutes into the lecture?

[@shankyxyz]

Just a general question that I always wondered. Can we go through this flow chart? Curl E = -dB/dt. From here we get induced current I (or current denisty J = sigma *E). From this we get from Amperes Law Curl H = J.

[@lecturesbywalterlewin.they9259]

Ampere's Law is NOT connected to Faraday's Law. They are independent eqs. You cannot derive Amp ;aw from Faraday's Law. But you can calculate the current that is due to Faraday in a given circuit and that allows you then to calculate the B field produced by that current. The dB/dt of that B-field, in general is VERY low. If you have no inductor in your circuit. Only R's and C's, and you use AC power. The dB/dt through an OPEN surface attached to a closed loop caused by the variable B field depends ONLY on the inductor value of your circuit and that is near zero of there is no L in the circuit.

[@geraltrivia3796]

Sir, we know that due to the hall effect, the lorentz force is neutralised by the electric field within a conductor. Thus a charge travelling will feel 0 N net force.My question is if a charge don't feel any force.Then why the conducting rod will feel the magnetic lorentz force?

[@lecturesbywalterlewin.they9259]

>>>Thus a charge travelling will feel 0 N>>> *not true* watch kzbin.info/www/bejne/hnvUgJR-d8ylaZI

[@geraltrivia3796]

But the hall effect tells that

[@AhmedAli-sn3ty]

Hey guess what he replies as well. Thank you. If i ever watched videos about EMT is only because of you.

[@lecturesbywalterlewin.they9259]

:)

[@n5sdm]

thank you sir.

[@lecturesbywalterlewin.they9259]

:)

[@physl2787]

Sir suppose there is a magnet. The magnet is oscillating. Hence the magnetic field at any point in the space will be changing. Sir sir is changing magnetic field at a point in space will produce electric field at that point. Sir it means that when we move the magnet in a particular space than electric field also produced at that space. Sir please please explain

[@lecturesbywalterlewin.they9259]

Yes, a moving magnetic field produces an electric field. Curl(E)=-dB/dt

[@sharudeva]

At 38:10 you told that the lorentz force pushes the metal out of the field. But when i applied Left hand rule, the force turned out to be towards the right side. Why is that so ?

[@lecturesbywalterlewin.they9259]

You must have made a mistake. The B field in which we move the disc is upwards!

[@sharudeva]

yeah! Got it Prof. Thank you very much for your spontaneous replies.

[@lecturesbywalterlewin.they9259]

Any Eddy current requires ENERGY and that energy is provided by gravity (that is the ONLY energy source). Thus the object MUST slow down in its fall.

[@kanadkrishanusengupta2578]

Thank you sir.

[@aradhyadubey6653]

Won't there be induced emf in case of eddy current because they also follow Faraday law??

[@lecturesbywalterlewin.they9259]

Yash, yes both are induced, use google for the definition of Eddy currents

[@diwashkarki8408]

how does the flying aeroplane develop potential between two wings?? luv ur lecture. .

[@lecturesbywalterlewin.they9259]

use google

[@pranjalverma3501]

Actually jee adcance 2020 question came from your eddy currents demonstration.