8.02x - Module 07.01 - Discplacement Current.

[@hiccup3.14]

I can't believe I still haven't watched all your lectures sir, You truly are one of the best teachers of all time, I'll continue binging your lectures after completing my honours. Have to solve all 150+ problems as well

[@unchainedfemaleyoucan6206]

He is just beyond anyone 🤩

[@andreathecat100]

Discussion of P3 is TERRIFIC FANTASTIC!Thanks Professor! Thanks so much!

[@arantheo8607]

No sophisticated computer generated images , no music, some pens, paper and a great professor.

[@lecturesbywalterlewin.they9259]

I do not do PowerPoint, I teach physics.

[@chemicallystimulated476]

it is really great that these contents produced years ago is helping many now

[@dybydx31]

One of the finest explanation of displacement current, I have ever had.

[@andreasstrauman3261]

Thank you! Interesting and mathematically intuitive explanation of displacement current! You make it all seem so easy!

[@lecturesbywalterlewin.they9259]

:)

[@softwarephil1709]

Brilliant, clear, thorough explanation. 👏 This is an absolute gem of a lecture. Thank God this was recorded and can be watched by future generations.

[@johncgibson4720]

Thanks for saying the "displacement current" is a misnomer. I was about to freak out trying to figure out how the accumulation of electrons on the plate can be viewed as current.

[@mayurm9917]

We need more professors like him!!

[@antiferroelectrica]

I have never seen such a great explanation. Congratulations!!1

[@aditya_p_mishra]

Best teacher ever seen .

[@northernskies86]

Best explanation I've ever heard 😊

[@wymchihiro]

best version to understand displacement current

[@cyriactalus1865]

Thank you so much professor. You're a god.

[@lecturesbywalterlewin.they9259]

:)

[@aourabindorout861]

Sir, you are no less than genius... Love from India...

[@teberer3246]

This was a perfect explanation of E, B and et al.

[@physicsismyfiancee...1353]

Marvelous ,fabulous ,fantabulous lovely and mysterious.💞💞💞💞🙏🙏🙏

[@lavanyashanwal842]

thank god somebody said loud and clear THAT IT IS NOT ACTUALLY CURRENT. THAT DISPLACEMENT CURRENT IS A MISNOMER. when we were taught, the teacher said there is actually current through the air bw the plates of capacitor...and i would get so confused ...bcoz there is nothing there that conducts..so i would try to come up with explanations like electric breakdown of air.........-__-. THANK YOUUUU SIR :}

[@aidanabregov1412]

I understand this now. Thank you so much!

[@brianfrancis5548]

He did not explain how the closed line integral of B.dL is evaluated as B*2*pi*r. Since B and the differential length element dL are parallel at all points along the chosen concentric circle I think the dot product of vectors B and dL (B.dL) reduces to BdL since the cosine of the angle between the two vectors is 1. Since B is not a function of position along the loop I further assume we can treat it as a constant for the purposes of evaluating the integral. The closed line integral of B.dL would then become B times the closed line integral of dL. The remaining definite integral then evaluates to 2*pi*r - 0 = 2*pi*r. Leaving us with B*2*pi*r.

[@armstrongtixid6873]

He didn't explain it because he explained it in his lectures about Ampère's Law, which he assumes the viewer has watched.

[@Markoul11]

@Walter Lewin According to Maxwell's thinking and his epistemic invention and interpretation of the displacement current, must be there always be a displacement current present in order the charge of an electric capacitor circuit to be ideally conserved and maintained?

[@lennysusskind2995]

at 11:50 you said if current was constant then charge would increase linearly with time. I thought capacitors charged exponentially? Could you please clarify?

[@lecturesbywalterlewin.they9259]

>>>>>if current was constant then charge would increase linearly>>>> that is a correct statement. On a different issue, charge C with R and battery in series. I cover that in my lectures.

[@HarshaVardhan-we1wc]

@@lecturesbywalterlewin.they9259 Sir but if current is constant and charge across the capacitor increases, the potential drop across the capacitor too should increase. This should lead to a decrease in current, right sir?

[@aliberk3112]

Very good explanation. I was a little sleepy during the last physics class so this really helped me :)

[@lecturesbywalterlewin.they9259]

:)

[@aaryanporwal]

Please someone guide me to become a physics teacher!!!

[@lecturesbywalterlewin.they9259]

First: get a bachelor's in Physics. Then, of possible also get a Master's degree or a PhD.

[@geekyhelps8752]

@@lecturesbywalterlewin.they9259 that's the easy part he is asking how to become someone like you

[@sahilsagwekar]

@@geekyhelps8752 just be yourself that's the best that you can do

[@mosesmccabe8983]

thanks for making these videos

[@ssvaph]

Hi sir ,from 10:00 by finding the Magnetic field outside a capacitor P2 I don't understand why when we calculate by creating the Open Surface the why the Electric flux is not changing meanwhile when we calculate by creating the Flat Surface there is the changing of the Electric Flux ?

[@lecturesbywalterlewin.they9259]

kzbin.info/www/bejne/aaSzap6ead2ngdE

[@ssvaph]

Ohhh thank you sir ;D

[@bachirnaitrabah2889]

Good explanation. Thanks !

[@lecturesbywalterlewin.they9259]

:)

[@mahmoudeldesokey147]

Will not the magnetic field be zero at point 3 if the change in electric field is zero? That is why i asked if you consider the capacitor is being charged.

[@frix123]

Does charging capacitor depends on amperage of the charging battery? And does displacement current drain the battery?

[@bharatchoudhury3383]

I love physics and you sir

[@weilam]

Can I be a physics teacher with a bachelor in Mathematics?

[@EnergySeeker]

is it possible the displacement current does a flux? like in the sandwich coil, making flux go to 2nd on transformer making real power

[@lecturesbywalterlewin.they9259]

displacement current is NOT a real current. *The name is a misnomer.*

[@TharunRPrakash]

Respected Sir, Thank you for this wonderful video and no offence but you do look like Benedict Cumberbatch :)

[@greece4surf]

are Help Sessions for 8.03 avaibabel?

[@lecturesbywalterlewin.they9259]

no

[@omarmuhamed6637]

Thank you for this good video but how there is current go throw inslutor capacitor ?

[@lecturesbywalterlewin.they9259]

one plate turns positive and the other turns negative. That is equivalent to a current.

[@omarmuhamed6637]

Lectures by Walter Lewin. They will make you ♥ Physics. Thank you very much 💜

[@zeyadomar200]

Thank you sir for this amazing effort, but I have a question, Can I use ampere's circitual law although the wire is of finite length?

[@lecturesbywalterlewin.they9259]

THINKKKKK and answer for yourself!!

[@zeyadomar200]

@@lecturesbywalterlewin.they9259 I guess it's no but why we used it in this video

[@lecturesbywalterlewin.they9259]

@@zeyadomar200 if a wire is 1 m long, Amp Law will get *quite accurate values* for B if you are near the middle 20 cm of the wire and not further away from the wire than about 10 cm.

[@zeyadomar200]

@@lecturesbywalterlewin.they9259 oh okay thank you 😅

[@ammarbinhashim8547]

shouldn't the electric "flux" be σ/∊₀? 13:01

[@zishangwu1916]

That is just electric field.

[@shyamdas6231]

Thank you, Sir.

[@vijayr1485]

How can the conduction current be taken as constant? If capacitors of finite area are charged then shouldn’t the conduction current decrease?

[@lecturesbywalterlewin.they9259]

A change in I is irrrelevant. Watch this video again. Choose I(t) at any moment of time t.

[@abhiksasmal3304]

Professor, every time-varying electric or magnetic field that exists in vaccum satisfies Maxwell's equation and hence they must satisfy travelling wave equation. So, my question is- If I hold a charged comb and move it in vacuum in a random way, then does its electric field become a travelling sin wave ?

[@lecturesbywalterlewin.they9259]

moving acceleration any electic charge will generate EM radiation. For more info watch my 8.02 lectures or use google

[@Hgkbukk]

2:53 "This is a 'ELECTRIC FLUX', and this is the time derivative of that 'ELECTRIC FLUX'"

[@lecturesbywalterlewin.they9259]

there is no electric flux at 2:53. there is a derivative of electric flux.

[@Hgkbukk]

Thanks but all I was talking about was the hillarious dubbing of the video. You can clearly hear how the word Electric flux has been added to patch over the original video XD

[@mahmouddesokey3818]

Please sir do you have a video about lc circuits?

[@andonipulido61]

Good methodology!!!

[@arifullah7395]

Sir can we say that displacment current is due to Ac source bz if we connect the capicator with dc source than we the capicator is fully charged then it acts as an open ckt nd current wl be zero so if there is no current there wl no B.field but if we connect Ac source then alternate charging nd discharging takes place nd as result inside the plates displacment current will be produce due to which B.field is produce..am i right sir??

[@lecturesbywalterlewin.they9259]

en.wikipedia.org/wiki/Displacement_current

[@arifullah7395]

@@lecturesbywalterlewin.they9259 tnx sir..lots of love fr u respected sir

[@seemaphadke178]

what is the provision of choosing the hat like structure ? i did not follow kind ly explain?

[@lecturesbywalterlewin.they9259]

If you choose any other closed surface you will not be bale to prove what my goal was.

[@seemaphadke178]

i am not at all following this sir?

[@lecturesbywalterlewin.they9259]

watch the video (if needed more than once) and find out what my objective was. If you can do that you will understand why I chose the closed surface the way I did, if you cannot do that, than I cannot help you any further.

[@tigerresearch2665]

We are working with Ampere's Law and Stokes' Theorem here. Take a look at the equation at 1:49. The righthand side has two expressions which are added up (+). The first expression is a contribution to your B-Field due to a flowing current (I_C). The second part of the equation deals with an E-Field, which changes over time (that's what we call the Displacement Current I_D). The hat like structure was chosen as such that we get rid of the contribution due to the E-Field. That means, we took a funny surface (which we can do, because Stokes Theorem still holds) just to prevent any contribution of the E-Field and shift the problem to the calculation of the contribution to the B Field only caused by the current (I_C).

[@harshaggarwal6632]

Thank you sir...for such a great explanation..but i have a doubt..if we are close to the plates of the capacitor then can we use ampere's law to calculate magnetic field as now the wire will no longer be infinite ??

[@lecturesbywalterlewin.they9259]

if the plates of the cap touch each other the capacitance becomes negligibly small and Ampere's Law alone (w/o the displacement current) will be fine

[@harshaggarwal6632]

Ok sir...thank you so much

[@483_saurabhjindal6]

Sir, the magnetic field due to conduction current is constant with time but magnetic field between the plates is changing with time, then how can we apply#Ampere law to it. And also why we assume both fields to be same

[@lecturesbywalterlewin.they9259]

If you charge a capacitor with a batterry with internal resistance then the current is high at first and then becomes zero as the capacitor is fully charged (exponential decay of the current). Take any moment in time when the current is I(t) you can then apply Amepere's Law to find the B field caused by the wire by choosing a closed loop around the wire (and an open surface) attached to that loop. However, if you now use the closed loop inside the capacitor and an open surface that cuts through the wire (as I did here), then we must find the same value for the magnetic field as we did before. That's why Maxwell adjusted Ampere's Law (with the *displacement current)* so that the same value is found for the B field.

[@lecturesbywalterlewin.they9259]

frotget that I is constant - I should not have said that but it is irrelevant. Take any I at any moment in time then the two measuremements of the B field must give the same values for B produced by the current I in the wire. That is only possible because of Maxwell's adjustment of Amepere's Law with the *dispalcement current.*

[@borisfilipovic5253]

​@@lecturesbywalterlewin.they9259maybe you have to suggest that there is constant current source

[@mahmoudeldesokey147]

Please sir, do you consider the capacitor is being charged while doing your calculations? And if we used the bag surface to calculate magnetic field at p. 3 after the capacitor is being charged, will the time derivative of electric flux be zero?

[@lecturesbywalterlewin.they9259]

I cannot add to the clarity of this video if you have a specific question refer to how many minutes into the video

[@mahmoudeldesokey147]

20.25 do you consider the capacitor is being charged or fully charged?

[@lecturesbywalterlewin.they9259]

your question is meaningless as B does not depend on the charge on the capacitor.

[@mahmoudeldesokey147]

Will not the magnetic field at point p3 be zero, if the change in electric field is zero? That is why i asked if you consider the capacitor is being charged

[@lecturesbywalterlewin.they9259]

B field at P3 is independent of charge on Capacitor. The charge can be ZERO. B at P3 only depends on I and r_P. This is my last msg.

[@EnergySeeker]

can this displacement current be part of real power? or only reactive power?

[@lecturesbywalterlewin.they9259]

question unclear.

[@EnergySeeker]

@@lecturesbywalterlewin.they9259 Can these current cause a flux? let say 2 wires coming from the outlet the 115 v AC placed on 2 flat copper as sandwich coil and the 2 copper flat area are separated by a plastic tape or Myler which is like a capacitor and placed on top of transformer core can this flux make a real power from the 2nadry of that transformer core

[@lecturesbywalterlewin.they9259]

*displacement current is not a current; it's a time varying electric field* en.wikipedia.org/wiki/Displacement_current

[@EnergySeeker]

@@lecturesbywalterlewin.they9259 but can it change the flux , if yes then it can generate power on the 2ndary of the transformer

[@EnergySeeker]

@@lecturesbywalterlewin.they9259 but it creates a magnetic filed and change of flux , if this capacitor lay down on a transformer core can it generate real power

[@santoshkamar3082]

Sir if I choose such open surface which is being not penetrated by conduction current then what happened

[@lecturesbywalterlewin.they9259]

that surface must be inside the capacitor. You will then find the same answer (displacement current) as the surface through which current is flowing.

[@seemaphadke178]

why is it called displacement current what displaces there?

[@lecturesbywalterlewin.they9259]

Ask Maxwell! WHY! It's not even as current! Thus the name is a misnomer.

[@seemaphadke178]

i did not understand what you are saying? how is it a misnomer?

[@lecturesbywalterlewin.they9259]

It's not even a current. !!

[@seemaphadke178]

+ALRU no I got the answer the conventional current is displaced by the changing electric flux and hence the name displacement. I didn't ask Maxwell

[@lecturesbywalterlewin.they9259]

In electromagnetism, displacement current is a quantity appearing in Maxwell's equations that is defined in terms of the rate of change of electric displacement field. Displacement current has the units of electric current density, and it has an associated magnetic field just as actual currents do. However it is not an electric current of moving charges, but a time-varying electric field. en.wikipedia.org/wiki/Displacement_current

[@tashi2009]

Sir, if current is AC then even in P1 there is changing electric field(so changing electric flux) and so current is not just conduction current alone because there is displacement current as well. Sir please help me regarding P1. .thank you

[@lecturesbywalterlewin.they9259]

displacement current is not a real current.

[@priyadubey3127]

Thanks sir please tell me how can we get the admission in MIT

[@lecturesbywalterlewin.they9259]

read MIT's admission site

[@yashijainj8032]

Sir, can u pls explain why integration of B.dl will be same for both of open surface and the closed circular surface???

[@lecturesbywalterlewin.they9259]

how many minutes into the video?

[@anjumsadiq6760]

Need explanation for all content of physics

[@lecturesbywalterlewin.they9259]

you should take my 8.02 course - it's all there

[@sliut1]

what happens if the current isnt constant?

[@bensonkwok951]

I'm just a high school student but heres what I think: As long as you can find dE/dt, you can just use the equation. Its just that in that case dE/dt isn't a constant and hence the magnetic field would change over time

[@s.s.l2858]

@@bensonkwok951 *I'm also in high school now, 🤔In that case, EMW generates by the capacitor. When dE/dt itself varying with time. Because Now, the both time varying field change into each other. Like in the case of LC Ossilations. We got the EMW from the LC system.* *But When Electric field between a capacitor doesn't vary with time 🤓.* *So, this can't be the case for a capacitor.*

[@albadarqamar7380]

Looking handsome 😀

[@aaryanporwal]

Oh god! I just loooovee physics! I wanna become a physics teacher so badly! 😭😭 It's so euphoric !

[@MeenaMeena-tk1lt]

Hey I loveeee..,........... Physics tooo so much

[@momentumclassesbyAvaneeshsir]

No one teach displacement current like this and say AHA ! 🙏😊

[@jfkciudadano]

❤

[@kousesri8769]

I didn't understand ..he didn't explain from fundamental things he did not say any reason for the statement he is saying so I got confused and overall didn't understand anything

[@andyeverett1957]

The professor only made one mistake in an otherwise great lecture, he keep putting the marker caps back on. Thank you.

[@gntkllrrbt5]

:)

[@charlessmith833]

Without all the wicked math it appears to me that when plate #1 is charged with a lack of electrons plate #2 accumulates electrons to match on it's plate but none pass over. When the opposite occurs and #1 is charged with a surplus of electrons plate #2 will unload it's plate back into it's connection. All happens because of magnetic attraction. No electrons actually pass over directly. This explains why a capacitor can retain a charge when the circuit is interrupted. One of it's plates will still be loaded with electrons with no place to go. The other plate will also be on hold as long as the electrons are piled up on the other plate. How's that for an easier explanation. Forget the confusing math crap. You eggheads are always trying to super explain everything.

[@lecturesbywalterlewin.they9259]

>>>All happens because of magnetic attraction.>>> *that is incorrect*