Lec 16 | MIT 8.02 Electricity and Magnetism, Spring 2002

[simysimss]

some great inforamtion here thanks

[elysianfury]

heads up, in the next video (17) he refers to this lecture (16) as the most important of all his lectures. pass this one around

[famster92]

there a very beautiful girls in this lecture

[muhammadzahmad]

the lecture is interesting while prof. makes sense...

[MrDBism]

I love how tries his best for his students

[mdgreg]

It's Good that he Emphasizes Understanding. Knowledge is Nice, but Understanding is Key.

[OKMUNWURX]

His demonstration at the last 6 minutes DOES make sense. He is testing the voltage across half turns of the 'transformer', which consist of resistors.

[nextblain]

homework homework we want homework XD

[Ghaiyst]

@ 0:36 "you are complete idiots!" lol I love this guy! He's right though.

[rinwhr]

2:13 that girl is like "yeah I am one of those people who found the exam too hard because it was easy"

2:30 that adult student is like "well screw you!"

[eddychaar]

Lol he brought a used gigantic condom to class

[qurabaguy]

chasteen it is very true .i am not an elite kid to make MIT and exploit my talent .i think the world is unfair in these centuries which i am hating it so much

[Osman9100]

Thank You.

Best regards from Bihac town,

Bosnia and Herzegovina

[calimat32]

Another mistake:

10cm^2 is 10^-3m^2 not 10^-2m^2

No student in MIT notice that? Why doesn't anyone asks questions to the teacher?

[cripling123]

@calimat32 Because it isn't the math that actually matters...

[Phnxdragon01]

@calimat32

wouldn't it be 10^-4 m^2 or 1^-3 m^2

[0Elijah0]

@Phnxdragon01

to convert cm^2 into m^2, multily 10cm^2 by 1m/100cm twice. When the units cancel, you will be left with .001m^2, which is 10^-3m^2

[condestable777]

I'm a little bit confused, he uses the solenoid, and he attach the crazy shape loop, first he does the measurement of the current and the galvanometer shows the same value of the current in both directions, but when he turns the wire to a smaller enclosed area and, he does the measurement of the current, the galvanometer shows a larger value of the current, why this happend? is right that the current value will no change but in this case it does. I can´t find yet wich are the causes of this.

[TheRoadsidePicnic]

Very helpful stuff. All power to MIT and the Prof!

[walle1976]

Great teaching indeed. I hear a dutch accent?



[brayosaintdead]

I dont understand why the students did not correct the lecturer when he made those mistakes such as ohms law on 36: 10

[SaMoVArr]

My brain, it burns!

[skatercdub16]

its really too bad over 90% of people who apply to MIT get rejected. the world would be a better place if more people got to go to schools like this

[aupStar]

Misspelling in the translation:

Hans Christian Oersted

Great vid..:)

[Huapua]

YEA SON, WHO GIVES A SHIT ABOUT KIRCHHOFF? NOT ME!

[theonelybrown]

Why is the end point so mind blowing? He simply made two circuits with the volt meters... so that the volt meters only read the current through one half of the original circuit...he merely changes the voltage by passing the current through only one resistor. Don't get me wrong i like his demonstrations and it shows the process clearly but leaves out the reality of the two circuits. Is this really hard to get?

[digicho]

@theonelybrown it's mind blowing because voltmeter has very high inner resistance so it is strange why voltmeter does not get the all 1V?

[theonelybrown]

Why is the end point so mind blowing? He simply made two circuits with the volt meters... so that the volt meters only read the current through one half of the original circuit...he merely changes the voltage by passing the current through only one resistor. Don't get me wrong i like his demonstrations and it shows the process clearly but leaves out the reality of the two circuits. Is this really hard to get?

[Xenthoid]

im glad they took the time to record these lectures...priceless

[mst7eel]

What a great prof, i wish i had one like that.

however, there's something wrong (48:00)

10 cm^2 = 10^-3 m^2 ,,

[lennyprankster]

1:30

[Mauroxm16]

Who cares about Kirchoff??!?!?!? =D

[Compact3]

I will accept that the math will give you different answers for different paths. But I just can't accept that if you simply flip the voltmeter you will get a different answer. You can't influence which path the voltmeter takes, or can you?

Can someone please explain?

[plantran1]

my professor at UCLA has the same idea. teach concepts. Fuck numbers, entire tests are variables only so there is no need for calculators. Too bad no one can understand his extremely heavy accent.

[KTLZ]

@plantran1 why do I have a feeling you are talking about Kumar Patel....

[406356690]

much better than the lectures here at UCSB.

[amodedude]

This teacher is amazing! So good at teaching.

[amodedude]

this teacher is amazing!

[x6801]

He is a very unique teacher. I have a degree in electrical and computer engineering. I have never seen something quite like this. Extraordinary!!!!

[vitron11]

@x6801 i want her name please

[AColonDashSix]

"I'm here to teach you physics not math."

Lol my chem prof says the same thing, about chem of course.

He's right though, anybody can plug numbers into an equation, but the goal of the Professor is to make you understand why that equation works, and when/how it can be used.

[FuFu4ever]

isnt the direction of the magnetic field at 6:22 wrong? it should go from right to left if you look how he draw the solenoid.

correct me if i'm wrong!

[ares213]

@FuFu4ever Yeah, it's kind of hard to tell from the drawing. The direction of the magnetic field could travel in either direction depending on your interpretation of the diagram. In his diagram, the field is pointing to the right, meaning that the solenoid would be coiled in a counter-clockwise when viewing it from the right side.

[fri5sko]

Isn't 10cm^2=10^(-3)m^2

[KClaisse2]

@fri5sko

10cm = 0.1m

10^(-3) m = 0.1 cm

[1Mperios]

@fri5sko No it's square, 10cm = 10^-3m but in square you have to double it.

[fri5sko]

@1Mperios That's exactly what i said :P

[1Mperios]

@fri5sko No, you have put square as a power...

[stomachtalk]

our physics deparment at columbia is no where near as helpful as MITs

[luzzie9]

Absolutely first class teacher; he makes my old physics teachers look sick. Some have it, some don't, that's life,as Prof. Lewin states.

[subash3]

they clap when he reduce the assignment. funny

[Sk82beast]

Did.. did he just call me an idiot :'(

[LogInForPaper]

I love when he says "You are complete IDIOTS!"

Great video.

[tkshoe]

man i wish this guy was my physics professor. Too bad i wasnt born into an elite family that could afford MIT. Currently Im taking this same class at the university of utah.

[kevrs2]

hey i wouldnt worry ,my education began the day i left school,:)

[chasteenfamily]

You can't go to MIT because you weren't "born into an elite family that could afford MIT..."?? The fact that you'd bother to think this way is the real reason you can't go.

[burnmothafucka]

sometimes we have to keep reality in mind. if your income is 30k/ year and you have two kids that means you can barely survive. so going to A community college is far beyond impossible.

[westz36]

@chasteenfamily ur 1 of those fags who gives ppl attitude. fag shit

[lingojac]

@westz36 He was not giving attitude. He was being realistic. Assuming that MIT students are all children of rich families that bought their way into MIT undermines the vast majority of the students that worked extraordinarily hard to be accepted by that school. And calling someone "fag shit" suggests that you don't have the maturity level to go to any school at all.

[westz36]

@lingojac i go to a good school, and wat about u? fagget shit

[EJNewbury]

@westz36 Then go to your "good school"'s english department and ask them to spell Faggot for you, Retard.

[westz36]

@EJNewbury ur still a fagget lol

[EJNewbury]

@westz36 It's more laughable that you actually think that's a come-back, n00b.

[westz36]

@EJNewbury tats wat she said, comeback

[westz36]

@EJNewbury tats wat she said, 'come back'

[JohnBreadForger]

Most elite universities have great financial aid packages for single students. The trouble is getting them to pay attention if your a white male from public schools of modest upbringing.

[sparkyinwa]

why, over a hundred years after the discovery of the electron are you still showing conventional current flow?

[v1taly1988]

Because electricity became a huge thing after it was discovered, and later when an electron was discovered they thought it would be too much work to change the notation to a more intuitive one. But apart from that, it's a perfectly good notation, as one can think of this as +ve charge unbalance flow, i.e. the amount of protons left without electrons. The math is the same and no-one really bothers about it, as long as you know the convention.

[dawesy38]

you are an idiot

[blumpkin123456789]

U have to be the greatest physics teacher that has ever lived

[cleocool]

ahahaha lmao 2:30 wtfff

[techieatwork]

HI Professor. Enlighten me please. If there is no magnetic monopole on a closed loop surface (22nd minute of your lecture), how come the magnet's surface particles are monopoles by themselves part of a larger monopole called the magnet itself? What you call a monopole then? Each atom is in itself a monopole. Everything in the universe is made of monopoles.

A nullification of magnetic field on the neighbor areas does not imply that the monopoles are not there.

Thank you professor.

techieatwork

[1V10ntyPyth0n]

@

techieatwork

r u mixing E with B-fields? A magnet is no monopol, it aktually has always a south and a north pole..., take a look at the wiki artikles witch are giving more elemental information.

[panchoavendano]

¿?¿?¿?¿? wtf???

[1V10ntyPyth0n]

your engaging me to study this. thank you

[lordennis01]

2:30 can anyone explain that to me please...

[Cloxxki]

Just like another poster, I distinctly hear a Dutch accent here. I here myself babbling. Aweful accent we have, and so little to do about it. But at least we make sense :-)

[spinynorman1982]

Mmm. Where can I find the equivalent of recitations online for MIT? Ah I'm just kidding. I'll read more books :)

[92yara]

I don't get how a changing magnetic field could induce a current, because that means the magnetic field is actually affective electrons that were initially not moving in the wire. How could a magnetic field affect a stationary charge?

Is it possible that it is actually the electric field of the wire that creates the magnetic field that is inducing the current in the second wire?

[scf84]

If that was the case - you would've had a current in every conductor, everywhere.

The change in magnetic flux DOES create a non-conservative electric field:

curl(E)=-dB/dt

This electric field is what moves the charges and causes the induced emf.

[92yara]

why would there be a current in every conductor?

Consider this: The electric field at a point near a conducting wire is zero in the direction parallel to the wire because there is the same number of electrons below and above that point. However, as you increase or decrease the current, the number of charges entering the wire is always greater or less, respectively, than the number of charges leaving and thus you get a net force that pushes electrons forward or backward in the secondary wire.

[scf84]

The charges never enter/leave the wire.

The direction of the electric field is clockwise/anticlockwise, it drives the current carriers "around" the change in magnetic field.

That means that if you have a conducting loop around the magnetic flux change - you will get a current in it, because the electric force will make the electrons move in the direction of the wire (not perpendicular to it).

[v1taly1988]

Electrons are always moving inside any conductor, there is like a "sea" of electrons going in random directions due to random thermal motion (because they are effectively unbound to the nucleus, last electron in particular of each atom). And when you apply an E or Magnetic (B)-field, a force appears that gives this random motion a defined direction (but they still have their random motion, just overall there is a net movement in the direction of the force). Which produces net charge flow:current

[goodluckpeace44]

physics is both mathematics and deep concepts that has a different decency than mathematics i would say. a person can have a very deep intitution power that he will feel everything in the world without knowing to prove them.

[kazumawarren]

? what...? He just stated that this stuff is incredibly non-intuitive. Lots of things in physics go beyond intuition; quantum physics, relativity, etc..

[goodluckpeace44]

I was just saying about an excentric case where a man would be able to predict things like the mental calculator. But in mathematics for example, Ecluid had a big place about his ancient theories in the history of knowlage, because he was so idealisticaly interested in proofs ignoring intitutions. Greeks didnt know about 0, and Eclid proved lot of things with 5 axioms. But in physics you could be a great guesser. The fact that you are not makes you spend millions of dollars on experiments.

[shihui1]

wth are u talking abt man... lol you just dont understand...

[goodluckpeace44]

you are the one who don't understand.

People cannot intituitively speak about things. That is a problem and it is also a gift. Non intitution gave people mathematics and proof and Euclid was a great man, who reffused to believe many things others believed. But we have limits like we cannot see atoms, so we had to do experiments.

[ghaffuarbhai]

WOW !!!!!!! Hez DAmn GoOD

[dasgregorian]

Also, one thing I don't think he ever specified... is this discussing DC electricity or AC? This is only for a 10MS duration, so it looks like he's using DC?  But only while the DC is kicking on?

So, wouldn't this be a rather constant effect in AC electricity since the electricity is constantly changing directions, therefore the magnetic field is also? Or is the "change" he's talking about a change in magnitude, rather than direction?

[kazumawarren]

He is talking about a change in magnitude. You would see a magnetic field with an AC circuit as well, but it would constantly flip directions, as opposed to a DC circuit, where a magnetic field changes in magnitude (as long as the currents magnitude is changing with time) but retains its direction.

[CCbball23]

This guy is badass...I will no longer attend lecture because this guy is so much better!!

[hughbacca89]

WOW ... this guy is sooooooo much better than my physics lecturer, not to mention my physics lecturer based his course on this course!

I wish i knew about these vids before now, i would have skipped my leatures and watched these vids, and then maybe i wouldnt feel like i might fail in a months time !!!

[v1taly1988]

(See comment below 1st). And his lecture notes support this. You see I1 is going clockwise relative to V1, so its top terminal is negative and connected to D. Whereas I2 "meets" V2 from the top, so it's top terminal is positive and is also connected to D. So they are measuring "integration" over different paths.

[goodluckpeace44]

yeah those stuff is just trivial mistakes, everybody makes mistakes. you are right about direction.

[v1taly1988]

(comment about the last few mins of the lecture). I accept the fact that the emf induced in the wire depends on the direction and the path of the integration. And surely emf in one direction will be different to the emf in the other direction. But I disagree that if you connect two identical voltmeters with + joined to + at one point and - to - to the another point. I think they are connected + on one voltmeter to - on the other voltmeter (which will obviously "integrate" over different paths).

[psychobollox]

36:00ish... huge brainfart ... but what i like about htis guy is how he puts in those little insets to point out his own brainfarts! :D

I'm a fan of TWO Lewins:

Kurt Lewin - social psychologist

Walter Lewin - physicist

[mamagdy64]

He is a great teacher of physics. He teachs students how to think physics and not to solve problems. He is an ideal excellent teacher of physics and I am follow his path in teaching.

[b0brenjc93]

teaches*

[insightllc]

@mamagdy64 Teach me how a world would not notice that in 1899 Nikola Tesla made Earthquake Milne Shide #333 British Association Record No. 333 on September 3 at 3:03 p.m. a 27 day swarm of earthquakes that cracked off musket shots at you before up hundreds of earthquakes and he spent last ten years and died in Room #3327

[farestabs]

@mamagdy64 God bless

[harshvgaus]

the 2:14 is gorgeous. but i live at sydney uni, there are 1000's of chicks like that to scope out during lecture

[RomeoTurbo]

So the students to instructor ratio in Australian university is 1001 : 1 ... and probably you are the only 1 male in 1001 students during the lecture!!

[hasnain721]

At 28:32, he has got the direction of the current wrong. If the magnetic field is coming out of the page then the current flows anti clockwise using the corkscrew rule.

[pepomk123]

You are wrong , the current will be anticlockwise if the wire was producing the field. In this case the solenoid is producing the field , so ITS field is anti clockwise. The field is induced to the wire , so the current is right. :-P

[hasnain721]

do u mean that the current is induced by the solenoid? is this the reason why the direction of the current is to oppose the flux change that caused it ? (lenz's law).

Thanks.

[pepomk123]

Yes , that is why there is a minus in the equation.

[hasnain721]

Kool.

Thanks!

[thefullmonte03]

Lenz's law says that the induced current will act to oppose the change in flux. Since the magnetic field is increasing out of the board, the induced magnetic field should point into the board. This would be true if the induced current flowed clockwise (by the right hand rule)

[rockYhre]

I think it's because it has different direction (I could be wrong)

[mademedoit]

ok since my comment didnt seem to get through :(

if those volt meters are attached to the same two points. how do they know which is which? which goes left and which goes right? if they are attached to the same points, could i just use one volt meter, take it and bend it over to the other side? would it read a different value? this cannot be possible...

[mademedoit]

are comments not allowed?

[notToast]

02:30 lol

[nangbutinang]

how lucky to be in this lect with that cutie>..

this murderous course could easily understood

with her aura...

[ldefranchi]

omg, the girl at 2:14 is truly beautiful. kisses to her from Chile. :*

[mysciencenow]

stop grumbling

[Lucky1Lucky1]

Walter Lewin is an excellent professor! Are all MIT professor of his calibre in lecture? If so, MIT students definitely have one less excuse for any poor academic performances.

[darijo203]

more you server to the people, more they are lazy to explore. Really, science is far more about exploring then pure study. You don't have to attend collage to become an expert.

[einarabelc5]

OMG its the Doc!!! Lmao, this guy reminds me of the definition of an scientist they gave me when I got into college. I bet it has something to do with what he does.

[ratselvolle]

Wish my teacher was like this. I learned Biot Savart with this guy on youtube, not in my classroom...quite sad in a way...

[nagamora]

he is morrocan hhhhhhh

[SlykeThePhoxenix]

This guy is awesome.

[ofey137]

is the man at 2:30 a maths prof maybe?:D

[villeelliv]

Hahahaa fucking lol

[crack700]

iam glad to know english, thanks it i can understand his teaching, thnxs MIT.

[4pharaoh]

I can't wrap my mind around the demonstration given on the last few minutes on this lecture. Does anyone have the details of exactly what was done? It would seem that two separate inputs fed from exactly the same source read completely different voltages, and that can't be the case. I have to assume there is something not seen here

[ernzzz]

maybe it was because the current was not generted by a battery, but thru electromagnetic induction, using a solenoid and a magnet instead...

that dont means i actually understand it tho.. kirchoff and complex magnetism is exactly the point i could not advance anymore highschool's electrotechnic class, like 6 years ago; from then on it was all cheating exams lol.. i wonder how could they teach this to 16yo kids like me..

[landin048]

If you click on the blue link in the description at the top, it takes you to the lecture page. On the left hand side is a list of resources you can click on and download in PDF files and print out.

[decade03]

Me brother had this guy a year ago, said he was a conservative looney meaning he didn't show the full potential, but a great professor

[theGreatSheva]

my textbook makes that mistake...with the non conservative fields...

damn book manipulates the circuit to make Kirchoff's laws seem right...a fucking textbook! that i paid a 100 bucks for...

-_-

[flashback1234]

His accent sounds dutch. Does somebody know if he is?

[henan8]

Yeah, he is..

[flashback1234]

cool

[jonahansen]

It's neat that prof lewin took the time to review these and correct mistakes - it makes them even better/

[elassowipeo12]

28:45 The flux remains the same, no matter what shape the closed loop is; of what consequence is this?

38:55 Solenoid replaces battery, solenoid seems to be placed in not wired into curcuit???

[cgibbard]

The magnetic field is (almost) all inside the solenoid, and so the magnetic flux through the surface of any closed loop that goes once around the solenoid will be the same, and so the induced current will be the same. However, when he makes a different closed loop, which winds multiple times around the solenoid, the flux through the surface on that loop is different, and the induced current is larger.

The solenoid is indeed not wired into the circuit. It's the magnetic flux which counts.

[Astontower]

How can all the flux be inside the solenoid! Flux lines must be closed loops. They must come out of one end of the solenoid and return to the other end.

If the external loop is very large, then there will be very little net flux linked to it: an equal amount of flux will go into and out of the arbitray surface.

[vulcancountry5]

what you say is true, however the magnetic flux lines outside of the solenoid are so sparse and so weak that they are effectively zero in a region that is just outside the tube itself. You are correct that a very large loop would read zero volts but that loop would have to be impractically large. I hope this helps.

[cgibbard]

(Hmm, my previous comment didn't seem to post, here's another attempt.)

The flux through the attached surface for any loop that goes once around the solenoid is the same, regardless of its shape, but when the loop is wound multiple times around the solenoid, the surface passes multiple times through the solenoid, and the flux and hence the induced current is increased.

The solenoid is indeed not wired into the circuit. It's the changing magnetic field it provides which induces a current.

[sgoo7]

The girl at 2:15 was awesome. mwah to her

[kermittherabbit]

Yeah, a very big mwah to the pretty princess!!!

[robakerson]

This guy is great.

[RobertKubica1121]

very good. Thank You