Lec 4 | MIT 8.02 Electricity and Magnetism, Spring 2002

[z0il]

47:42 : "and then, if I feel very good, I will do that again" - priceless face

[ZackFair524]

Can someone help explain the concept of where a volt comes from? I did my best to follow his explanation but when he started doing all the math and everything else I lost it. Just a conceptual understanding is what I'm asking cause right now that's a huge block in my way to moving onto other E&M concepts. Thanks

[chesterlott821]

This video is a favorite on Tbilisi

[OdaHanadw389]

hiya ich bin total naja wie soll ich sagen reich

[EclecticSceptic]

Thank you Walter Lewin! Voltage was presented to me very vaguely and as a concept I should accept on face value, I appreciate it being explained from first principles and then named at the end of the process. That is real teaching.

[adirajuadithya]

genius@@

walter levin

[foxconcept]

lol...Walter Lewin force F WL

[LegIsLate]

the camera man picks the good-looking students

[Thebuterflygurl]

the resolution is low... bt walter lewin is worth it.

[dimpubeckylu]

but if the work done by Walter and the electric feild are same then how is it possible to bring the charge closer to q1

[EclecticSceptic]

@dimpubeckylu The charge is pushing back Walter. Therefore he has to do work to overcome this. The force he needs to overcome the charge is arbitrarily greater than the charge's force. If you make an analogy with gravity, it is easier.

[EclecticSceptic]

@dimpubeckylu Lift an object of mass m a height h above the ground. The work done by you, in lifting that object will be mgh, where g = gravitational acceleration. This is more intuitive to us. Once you match gravity, all you need to do is be just, just stronger.

[moritzkremb]

I dont understand the light experiments in the end...For electrons to flow, doesn't there always have to be a loop? How can the electrons be flowing infinitely into one direction, wouldnt they be depleted at some point? And how can the electrons flow through the Professors body into the ground, when I dont see a way where they can flow back from the other side?

[Maelkenar]

@moritzkremb think non restrictive. You don't need a circuit for electrons, you just need positive, and negative charges. Electrons flow thru these charges, until there is no tension drop, or potential difference between them. I suppose you know this already. Imagine that negative and positive charges "float" thru the air. Because of this, when a negative, or positive charge is created, it will always find another complementary charge to wich it will discharge.Such charges exist everywhere

[Maelkenar]

@moritzkremb every molecule has charges, that's why electrons seem to travel indefinetly, when they are just closing a gap between 2 points. Same with the electrons traveling to the ground, the earth's core being a magnetic field generator it is the biggest generator you can have at the moment, every charge on Earth's surface, and close to it, will TEND to discharge towards it, unless it has somewhere better/closer to discharge it its vicinity. I cannot properly explain this to you

[Maelkenar]

@moritzkremb it is alien for me to explain physics in english, because western conventions differ from eastern ones, and simply because of the language barrier present when trying to explain something somewhat complex, yet simple

[danish2baswal]

the best teacher of the century! thank you so much, really



[devyanishrm]

amazing teacher,u learn even more when u see ur teacher so dedicated........ :)

[che517]

Muy bueno.. lo vi muy bien representado..! excelente clase.! como se llama el profesor?¿

[LV07TSK]

Professor Lewin has a NEW BOOK OUT........get it while its hot.....

[death666label]

Finally one professor that asks himself directly WHY they put the potential as Vb-Va with a minus on the integral. I mean, it's taken not as the work made by the electric field, but the work made by an external force that has the same magnitude but opposite direction... but at the end what's the difference? It's confusing sometimes. You've said it Walter, for reasons you don't understand yet ;)

[Zeykieee]

I love these chalkboards. So amazing. ^-^

[LegendLength]

@Zeykieee

Heh yes they mesmerize me every time with their complex positioning systems.

[Osman9100]

genius

[lthroy]

The girl on 4:54 is so damn pretty!

[EdEditz]

@lthroy She sure is! :)

[BeLieVeR11111]

@lthroy we can make a fan club :D

[EdEditz]

He sounds like he is a countryman of mine. Dutch that is.

[nikki0mitra]

@EdEditz yup...he is a dutch....i heard him say in one of his lectures...!

[EdEditz]

@nikki0mitra Cool! :) Thanks ^^

[GregTom2]

21:35, where the heck did he take that ballon from???

[MenialWitnesl1l]

OMG, interactive transcript is so awesome. check it out right under the video next to the description.

[junior1984able]

electric potential energy= q1q2/d^2 times kc

[AvatarOfGames]

36:48 hey baby, why dont we go release some potential energy? lol

[digicho]

In 5:50 doc's not speaking the whole truth.

Potential work in electrostatic field is a static figure. It's "conservative" for STATIC, "zero-duration periods", when no move nor event modifies the field! Can't move real charge statically (i.e. without changing rest of the e-field). Potential work can be "conservative". Work in dynamic field -not! So work depend of the path and events. In real world we can achieve the same point doing different amounts of work. The same for gravity. Free energy!

[MaximPodolsky]

@digicho whoa, wait. "Can't move real charge statically (i.e. without changing rest of the e-field)", a charge doesn't feel its own field! The total field changes, but the charge only feels the field from other charges.

"In real world we can achieve the same point doing different amounts of work." Example please!

[digicho]

@MaximPodolsky U r talking about movement through e-field frozen in time. It's not real. Moving charge changes its field, field changes position of near charges, they change their influence on our charge and so on. Moving means dynamic not static. Moving of charge is dynamically not conservative. So concept of work (moving from infinity) in static e-field is simplified or ill-defined.

Example: work depends on path in non-conservative field -> go to lecture 16 of Lewin /watch?v=G3eI4SVDyME

[yrsnkd]

@digicho u jelly physics? trololol

[dfnmartins]

MIT is MIT, actually !

[DormRoomMonkey]

I'm so glad I found these lectures. My professor for 1B at UCLA sucks! Nothing but derivations. And I like that I finally have something to compliment the book. Thank you MIT!

[zulek87]

he said that he has already discussed it in previous lecture ... explanation is easy... inside the sphere E=0 so it means that there is no force, if there is no force so u can rest inside... even if you have a little charge in your pocket and you walk there you don't feel any force, so if you don't feel any force you don't do any work if you don't do any work potential every where inside must be constant :) work = q*(Vb-Va)

[143mounica]

he never explains why electric potential when r<R is constant n maximum. around 11:30mins

[143mounica]

he never explains why electric potential when r<R is constant n maximum

[woodydrn2]

If he was my teacher when I was young, I would have a much better job now ;)

[JDBolt1958]

teaching genius

[JDBolt1958]

genius

[dankiewfc]

these 4 first lectures are really boring and not getting to me , I have to watch this 3 times over to get to grips of whats he talking about , are they really that necessary to understand the basics of Tesla ?

I cannot wait till I assimilated this well enough to move towards impedance and circuit analysis type of things .

[abelgz]

I am taking General Physics II, as it is called in my school, this semester. Thanks to professor Walter Lewin, I am able to understand the basic concept of the material. Physics always creats panic and tension in me, but the lectures are making me love it. Thanks MIT for making these lectures available on you tube. Profesor WL, thanks again for making physics come to life.

[whiterican74]

I like this guy he reminds me off Doc in the movie "Back to the Future"

[otTbc123]

we use a g Greek letter φ for electric potential and V for voltage, and voltage is difference in potentials so V= φ1 - φ2...and how some charges can have negative energy???

[endothermique]

@otTbc123 Actuallly,Potential is related to the work to move a "Positive" test charge. so imagine if you had to move a negative charge towards a positive charge. Obviously, you wouldn't have to put energy in the system to do that work, since the two charges are attracted by each other. In that case the system is giving YOU energy.

[otTbc123]

@endothermique thanks,it was very helpful;

[farestabs]

@otTbc123 too bad 4 u

[Angadude]

Saw many people ask this so I figured I would help - It doesn't matter which way the electrons go, the tube would still light up! Positive or negative, its all just convention.

If I call something positive, and it attracts something else, then that automatically becomes negative, because two identicals of my original object happen to repel.

[wittvk]

I don't understand a little bit at last experiment with neon tube.

If we make a constant flow of electrons in neon tube - then

electrons constantly appear at one end of tube and dissapear at other end.

Where from they come and where they go?

From air?

If so - then air is also have ionized with one charge near vandergraph,

and other charge farther away?

And what happens if we shut down vandergraph? Another flow of electrons

that restore charge balance of air?

[Ziacer]

@wittvk I think that when the electrons move, the gas got into an excited state and release photons (which gives us light), and then they become normal again

[feinsteing]

11:11 is kind of impressive

[AgentChaos1337]

Hehh....I love these lectures. The only annoying thing is that they pan to random students' faces when he's talking. I DON'T WANNA LOOK AT YOUR MUG FOO!

[djpatonics]

@AgentChaos1337 some of them poking their noses lol

[azezel2311]

this has implications on the orbital elevator. Half the challenge is getting above 20 miles. Then it gets much easier im thinking.

[Goatseeee]

Oh, and these video lectures are AWESOME since my high school teacher sucks at teaching...

[xenoepist]

@Goatseeee It doesn't get any better in college (unless you are at MIT, of course)

[Tyrann90]

lol he calls his force Fwl^^

[dan020350]

If one knows everything, then what must one have to know ? These calculations are hard to understand , i am just too ignorant :)

[dan020350]

free education bootleg from universities so we don't have to pay tution anymore ?

[Koolafeer]

easy to understand awesome lectures

ive never seen a professor like him.....

hezzzz great ...................

tributes to him

[nit323]

ONE word for Prof.Walter Lewin : AWESOME!!!!

[purememory]

not that im saying american education is bad.. infact, his traditional way of teaching is so much more easier to absorb than prepared slides

[purememory]

in asia, this is learnt in grade 11... i wonder which is better.. learning everything at rapid pace or slow paced learning like america's...

[PentagramSchematic]

In college, they always assume you haven't learned anything and teach it all over again.

[pLaSmAGrEEn]

@purememory not suprised, u guys r like 19 by that time lol

[purememory]

not surprised abt what exactly?

n wow i was shocked at the number of 'thumbs down ' my comment had. what i wanted to say was that i myself prefer relaxed american education system.

[pLaSmAGrEEn]

@purememory u shouldve made it more clear ..... lol

and in which case then i agree w/ u

[purememory]

aren't u guys 19 by the time u enter uni?

[Goatseeee]

@purememory

no, we're usually 18

[purememory]

well, thats 19 in some asian countries cuz we count extra year u spend in ur mother's womb.

[pLaSmAGrEEn]

@purememory 18 is normal here

[purememory]

yeah 18 is the normal age for universities for most countries not just USA, unless u r a guy n have to serve national service at age 18.

[pLaSmAGrEEn]

@purememory ohhhhh kool

[darkoriginhunter]

@purememory 18 or 19 usually

[dafatabacata]

hey when he places that gas tube parallel to the field lines with the vandergraph switched on,can someone please explain why the electrons within the tube are said to travel from higher potential to lower potential (left to right) because i thought the opposite was true.

[Rastamanchanter]

they go from high to low cause all things want to be at rest

newton's first law ... it want's to maintain a balence same is true for air pressure High pressure flows into Low never from low to high cause it want's to be at rest or balence not be forced into a situatuion where overall work is increased just remember newton's first law inertia and it all makes sence

forgive the spelling hope it helped

[ofey137]

dont listen to Rastamanchanter's baloney...

you thought right, (29:09) in fact the electrons moved from lower potential to higher. The potential caused by a charge: V=Q/4pi epsilon0 r, so if Q is negative so is the potential. Thus the farther you are, the higher the potential. So electrons want to move farther. But you can clearly see it if you consider the forces not potentials...

[mryahooshah]

u can download his lectures from itunes u for free

[helenbask]

I love his lectures. He goes into much more detail than I'm taught in my lectures and still it is so much easier to understand.

[esquadmiller]

Very very good. Much better explanation that both of my textbooks (Young & Freedman and Purcell).

The only ANNOYING thing is all of the sudden a student's face is shown. Why?!! It bothers the HECK out of me! I'm trying to pay attention to the instructor not to the surroundings. Very distracting way to film the lecture.

Overall, awesome. Thanks for the vid MIT.

[errold32]

I agree. Please keep the camera on the professor. Thanks for the good work!

[astrinick]

really very helpfull!!!

[inertialcapacity]

So Voltaire erred, and the result was we ended up with the incredibly complex electrical theories of today -- they hamper true understanding and progress. We should define resistance as (v.max - v.min) / ( vmax + vmin) -- where v is velocity of the electron front in the CSA.

This then makes mv-squared, in this case Rv-squed, equal to power. I am sure you will all see this is a more advanced system because simpler and easier to use. I hope many work on the theory independently of each other.

[NoticeOfDefault]

that because he stole the knowledge from the Egyptians and did not fully understand it ....

[bushyblowfish]

Clearly this isn't the best of all possible worlds...don't you mean Volta?

[srofficial06]

Perhaps if the metal of the vdg globe were thin enough and the room was made sufficiently dark such non conductive optical equipment as i've described could record the position of the sparks on the outside of the globe and also simultaneously observe and or record any phenomona on the opposite side of the globe from each spark ? or " on the axis " ? more or less ? every action has an equal and opposite reaction ?

[orvallo]

The professor is really good. He knows to show all the conceps using good tools for that. (excuse me if my english isn't good)

El profesor si que es bueno, él sabe mostrar y transmitir todos los conceptos con un adecuado uso de las herramientas a su alcance.

[lifeskills81]

At 21:21, to me the shape looks like a lightbulb. Where the electric field is zero there is a mobius strip that serves as a little reminder that there is infinite energy available for free. The green and red colors correspond to christmas and remind me that free energy is the ultimate gift TO BE GIVEN to humanity. Long live Nikola Tesla!

[Matice21]

I though he was the scientist from back to the future!!!

[srofficial06]

Might it be possible with the advent of fiber optics and very small cameras to place a device inside what has always traditionally been the " hollow" sphere of a van de graff generator such that "observations" could be made of the interior of the sphere whilst the sparks are jumping on the outside ? Perhaps there might be something interesting or worth observing , if not with optical equipment then perhaps with some other measuring apparatus ?

[janton08]

interesting question, my guess is that nothing would be going on inside of the hollow sphere as all of the charge is held on the surface.

[333irani333]

what makes a school great is its professors

[rajansh2]

really good presentation .........

[HomerWells007]

Does anyone how he got from the integral of Fwl dot dr to q1q2/4pi*e times the integral of dr/r^2 ....Sorry I am a little confused at the derivation

[janton08]

The fwl integral is equal to the fel integral because of conservation of energy; energy he puts in by pushing one charge toward the other is the same as the as the energy used by the electrical charges to push away. Then, because the force vector Fel and vector dr are parallel the dot product is just multiplication (cos(0) = 1). Next he uses Coulomb's law (F = (q1*q2)/(r^2*4pi*e) to substitute for Fel in the integral. Take out the constant term (q1*q2)/(4pi*e) and integrate dr/r^2.

[Richardwinkel]

what amazes me more then anything about magnetic fields is. It is theoretically possible to detect all other things in the universe, if you have the right tools. It the field is able to be detected up to infinity. Then you should be able to have a magnetics field in mars and be able to detect it on earth providing that you can filter out the other changes and are able to detect it.

[romanuh]

Why Fwl=Fel ? if they are equal the q are in equilibrium so there is no movement; it means that W=0 ?

[koningrobot]

If the net force on an object is zero, its velocity is constant. The acceleration is zero, but the velocity can be nonzero. So Fwl is the force needed to keep the object moving at constant velocity.

[yamaha893]

he makes it easy to know what lecture hes on by what kind of food is pinned to his shirt. convenient!

[dclair12]

This guy is awesome. Makes things very clear! Not to mention he's a boss

[alxmagic]

He is very imediate "teacher". This is best for a physicist.

[Zetimenvec]

when he says

⌠

⌡ E dot dr

does the "dot" represent a times symbol or a period? because if it's a period, I have no idea what that means in the context of mathematics.

[darthelessar]

none of them, it is dot product (scalar) of vectors. you can find further info about this easily i guess.

[alxmagic]

I dont usualy do this, but....pffff

[symphoma]

dot product of two vectors = the product of their magnitudes times the cosine of the angle between them.

So, if the angle is 90 degrees, dot product is zero.

:)

[Dynamics18]

look it up on the internet, there are many resources to explain this =)

[goodluckpeace44]

well if its a dot then you are calculting a line integral. either way you get same result, its the secret behind comnsecutive forces and line integral. u can find free e book abouit it.

[hopher10]

@Zetimenvec the dot between the E and dr means you use the dot product which is E x dr x cos θ (where θ is the angle between the two vectors E & dr) but in that instance E & dr are parallel so cos θ = 1 so you ignore it

[DexteriousDex]

The last demonstration was a good example, liked it

[sukardihenry]

good guy

[thefullmonte03]

I always understood that we choose the 0 of potential. That any particular reigon is not confined to have any particular potential like he said, but instead it is the change of potential that is meaningful... Loved the lecture though

[pimborman]

Please come to Utrecht. We really really need a good professor...

[fkafka64]

Dr. Lewin is originally from the Netherlands.

[Globalizator1]

Strange but USA lectures I understand better(100 %) than Russian.WTF

[notToast]

yea me too

i dont speak russian

[dodoshlodo]

lol

[ThePrivateBenReel]

I'm pretty sure he's Dutch...or German...definitely not Russian

[lordennis01]

This professor isn't typical for USA, infact if you look at most lectures, they cannot compare with this person. It is not the University, nor the country, but more so the person, this one in particular is special.

[Resonant2Frequency]

Sweet.....so much better than QUT first year physics....! he just makes it interesting instead of throwing tones of formulea down ur neck!....fantastic

[emptywallet]

and how do you upload vids which over 10 minutes of length?

[clerlic]

You have to be a Youtube partner, it was just a matter of changing a setting like a year ago. Now it's impossible as far as I know.

[Sixnend]

OMG!! School in the internet!! I want to die!!!

[juanespinosarsr]

this profesor is awasome. he's smart, and extremelly funny which in turns keeps the audience stuck and on track to the subject. I really enjoy watching this vdieo plus brings me back old mmemories when I went to college. I'm not an MIT graduate. but go MIT!!!

[clerlic]

Our lecturer is almost as good, except sometimes he gets excited and starts talking really fast. Also we only have maybe 2-3 demonstrations per month

[Wuzzi55]

fuckign genius...w/e he said i got it instantly..holy fucking shit! FUCK HIGH SCHOOL, I MUST GET OUT OF THIS HELL HOLE!

[jplonmars]

unfortuntely, not every college professor is as clear as he is......I guess that's why they charge $35,000 /year at MIT...good thing I get listen to his lecture for free. =)

[juanCarlosvp7]

This are gr8 explanations. Amazing, MIT and this Professor owns.

[BGasperov]

This doesn't seem to be too difficult, its a simple calculus!! But I still love it.

[simkodedon]

omg he owns

[MarcAntoni07]

Where does the charge needed for the current that lights up the fluorescent bulb come from, since there are no wires? And where does the charge escape from the farther end of the bulb?

[4txx]

charges probably just build up at ends of tube. when new electric field created by that charges cancels that of vandergraff, current drops to 0.

[jplonmars]

you probably didn't understand the concept...

the whole concept is that when there is a differnce in "potential electric energy"

electrons will automatically flow from lower potential place to the higher one .

of course, if you ground 1 end of the light, the electrons will be able to flow MUCH MORE FREELY and therefore will create a much brighter light. otherwise, electrons can still escape thru the AIR. (just not as quick)

[TIMESTREAMER12]

this...man ... is...a god in my eyes, he makes you really love science . he reminds me of one of my idols Nikolai Tesla

[sn1pe352]

Giancoli, D. C. Physics for Scientists & Engineers. Vol. 2. Prentice Hall aka screened and approved by government persons to make sure it doesn't teach too much!

[NicePeopleGive]

well said sn1pe352 -- irritating and true

[solmyr2]

ok good lecture .. btw do u know what textbook the students use ? feynman maybe ?

[Forsaken87]

Go to the MIT OCW website for things like this. Here is the textbook they use:

Giancoli, D. C. Physics for Scientists & Engineers. Vol. 2. Prentice Hall.

[xelloschan]

it is very popular, i use this book, i study physics in the netherlands

[harkken]

simply awesome, i wish my lecturer is as good as him

[gustgr]

excellent lecture. I love the demonstrations.

[nekrataal7]

around 14:50 i'm not sure what he says "lobby 7?" and i can't read what's on the board because the quality of the video makes it a little blurry. can someone clarify that? if it was lobby 7, what does that mean?

great lecture btw! =)

[ronron858]

lobby 7 is the next building over. he replied to lobby 7 as if we were at an infinite distance away from the charge

[azben1428]

I realy like it