Lec 9 | MIT 8.02 Electricity and Magnetism, Spring 2002

[ericajjful]

brilliant video

[distractionxx]

some really good stuff here

[zuproc66]

nice... so informative.. if you were my professor in physics here in the Philippines, i would probably be a physics enthusiast and a physics prof like you!.. you are an inspiration to the young generation!

[PROFFESORSEXY]

so thats why those schools are so expensive...but they are the smart kids why do they get the good teacher?

[mdgreg]

What Else is Happening With the Candle?

[haienkmenk]

THANK YOU!!!!!!!! :)

[amrosik]

something you should know in this lecture, is the real imagination of current flow.

take for example a long wire, that is miles long, and swirled up at some points.

now try ti realize how the electric field of an Potential difference, that you apply, will develope through this wire. The current signal would reach the other side of the wire a bit later, so the other side of the wire doenst know yet that you applied a voltage. why is that? the electric field of the potential source...

[amrosik]

@amrosik ... the electric field of the potential source, for example a charged sphere, or a simple battery is not simply creating a electric field through the wire... it is only working on the immediate section of the wire, by repelling/atracting the immediate charges.... and as this charges move forward, they create a charge uncalance, thereby repelling the next charges and so on... and this movement would look like a wave which is moving along the wire.

[MrPriyantgupta]

look such a big teacher also writes m/sec instead of m/s . it becomes a habit .

[bobsinclair6]

since we're doing jokes, what does a resistor say when things get hot and heavy? ans : "ohm" my god

[rinwhr]

Why is half the class empty? 25:45

[northbeachfilms]

"A turtle would go much faster than these electrons." Love these lectures.

[ampedee]

It might be here useful to point out that magnetism is the effect of the divine ray and manifestation in the same sense the electricity is manifest effect of the primordial way of active intelligence.

[DotAMastA4Life]

His hair makes him look like einstein

[md65000]

I've watched all the lectures up to this point and not one person has asked him a question. Doesn't this guy allow people to ask him questions in class?

[interstitialofficial]

@md65000 Only about what he's covering, and what is the probability that you have a question that happens to correspond to his lecture? Also, if they're able to follow it, they don't have questions, and if they can't follow it, they wouldn't know what to ask that wouldn't embarrass them.

[sgtbigs22]

It's a lecture bro. No questions just notes. Plus he is streaming it so people asking questions would waste class time. Finally, he explains it very well, and MIT students who are taking this review course most likely understand this information from high school anyway.

[rahulilrplac]

I was thinking the exact same thing. Why are these sessions a one-way ball game. No interaction :(

[interstitialofficial]

If you follow this, you dont' need it. If you don't follow it, it doesn't help you. You first have to work up to the level where you can follow it... but then you won't need it. Lectures, therefore, amount to just showing off.

[LegendLength]

@interstitialofficial

Erm no. I have learned *tonnes* from these first 9 lectures.

[riflemaster]

@rexineffex Thats photons, not electons

[riflemaster]

How does that blackboard system work. Its amazing how he pushes one board up with little force and it never falls back down when he lets go. Do they employ stoogies to hold them for him behind the scenes

[erubiza]

If we keep using classical point of view that treating electrons as particles, we can never fully understand this phenomenon. In order to understand it, treating electrons as wavticles (wave and particle like) is important and the key. Although quantum does not show a picture how electrons "look" like, and we currently are not able to imagine electrons neither, the mathematical result from quantum proves that electrons are definitely NOT particles. Otherwise, we would reach a contradiction.

[92yara]

if it takes half an hour for electrons to move 10m in a copper wire how come electricity moves so fast (for example when I flick the switch the light bulb lights up almost immediately)?

[invalidkin]

think of current as water in a hose. If the hose is full, then as soon as you turn the water on, the pressure causes it to instantly come out of the hose. That seems to be the case with current, free electrons are already in the wire, so their flow is almost instant, as soon as you flick the switch.

[nathananise]

imagine you are pushing a line of balls aligned in a small tube. The ball you are touching takes n times the time more than the last ball in the line, if there are n balls you are pushing. Here, every ball are capable of conducting your force or energy (analogous to electricity here)

[lethalevi7]

How is it that I look for an exhaust system for a car on youtube, and this is the video that comes up? That's one question that can't be answered by this video.

[tchonia]

44:10

[brngva]

Finally understood how to charge myself up with an electrostatic potential, starting in 43:10!

[tchonia]

43:10!

[phj1990]

I like his breakfast badges!

[JonThm]

Water is high electrical or magnetic feilds, under pressure, does nuclear fusion!

[FlavoredCoffeeGuy]

If I placed a charged sphere inside of a hollow conductive sphere, and the diameter of the smaller sphere was at breakdown, would the larger diameter sphere charge up like a Van De Graff?

[veryrice]

When he uses the oscilloscope on the light bulb, shouldn't the path of the oscilloscope change as the light bulb turns on and off? There's bound to be leftover heat from the light bulb, so why isn't the curve of the oscilloscope starting higher and higher? I don't think that the heat from the light bulb would dissipate that fast...

[riflemaster]

@veryrice Thermodynamics

[bucles2000]

The resistence in the light bulb is a semiconductor resistent?..and the resistence in the distil water just don't allow electrons to move on?...in other words a semi conductor resistence the electrons move but with caos and the resistor like distil water there is not such a movement,,,,am I right?

[MuleXC09]

I'm taking an online class and I've had to watch these videos all year long. I also had to watch the videos for 8.01 (which by the way he references in the beginning of this lecture)

I was afraid when our class came to the end of mechanics that the new set of lectures wouldn't come from Prof. Lewin. His way of explaining things really helps, the demos are cool, and it's just a really great tool for learning.

[Orange19viollett11]

nice

[scozzahisee]

has he got a big enoigh black board lol?

[lowfat422]

his handwriting is flawless, don't you agree?

[Adrianrff]

Not only that, his schemes, equations order and hability to explain are also flawless. Excelent teacher, I wish I had him as a teacher when I took Electromagnetism in collage, it would have made my understanding of it much easier

[surfshark13]

@Adrianrff It would have been even greater if said teacher taught you how to spell.

[Adrianrff]

@surfshark13 I'm sorry if I misspelled something you pedantic hater, but english is not my mother tongue. I taught my self the lenguage

[mifster83]

I have another great joke, laughed alot first time I heard it.

A scalar particle of the quintessence field is a Quinton, what's its superpartner called?

the Tarantino!

[jonahansen]

How about: One atom is talking to another, and says - "Jeez, I lost an electron!". And the other atom says - "Are you sure?" to which the first retorts "I'm positive!".

[killabilla]

hehe nice

[mohab1989]

@jonahansen fall out 3 :D

[TheBochas11]

@jonahansen hahahahahaha nice

[Adrianrff]

@mifster83 I have a better one!

Heisenberg is stopped by a traffic cop who askes: "Do you know how fast

you were going?"

Heisenberg replies: "No, but I know exactly where I am"

[Liam2020]

Ahaha, funny physics joke! I got another one. God is reading a book called "Phyisics laws" and he is laughing.

[Adrianrff]

LOL

[killabilla]

thats a diss man :(

[anony913]

I don't get it

[Gicior]

I don't like the idea that Ohm's law breaks down. Ohm's law holds, what breaks down is our assumption that the resistence of a particular object is constant.

[clerlic]

Well you could think of it as the gravity law, which only applies for big masses and small speeds, good for most applications but not so good for really precise measurements.

Ohm's law will probably hold precisely enough in most temperatures that modern electronics have to endure, the tolerances on components also add up to the uncertainty so it's not really much of a big deal, digital circuits don't really need the accuracy in millivolts when it's +5 volts for 1 and 0 volts for 0.

[rebirthatthefountain]

ohm's law does break down and i'm glad it does because if it didn't, we wouldn't have all the wonderful circuits in our TVs and computers : )

[Adrianrff]

I disagree, it does break down, ohms law states that the difference in potencial is linnear with the resistance, that is just not true, the "law" works for a short range of temperatures (with acceptable precision) but the temperature itself depends on the current, it's somwhat like newton's law, it doesnt hold at high speeds, it's not a law anymore. Lewis states it clearly, the relation between V and I, becomes a function of Temperature, and at the same time a function of I since T = funct(I)

[yetidh]

All you say it's true but remember that this is a basic physics course. The things you say are surely taught in more advanced physics courses.

[riflemaster]

@yetidh Try Berkeley then

[Gicior]

I don't like the idea that Ohm's law breaks down with temperature. Ohm's law holds, what breaks down is our assumption that resistance is constant.