Lec 23 | MIT 8.02 Electricity and Magnetism, Spring 2002

[surfax100]

My guess is that using a variable resistor is less efficient than using a variable inductor because it involves heat loss. as the second law of thermodynamics indicates there is no perfect engine.

[OKMUNWURX]

Since when does a 100 volt 100 ohm light bulb, which draws 1 ampere, operate at 50 watts? See 44:nn - 47.nn time

[TheRealBanana]

@OKMUNWURX

When you are dealing with AC circuits half the time the circuit is at 0V, which is why its 1/2*I^2*R

[OKMUNWURX]

Since when does a 100 volt 100 ohm light bulb, which draws 1 ampere, operate at 50 watts? See 44:nn time

[OKMUNWURX]

Since when does a 100 volt 100 ohm light bulbwhich draws 1 ampere, operate at 50 watts?

[kotofu]

Great review. I love how he focuses on the concepts rather than on the math.

[DrTruffaldino]

Dr. Lewin makes an error at very beginning of his lecture, writing a wrong Biot–Savart law: should be r^3 in the denominator, rather than r^2 as shown on the blackboard

[musicofthen]

@DrTruffaldino No, Dr. Lewin is correct. Notice that it is r-hat and not r-arrow.

[musicofthen]

@DrTruffaldino No, Dr. Lewin is correct. Notice that it is r-hat and not r-arrow.

[musicofthen]

@DrTruffaldino The form that he wrote it in is correct. Notice that in the numerator it is the unit vector r.

[mdinka]

@DrTruffaldino no the formula is correct as he uses the unit vector of r check wikipedia

[oshmunnies]

Does anyone know what lecture Dr. Lewin talks about inductance and magnetic field energy??

[shuwukong]

and hey, i like your hairstyle, makes me remember of Einstein

[henf5671]

@shuwukong he's a true genious like einstein. but without the eccentric nature. he is an awesome professor.

[kertzey]

so what does this thing do to our daily lives and how important it is??

[x6801]

@kertzey Electromagnetism is everywhere in our daily life. Lighting, motors, household devices, antennas (->tv, internet, mobile phones), lasers(->medicine), electronics, computers, the light itself is an electromagnetic disturbance propagating with, well, the speed of light :).

[kertzey]

@x6801 thanks.. now I know how important this subject is.. Im suck with math and science that's why somehow I don't have interest as such, too ironic life is nothing without it.. #LOL

[shuwukong]

@kertzey

life is much much more than just math and science

[Compact3]

How does he do the dashed line so fast?

[Xerangelo]

he sounds dutch XD

[Compact3]

@Xerangelo he is.

[yash91100]

how is he doing that thing with the chalk where he makes that broken line so easily??

[PivotStorm]

the only thing i know about MIT is Gordon Freeman

[7olusegun]

Hahahaha!! is this lecturer a comedian? He said those topics not covered will be 'covered' in the exam.

[emailbanoo]

@7olusegun : No he said there are many topics to cover for that particular day (which he already has taught them before ) that he cannot go through all of them in on day and the ones that he doesn't talk about them that day might show up in the exam.

[DumblyBrilliant]

What is great is that magnetic mono poles have now been discovered. I would love to have heard the Profs comments on that. I bet he buzzed for week.

[7olusegun]

Real? How was it discovered? Was it ever discovered or it was a hoax? Even as small as an electron spinning, there are two opposite poles (depending on which side you are looking at it).

[DumblyBrilliant]

Google this:

Magnetic Monopoles Detected In A Real Magnet For The First Time

Look for the Science Daily link

[Nicetry75]

"excuse me sir, um... how do i count cards?"

[RichardBass89]

B have to go out of the board, not in....it's a mistake..but, you teacher are an hero!

[goodluckpeace44]

Kids have brought their fathrs also in the lecture hall.

[FlavoredCoffeeGuy]

Why doesn't anyone use iron or low carbon construction steel in conjunction with permanent magnets to make high field magnets that are in the actual teslas? Can't you use permeability to enhance a permanent magnet?

[FlavoredCoffeeGuy]

If I take a round neodymium magnet 1 inch in diameter and 0.75 inches thick and place it inside of steel tube with an inside diameter of 1.01 inches that is 0.8125 inches high, and the top is north and the bottom south, then take a ring magnet with an inside diameter of 2 inches and an outside diameter of 3 inches, then place a steel ring the same height around it and continue until I have 5 steel rings and 4 magnets that are magnetized through the thickness, how strong will construction steel?

[ShinmaRyuu]

22:50 that was very very cool.

[hom3slicez]

Wow, I am in AP physics right now in high school and we have done pretty much the same thing with those Gaussian surfaces and electric flux and all that good stuff.

But there's no chance of me ever going to MIT so...darn

[goodluckpeace44]

you can start the University 1st year from quontum mechanics I suppose.

[JizzMasterPino]

holy shit 3:19

[1q1q1q1qjigsaw]

lmao

[kcmanc87]

I love his little digression into the threads of screws.

[bas3ballplaya24]

i live near MIT...

[soulreaverdoug]

so what... makes you that a genius

[CrashGames2108]

You well not talk good.

[georgesmeinders]

The guy has a Dutch accent, WE'RE EVERYWHERE lol

[aloha882]

You're right!

That is Professor Walter Lewin, received his PhD at the TU Delft.

Dat je het weet ;)

[Cypia]

I've got no idea what anything of this video is...

[anonymees]

the review lecture might not be the best place to start.